Flu drugs may help ward off the low-grade inflammation and related cognitive decline that can come with HIV infection, an early study suggests.

Upwards of 24% of people with HIV experience some degree of cognitive impairment that interferes with functions like attention, concentration and multitasking. These declines are often mild but can worsen quality of life, and they can happen even when a person consistently takes HIV medications that suppress the virus.

Understanding why this impairment emerges and how to prevent it is especially relevant as the population of people with HIV grows older.

"This is very important," study co-author Mohamed Abdel-Mohsen, an associate professor of medicine in the division of infectious diseases at Northwestern University Feinberg School of Medicine. "Despite the fact that people living with HIV live longer, so the lifespan is preserved, the 'health span' is not as preserved."

The new study, published Friday (June 5) in the journal Med, pinpoints a potential driver of this decline along with a strategy to reverse it. The scientists used mouse experiments and blood samples from HIV patients, so the work is at an early stage, but it could open the door to future treatments.

"Not only do they describe a mechanism, but they then report in animal models that this mechanism is something that potentially could be targeted by an intervention," said Dr. Alan Winston, a professor of HIV and genitourinary medicine at Imperial College and a consultant physician at St. Mary's Hospital in London. "That's really what's novel," said Winston, who was not involved in the study.

Anti-inflammatory sugars

HIV treatments are now so effective that people with the chronic viral infection can live nearly as long as people without HIV, provided they consistently take the medicines. In 2022 in the U.S., more than half of people living with HIV were over 50 years old, and that figure is expected to grow.

HIV treatment, called antiretroviral therapy (ART), suppresses the virus so that it can't multiply. This prevents the disease from progressing to AIDS while also driving down transmission rates, making the virus unable to spread via sex. But while ART wards off many symptoms of HIV/AIDS and extends an infected person's lifespan, it does not make the chronic infection completely benign. People with HIV still show various signs of accelerated aging, including earlier cognitive decline, in some cases.

"Before we had antiretroviral therapy, we saw profound effects of HIV in the brain," Winston noted. HIV-associated dementia is now rare, because its rates declined as the use of ART increased.

But some percentage of people still develop milder cognitive impairments, and "it can actually have quite a big impact on concentrating at work, for instance, and undertaking daily activities," he said. People have a higher risk of impairment when they've had a big gap between contracting HIV and starting ART, and interruptions in treatment can also raise the risk. However, the underlying cause of the impairments isn't fully understood, Winston said.

To unpack that mechanism, Abdel-Mohsen and colleagues investigated a clue that's recently emerged in the medical literature: sugar molecules called glycans.

These sugars are found throughout the body, including in the bloodstream, where they bind to proteins and fats. In circulation, they tag certain immune molecules to keep inflammation in check. But with age, the concentration of these inflammation-controlling glycans declines: This decline is fairly steady for males, while it has a sudden dip around menopause for females, Abdel-Mohsen said.

Scientists have previously explored how this decline might contribute to age-related diseases, including cancer, while Abdel-Mohsen and colleagues have investigated its relationship to HIV. In a 2024 study comparing people with and without HIV, they found that those with the infection show age-related shifts in their glycans at younger ages than usual. These shifts come with an increase in enzymes known to degrade the sugars, as well as increases in inflammation.

"While it's low-grade [inflammation], it's still damaging because it's continuous," Abdel-Mohsen said. "It's chronic; it doesn't get resolved."

Potential treatment?

In their latest study, the researchers wanted to see if they could connect the dots between the glycan loss, inflammation and cognitive decline.

They analyzed blood samples from more than 100 people with HIV who contributed data to the AIDS Clinical Trials Group, a global research network. All the individuals were taking ART and either had signs of cognitive impairment or did not, according to various assessments. The group with impairments showed greater losses in two glycans that help control inflammation: sialic acid and galactose. In particular, these losses were most pronounced in women, and they correlated with the degree of cognitive decline seen in a given person.

It's not clear why this pattern was more pronounced in women, and that remains a question for future study, Abdel-Mohsen noted. That said, it's known that glycans dip around menopause and that key immune cells bear estrogen receptors, so it's possible that estrogen levels play a role, he suggested. Winston wondered if the timing of diagnosis might also be relevant, since women tend to get diagnosed later than men and may therefore start ART later.

After studying the blood samples, the team used two models of HIV infection in lab mice. One enabled them to study the glycan changes and inflammation triggered by HIV, while the second let them examine signs of cognitive decline. They again found that infection was tied to a loss of glycans and increase in inflammation, as well as worse performance on cognitive tests.

Next, they set out to see if they could reverse those effects.

A class of drugs called sialidase inhibitors work by blocking the effects of enzymes that split sialic acid, one of the key inflammation-quenching glycans. These drugs include Tamiflu, because flu viruses rely on sialic acid to infect cells and spread through the body. The team found that, at least in mice, these flu drugs helped preserve sialic acid, prevent inflammation and improve cognition.

We saw that "this class of inhibitor could reduce inflammation caused by viral infection … and even more importantly, preserve cognition and and prevent memory deficits," Abdel-Mohsen said.

Abdel-Mohsen considers this work "proof-of-concept" that glycans may play a role in HIV-related cognitive decline, pointing to potential treatments. The end goal isn't necessarily to use flu drugs to treat cognitive impairment in HIV, but rather to identify compounds that could "either prevent the glycans from being degraded or just replace them," he said.

One limitation of the current study is that its definition of cognitive impairment was limited, Winston noted. It relied on cognitive test scores — how well participants solved various puzzles — and a standard "scoring" system that's used to assess a handful of cognitive functions. Including details of patients' symptoms and challenges they experience in daily life would have made the meaning of those scores more tangible, Winston said, and he would hope to see that in future work.

In the long run, Winston could see this research leading to a "precision medicine" approach in which patients' glycan levels are checked, and if they're low, they could get a treatment to address that. It's likely that different factors drive cognitive impairment in different people with HIV, so matching patients with the right treatment is imperative, he said.

And in theory, it may be that the loss of glycans also drives other issues seen in aging adults with HIV, such as increased rates of cardiovascular disease. If that's the case, the same treatments may serve multiple functions. "It will be definitely fantastic to study and see how targeting this mechanism could impact the multiple comorbidities," Abdel-Mohsen said.

This article is for informational purposes only and is not meant to offer medical advice.

If it feels like everyone around you has come down with the flu, you're not imagining it.

As of Jan. 9, the U.S. Centers for Disease Control and Prevention (CDC) estimates that at least 15 million people have gotten sick with the flu this season. Of those, 180,000 have been hospitalized, and 7,400 people have died from flu so far, including 17 children.

These numbers make the current flu season one of the worst the U.S. has seen in decades. According to Syra Madad, an epidemiologist at the Harvard Belfer Center and chief biopreparedness officer at NYC Health + Hospitals, who publishes a weekly analysis of respiratory virus trends in her newsletter Critical Health Voices, the country is experiencing the highest number of flu cases in nearly 30 years.

At Phoenix Children's Hospital, flu cases have been high since mid-December 2025, said pediatric infectious disease specialist Dr. Monica Abdelnour.

"Although the CDC [currently] estimates this flu season to be of moderate severity, the rates we are seeing are similar to, and in some cases slightly higher than, those observed in the 2024-2025 season, which was deemed high-severity," she told Live Science in an email. "It remains possible this flu season may ultimately be categorized as high severity as well."

While the CDC reported a slight dip in the number of lab-confirmed flu cases in the first week of 2026, compared to the last week of 2025, Madad told Live Science that it's "too early to declare we are past the peak." Flu activity in the U.S. usually peaks sometime between December and February. The CDC also noted that the small drop in confirmed cases could be linked to expected decreases in doctor visits and testing during the holidays, not an actual drop in flu spread.

The CDC recommends prompt treatment for people with suspected flu symptoms who are at increased risk of serious flu complications, including pregnant women and people with chronic health conditions, such as asthma, lung disease, diabetes, or heart disease.

Madad said that flu antivirals are "underused" and reminds people at high risk of severe flu or who are caring for someone who is high risk to ask their doctors about antivirals shortly after they suspect an infection. The good news is that current data show subclade K is still responsive to the standard antiviral medications doctors use for influenza, she said.

This article is for informational purposes only and is not meant to offer medical advice.

Scientists have developed a new, needle-free way to deliver vaccines: through the gumline.

In a new proof-of-concept study, researchers successfully vaccinated mice against influenza by cleaning their teeth with dental floss coated with inactive flu viruses.

Most vaccines are administered using a needle, an approach that has its drawbacks. For example, apprehension about pain from the injection and needle phobias can deter people from getting vaccinated. Additionally, injections require more medical expertise to administer than needle-free options, like mouth drops or nasal sprays, and are more challenging to store and distribute.

But a floss-based vaccine could eliminate the pain and logistical challenges surrounding injections and "even be distributed through the postal mail," the researchers behind the development wrote in their study, published July 22 in the journal Nature Biomedical Engineering.

Such a vaccine could potentially be deployed in "resource-limited settings with minimal training," they added, and be especially helpful in active outbreaks when vaccine coverage needs to be boosted quickly.

Previous studies have shown that vaccinations delivered in the cheek or under the tongue trigger satisfactory immune responses. But it can be difficult to deliver adequate doses of these vaccines through mucosal tissues in the mouth — the lining that acts as a barrier between our body and the environment.

Related: Acne vaccine: Experimental shot for common skin condition reaches clinical trials. Here's what you need to know.

The researchers behind the vaccine floss found a creative solution: Researchers focusing on gum disease have found particular areas in the mouth that are very permeable, meaning molecules are easily absorbed by the tissue. One of these areas is called the junctional epithelium (JE). The JE is found on the tissue between teeth, at the spot where the tooth's surface meets the gum line. By secreting different molecules, the JE detects and defends against pathogens that try to get in through the gums.

The study researchers thought that the JE's ability to allow molecules through and to stimulate an immune response made it a potential candidate for a vaccine site. To reach it, they needed something that could get into that small crevice between tooth and gum. So, they went out and bought some dental floss.

To explore this concept, the researchers tested their hypothesis in mice. Once they'd figured out how to floss a mouse's teeth — turns out, it's a two-person job — they set up a flossing schedule to expose 50 mice to an inactive flu virus. Killed, or "inactivated" viruses cannot cause infection and are a common component of vaccines; they're used to immunize humans against diseases such as hepatitis A and polio, for example, and are found in some types of flu shot.

One group of mice had their teeth flossed with the virus-coated floss three times, with two weeks between each dose. Then, a month after their final dose, they were exposed to an active flu virus. All survived, while a comparison group of mice that was left unvaccinated all died.

Further testing found that the mice that had been vaccinated via the floss had a strong immune response, producing ample antibodies and many immune cells. This immune defense was found throughout the body — known as systemic immunity — and in their saliva and feces.

"The floss-based vaccination induced both systemic and mucosal immunity, while conventional intramuscular shots largely stimulate systemic immunity," first study author Rohan Ingrole, a chemical engineer at Texas Tech University, told Live Science in an email. "Mucosal immunity is important because most of the pathogens enter through the mucosal routes," he emphasized.

In theory, vaccine floss could thus have an edge over syringe vaccines by triggering this additional protection, but the team would like to directly compare the two methods in the future to validate this idea.

Next, the researchers wanted to know if flossing could transfer compounds to the JE in humans. As an early test, they used a fluorescent marker and blue food coloring to coat a dental pick and had healthy volunteers floss their teeth with it. Photographs confirmed that a fair amount of the dye was transferred to the space between their teeth and gum, though just over 41% was left on the floss.

The next step, the researchers said, is to translate the research to larger animals, which "can then lay the foundation for human testing in the near future," senior study author Harvinder Gill, a bioengineer at North Carolina State University, told Live Science in an email.

This article is for informational purposes only and is not meant to offer medical advice.

Two might be better than one: A new vaccine that targets the viruses behind seasonal flu and COVID-19 triggers a stronger immune response than flu and COVID shots given separately, trial data suggest.

The combination shot, dubbed mRNA-1083, is made by the pharmaceutical company Moderna. The promising results of the Phase 3 clinical trial of the vaccine were published May 7 in the journal JAMA. This late-stage trial included two groups of adults ages 50 and older who were given either the new vaccine or a combination of previously approved flu and COVID-19 vaccines.

The trial runners looked at the quantity of antibodies, or protective immune proteins, made in response to each vaccine regimen. Antibody levels correlate with how well a shot should protect against a disease and how long that protection might last. They do not provide direct data on how well a shot drives down infection rates in real life — but ethically, such real-world data is difficult to gather when effective vaccines against an illness already exist.

While the U.S. Food and Drug Administration (FDA) has not yet given mRNA-1083 a final stamp of approval, experts told Live Science that the trial results are largely positive. However, political factors could hinder the shot's approval, some say.

Why do we need a combination vaccine?

The U.S. Centers for Disease Control and Prevention (CDC) already recommends that people get both their annual flu and updated COVID-19 vaccines at the same time. But at the moment, that requires two separate shots to be given at the same appointment.

The combination vaccine would be "a one-stop shop," said Dr. Peter Chin-Hong, an infectious-disease doctor at the University of California, San Francisco (UCSF) who was not involved in the Moderna trial. "A lot of times, people are not excited about needles. I mean, who likes getting a shot?" he told Live Science. With the decline in vaccination rates in the United States, a combination vaccine offers more convenience and helps people get on top of their immunization, he said.

The idea of combining multiple immunizations into one shot is not new.

"It is a very commonly used strategy, especially in children," said Dr. Monica Gandhi, an infectious-disease doctor at UCSF who was not involved in the study. An example of this is the measles, mumps and rubella (MMR) vaccine, which is a combination of three vaccines in one shot.

What is in mRNA-1083?

The new mRNA-1083 vaccine is based on the same messenger RNA (mRNA) technology that Moderna used in its FDA-approved vaccine against SARS-CoV-2, the virus behind COVID-19. mRNA is a genetic molecule that relays instructions for cells to build different proteins.

The new COVID-flu shot combines the company's updated SARS-CoV-2 vaccine, called mRNA-1283, and its newly developed flu vaccine, mRNA-1010. As of yet, there is no FDA-approved flu shot that contains mRNA, so this could be the first.

The culprits behind seasonal flu are influenza A and B viruses, which can be further classified into subtypes and lineages. Current flu vaccines in the U.S. guard against two subtypes of influenza A viruses, called H1N1 and H3N2, and one influenza B virus lineage, known as the Victoria lineage. (Until very recently, the shots also guarded against a second lineage, called "Yamagata," but it's likely extinct.)

The flu component of the new vaccine contains genetic instructions for human cells to make glycoproteins — molecules of protein and carbohydrate — found in these four flavors of influenza A and B viruses. These glycoproteins, known as hemagglutinins (HA), are located on the surface of flu viruses and enable the viruses to latch onto host cells and initiate infection. Once made, these proteins are shown to the immune system so it can recognize the viruses and fight them off.

Likewise, the COVID component of the new vaccine contains the genetic instructions to make the proteins displayed by SARS-CoV-2, called spike proteins.

"mRNA vaccine is the vaccine of the future, particularly when you want to respond to rapidly changing diseases or new variants," Chin-Hong said. Whereas the molecules in mRNA vaccines can be manufactured very quickly as a virus evolves, conventional flu vaccines take months to make. Chin-Hong described mRNA vaccine production as "cutting and pasting a code"; the mRNA that encodes the virus protein can be conveniently incorporated into the platform.

Moderna has already completed Phase 3 clinical trials for the individual flu and COVID components of its new combo vaccine.

On its own, the flu shot triggered higher immune responses than two conventional flu vaccines — Fluarix and the high-dose Fluzone — against A(H1N1) and A(H3N2) strains. It triggered comparable immune responses against influenza B viruses, based on the trial results. Moderna also showed that its new COVID vaccine induced greater immune responses than Spikevax — the FDA-approved shot — against XBB.1.5, a version of the omicron variant that's been circulating recently. Both vaccines were tested on adults ages 18 and older.

How did the combination vaccine perform?

The purpose of Moderna's latest Phase 3 clinical trial was to measure the robustness of the immune response the mRNA-1083 vaccine triggers in people. The trial runners measured this by looking at the number of antibodies vaccinated people produced against both the flu and the SARS-CoV-2 viruses. Real-world data, which would indicate whether the vaccine effectively lowers rates of hospitalization or emergency visits from the infections, is not yet available.

Consistent with these earlier studies on individual vaccines, the mRNA-1083 combination shot triggered a strong immune response against all four influenza viruses and XBB.1.5.

In the trial, the company recruited two groups of participants: one consisting of adults ages 50 to 64, and another including people 65 and older. Each cohort consisted of about 4,000 adults. The 50-to-64-year-olds received either the new combo shot or a combination of Fluarix and Spikevax. The 65-and-older cohort received either mRNA-1083 or a combination of high-dose Fluzone and Spikevax. (Fluzone, a high-dose flu shot, is approved for adults ages 65 and older and some younger people with weakened immune systems.)

The researchers sampled the trial participants' blood at Day 1 and Day 29 of the experiment to measure the levels of antibodies. They found that the mRNA-1083 vaccine triggered a more robust immune response — as reflected in higher levels of antibodies — than the other vaccine combinations against the SARS-CoV-2 variant.

It also triggered a stronger antibody response against all four influenza strains compared with Fluarix, and three influenza subtypes — H1N1, H3N2 and Victoria — compared with Fluzone.

Will the combo vaccine be tested in younger adults?

While the individual components of the new vaccine have been tested in individuals ages 18 to 49, the latest clinical trial for mRNA-1083 did not include this population.

"It was done in people who are at the highest risk of getting in trouble medically [from both the flu and COVID-19]," said Dr. Robert Schooley, an infectious-disease specialist at the University of California, San Diego. "But it doesn't mean that if you're under 50, you don't want to get a booster shot." Everyone 6 months and older is recommended to get an updated COVID-19 shot when new ones become available, typically on an annual basis.

In a recent statement, Moderna announced that the company is deprioritizing further research on mRNA-1083 testing in adults ages 18 to 49. In the same announcement, the company cited an effort to reduce its operational expenses as a factor in the decision. It is unclear if or when the company might resume testing in this age group.

Does the combo shot have more side effects?

Based on the clinical trial results, the likelihood of mild side effects — such as fever, fatigue and chills — was higher with the combination vaccine than with the currently available vaccines given separately.

"These are expected. The side effect is actually your immune system waking up, and likely shows that you'll get a very durable response," Chin-Hong said.

He also added that serious side effects are very uncommon for both the already-approved COVID and flu shots and in the trials of the new combo vaccine. There were no serious side effects related to the new vaccine, the trial runners reported.

When could the vaccine be approved?

Moderna initially applied for FDA approval back in 2024, using the preliminary results from the same Phase 3 trial. At the time, the FDA asked for more data to show efficacy against the flu. With more data in hand, the company is now "targeting approval" for the vaccine in 2026, according to a recent statement.

However, when it comes to mRNA vaccines, both scientific and political factors are at play, Chin-Hong said.

"The science is unmistakable: it [mRNA] is very nimble; it is durable; it is effective in general," he said. But he pointed out that mRNA vaccine technology itself has been the target of political criticism in the U.S. that traces back to the COVID-19 pandemic, when the vaccines' development was expedited and safety concerns were raised. Even though mRNA vaccines have proved to be very safe and effective, this history may pose a barrier to FDA approval of the new combination vaccine, Chin-Hong said.

In addition, the existence of already-approved separate vaccines for flu and COVID-19 may lessen the urgency in getting the mRNA-1083 approved, he added.

Research funding through the National Institutes of Health (NIH) has been widely cut or frozen, and research on vaccine hesitancy and the boosting of vaccination rates was specifically affected. Experts told NPR that they're concerned mRNA research will soon face similar cuts. NIH officials were cautioned to keep the term "mRNA" out of grant applications, KFF reported.

There are also uncertainties about whether mRNA-1083 will be subject to the new framework on vaccine approval. On May 1, a Department of Health and Human Services spokesperson told The Washington Post that "all new vaccines will undergo safety testing in placebo-controlled trials prior to licensure." A placebo is an inert or inactive substance, such as a saline shot, that new vaccines would have to be compared against during a clinical trial.

Many trials of brand-new vaccines already include placebos. But when there are already existing and effective vaccines for a given disease, comparing a new shot against a placebo isn't necessarily helpful or ethical. Scientists want to understand how much better the new shot works compared with the previous one.

"I don't think it's ethical to give someone a shot that is a placebo, that can in no way help them, when you know there's an existing technology that can," said Dr. Paul Offit, a virologist, immunologist and director of the Vaccine Education Center at Children's Hospital of Philadelphia.

Because mRNA-1083 is a modified version of an already-approved vaccine, "I'm not sure whether that counts as new," Offit said. He thinks a placebo-controlled trial would not be appropriate in this case.

Although the new combo shot still awaits approval, Gandhi said the current clinical trial results were "already convincing." The Phase 3 clinical trial demonstrated that the vaccine is safe and triggers a robust immune response, she added.

"I don't see any red flags at this point," Schooley said. "I'd be confident to take the vaccine myself."

This article is for informational purposes only and is not meant to offer medical advice.

The flu is a contagious viral infection of the respiratory system that affects the nose, throat and lungs. It is caused by influenza viruses.

Symptoms of mild flu infections can resemble those of the common cold, which is also caused by viruses; that said, flu symptoms tend to be more intense than cold symptoms and come on faster. And in severe cases, flu symptoms can be serious enough to lead to hospitalization and death.

Flu viruses are categorized into four types: A, B, C and D. Types A and B are the most harmful to humans and cause seasonal flu epidemics; in the Northern Hemisphere, flu season typically occurs from October to May, with peak activity from December to February.

Influenza C can also infect people but causes very mild illness and is not associated with epidemics, while type D affects cattle and other animals and is not known to cause illness in humans.

Flu spreads between people via viral particles in droplets expelled when an infected person coughs or sneezes, or through saliva exchanged via shared utensils or kissing.

Each year, seasonal flu sickens about 1 billion people worldwide and causes approximately 290,000 to 650,000 respiratory deaths. In a given season, the U.S. sees about 200,000 hospitalizations and 36,000 deaths from flu, although these numbers vary year to year.

With rare exceptions, seasonal vaccines are recommended for everyone 6 months and older to avoid illness and to prevent serious complications from flu. Hand-washing, avoiding contact with sick people and masking while in public can also reduce the risk of infection.

Facts about the flu

• Cause: Influenza viruses
• Early symptoms: Fever, chills, tiredness, muscle aches
• Prevention: Seasonal vaccinations

Everything you need to know about the flu

Glossary

• Sepsis: A life-threatening medical emergency caused by the body's extreme response to an infection. During sepsis, the immune system ceases to fight invading pathogens and begins attacking body tissues and organs.
• Pneumonia: An infection that causes inflammation in lung tissue, filling the lungs with pus or fluid and restricting breathing. Pneumonia can occur in one lung or in both, and it may be caused by viruses, bacteria or fungi.
• Rhinovirus: A type of virus that affects surface cells in the respiratory system. Rhinoviruses are the most frequent cause of the common cold. They spread through the air and through contact with contaminated surfaces.
• Viral load: How much of a virus a person is carrying in their body. The more the virus replicates in a person, the higher their viral load will be. This typically makes the person more likely to spread the infection, as well.

Flu pictures

Discover more about the flu

—How do people die of the flu?

—How long is the flu contagious?

—How to get better faster when you have the flu, according to science

This article is for informational purposes only and is not meant to offer medical advice.

Roughly 1 in 8 child flu deaths in the U.S. so far this year have been linked to brain damage, according to a report released by the Centers for Disease Control and Prevention (CDC).

The U.S. is experiencing its most severe flu season in more than a decade, Live Science previously reported. More than 19,000 people have died including 68 children, at least nine of whom had flu-associated brain complications, according to a CDC Morbidity and Mortality Weekly Report published on Feb. 27.

In January, the CDC received anecdotal reports of children becoming critically ill and dying with influenza-associated encephalopathy or encephalitis (IAE), a spectrum of neurological syndromes. Encephalopathy is a group of conditions that cause brain dysfunction while encephalitis causes brain swelling, according to the Cleveland Clinic. IAE symptoms include seizures, decreased level of consciousness, hallucinations and personality changes lasting more than 24 hours, according to the report.

The CDC doesn't normally track cases of IAE, so it's not clear whether the number of cases reported this season is higher than in previous years. However, they did warn doctors to be on the lookout for IAE.

"It is not known whether cases observed in the 2024–25 season vary from expected numbers," the CDC report authors wrote. "Clinicians should consider IAE in children with influenza and abnormal neurologic signs or symptoms."

Related: How do people die of the flu?

Flu is among the deadliest viruses in the world. While it kills around 1.8 people per 100,000 infections, flu infects enough people that thousands die from it every year in the U.S. Nervous system injury, including IAE, is one of the main causes of flu fatalities among children, according to a 2021 study published in the journal Medical Science Monitor.

To investigate anecdotal reports of IAE, the CDC contacted state health departments for data on influenza-associated deaths in children across flu seasons between 2010 and 2025. Flu can spread all year round, but there are more cases in the fall and winter (the flu season), with activity peaking between December and February, according to the CDC.

The CDC found that as of Feb. 8 in the 2024-2025 season, 68 children had died with flu, with IAE documented in nine (13%) of those cases. That compares to a rate of 9% across all years, but it varied season to season and peaked at 14% in the 2011-2012 season, according to the report.

While the CDC couldn't identify whether the current rate of IAE was abnormal, the report highlighted the importance of vaccination and recommended flu shots for all eligible people aged 6 months or older so long as influenza viruses are circulating.

"Influenza vaccination is an important tool for preventing influenza and its associated complications," the report authors wrote.

Flu vaccine uncertainty

Early data suggest that this year's flu shots have dramatically reduced seasonal flu hospitalizations. However, uncertainty hangs over the next vaccination round for the 2025-2026 season.

Flu mutates quickly. So every year, manufacturers use strains circulating in the Southern Hemisphere to predict the strains most likely to circulate in the coming season and create updated versions of the flu vaccine targeted to those strains. A U.S. Food and Drug Administration (FDA) advisory panel normally meets around this time of year to decide which strains should be included in the vaccines for the next season. That meeting was scheduled for Mar. 13 but was canceled without reason, Reuters reported on Feb. 28.

There is concern among experts that the cancellation could disrupt the production of flu vaccines, which could mean higher rates of serious illness and death, Live Science previously reported.

An FDA spokesperson told Reuters on Feb. 28 that it would make recommendations to manufacturers public in time for updated vaccines to be available for the 2025-2026 season.

This is the second time a vaccine advisory panel has been disrupted since Robert F. Kennedy Jr. took over as head of the U.S. Department of Health and Human Services, Reuters reported. Kennedy Jr. is a notable vaccine skeptic, though he's denied being anti-vaccine.

This article is for informational purposes only and is not meant to offer medical advice.

This year's flu shots have dramatically reduced hospitalizations for seasonal influenza, early data suggest.

For kids and teens, the shots were at least 63% effective against flu hospitalizations, and up to 78% in some regions. That's according to data pulled from two networks of health care centers that the Centers for Disease Control and Prevention (CDC) monitors to assess flu shot effectiveness.

"Vaccine effectiveness" reflects how much a shot lowers the chances of a particular health outcome in the real world, and it's calculated by comparing how often that outcome occurs in vaccinated versus unvaccinated people. So in this case, vaccinated kids had a 63% to 78% lower chance of being hospitalized for flu.

For people ages 18 and up, the shots were 41% and 55% effective against hospitalization, respectively, in two of the networks.

Related: Flu shots have changed this year — here's why

The shots have also reduced the likelihood of outpatient visits for flu, which include normal doctor's visits, as well as urgent care and emergency department visits. For kids and teens, the level of effectiveness was 32%, 59% and 60% in three networks, and for adults, it was 36% and 54% in two networks.

These estimates, shared Thursday (Feb. 28) in a Morbidity and Mortality Weekly Report (MMWR), are subject to change as flu season progresses.

At a glance — compared with estimates provided for last season's flu shots at this time last year — the 2024-2025 vaccines appear to be providing greater protection against pediatric hospitalizations. However, they may be less protective against outpatient visits for some kids. Adults' protection, on the other hand, appears to be slightly higher for both types of health care this season.

"Findings in this report show that vaccination with the 2024-2025 influenza vaccine reduced the likelihood of medically attended influenza and support CDC's recommendation that all persons aged ≥6 months be vaccinated against influenza," the MMWR noted.

"These findings also support the strong protective effect influenza vaccination has against influenza-associated hospitalization, demonstrating the importance of vaccination to reduce more severe influenza-associated complications," it concluded.

The authors of the MMWR proposed that a subtype of flu known as H3N2 may explain why the shots have not been as protective against pediatric outpatient visits this season.

Human influenza viruses come in two broad types — classified as either "A" or "B" — which are then divided into subtypes and variants. CDC data show that the vast majority of flu cases this year have been caused by influenza A viruses. Officials have genetically analyzed a subset of these A viruses and found that they fall into two subtypes: H1N1 and H3N2.

At a national level, H1N1 and H3N2 are each responsible for roughly half of the influenza A cases. However, the ratio of H1N1 to H3N2 varies between regions, and thus different networks. The MMWR suggests that the shots were less protective against pediatric outpatient cases in areas where H3N2 was more prevalent. Lab experiments conducted with ferrets had already suggested that the shots were not nearly as good a "match" for the circulating H3N2 viruses as they were for the H1N1 viruses.

Nonetheless, this season's flu shots are showing levels of protection "consistent with those from the 2023-24 season and seasons associated with higher VE [vaccine efficacy] over the last 15 years," the MMWR says. Flu season in the United States can run through May, so it's not too late to get a flu shot.

Next season's flu shots

Meanwhile, the fate of the next round of flu shots — for the 2025-2026 season — is uncertain.

Typically, an advisory panel to the U.S. Food and Drug Administration (FDA) meets around this time of year to determine which strains should be included in the upcoming season's flu shots. That meeting, scheduled for March 13, has now been canceled without a given reason, Reuters reported, though notably, the newly confirmed head of the U.S. Department of Health and Human Services, Robert F. Kennedy Jr., is a longtime vaccine skeptic.

Members of the FDA advisory panel and other experts have expressed concern that this cancellation could throw off the carefully timed planning and production cycle for flu shots, The New York Times reported.

Flu shot production involves growing viruses in chicken eggs, and that takes time — there's typically a six-month buffer between the FDA selecting strains and the start of the vaccine rollout in late July or early August, according to NBC. Delaying that strain selection for the U.S. market could mean that manufacturers don't have time to make enough vaccine for the season's start, which could translate to higher rates of serious illness and death.

The cancellation of the FDA advisory panel meeting follows the postponement of a different meeting of vaccine advisers to the CDC. That said, the CDC and FDA did recently participate in a World Health Organization (WHO)-led meeting about flu shots, despite the Trump administration's stated intention to withdraw from the WHO, STAT reported.

The WHO meeting typically guides the FDA's strain selection for the U.S. market, although the agency can make its own selection for America's vaccines if it chooses. Time will tell if the FDA's vaccine advisers will meet as usual, allowing a window of time for public comment, or if strain selection might otherwise proceed without the meeting.

This article is for informational purposes only and is not meant to offer medical advice.

The United States is experiencing its most severe flu season in more than a decade, with the U.S. Centers for Disease Control and Prevention (CDC) estimating that there have been at least 24 million influenza cases so far this season.

At least 310,000 Americans have been hospitalized with influenza and 13,000 people, including 57 children, have died of the infection, the CDC reported Feb. 7. Note that these CDC data are preliminary and subject to change as the season progresses.

"Influenza (flu) is widespread across most of the US," Dr. Robert Hopkins, Jr., medical director of the National Foundation for Infectious Diseases (NFID), a non-profit, told Live Science. "We are currently seeing the highest level of influenza hospitalizations since 2010."

For the week ending Feb 1, 2025, 7.8% of health care visits were due to a respiratory illness such as influenza, the CDC estimates, a number that has not been so high since the 2009-2010 season. The closest it has come in that 15-year period is the 2017-2018 flu season, which peaked at 7.5% of health care visits in the first week of February.

When people catch seasonal influenza, commonly referred to as "the flu," symptoms typically come on suddenly and last up to two weeks. However, the infection can cause serious complications, which can be fatal.

Related: How do people die of the flu?

For instance, influenza can cause pneumonia, inflammation of the heart, organ failure or sepsis, all of which can result in death. Those that are more at risk of these complications include older adults, pregnant people, young children, and individuals with chronic medical conditions, such as asthma, lung disease or kidney disorders.

This year, respiratory illness, including influenza, is increasing nationwide and has remained above the US's national baseline for the tenth consecutive week. During the week ending on Feb. 1, the CDC reported that 31.6% of respiratory illness tests conducted by clinical labs came back positive for influenza. That's compared to 28.5% the previous week, approximately 20% earlier in January and below 10% from the start of flu season till mid-December.

Ten children were reported to have died from the disease in the week ending Feb. 1, bringing the season's total to 57 pediatric deaths.

Out of 50 U.S. states, 32 states currently have "very high" levels of respiratory illness, including influenza cases, according to CDC data. The highest levels are being reported in Massachusetts, Michigan, Nebraska, New Hampshire, New Jersey, New Mexico, Ohio, South Carolina, Tennessee and Texas.

"While flu activity and flu season severity cannot be predicted and can vary from season to season, it is expected to see flu activity elevated and increasing at this time of year," Paul Prince, a CDC spokesperson, told Live Science in an email.

He added that the severity of a flu season could depend on several factors, including when it begins; the public's pre-existing immunity from prior flu seasons and vaccinations; the year's vaccine uptake and effectiveness of the shot; and the characteristics of circulating flu viruses.

"Flu severity varies from year to year as the strains in circulation change through a process of 'antigenic drift,'" Hopkins explained. "Some strains change more than others and those that do change more — and for which the population has less immunity — cause more severe disease."

Human influenza viruses come in two broad types — classed as either "A" or "B" — which are then divided into subtypes, and variants, with influenza A variants tending to change more quickly than influenza B and causing more severe disease.

The CDC reports that, of more than 4,300 flu specimens tested by public health laboratories in the week ending Feb. 1, only 113 were influenza B and the rest were influenza A.

"We are currently seeing widespread circulation of influenza A H1N1 and A H3N2 viruses primarily, with less circulation of influenza B viruses," Hopkins said. He explained that the characteristics of these strains, combined with low vaccination rates, are likely responsible for the severity of this flu season. Other potential factors, such as the impact of social distancing during the COVID-19 pandemic on influenza immunity, likely aren't as relevant, he said.

Prince added that elevated flu activity is expected to continue for weeks and even months to come. Flu season typically runs from around October to as late as May, peaking in February.

"There's still time for individuals and family to benefit from the flu vaccine this season," he said.

"As long as flu viruses are circulating in your community, it is not too late to get vaccinated," Hopkins emphasized. "I strongly encourage anyone who has not yet received an annual flu vaccine to do so immediately."

The CDC recommends annual influenza vaccines to everyone over the age of 6 months old.

A second type of bird flu has been detected among dairy cows in the United States, the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) has reported.

A broad subtype of bird flu called H5N1 has been circulating among U.S. dairy cows since at least 2024. The first cases were detected in cattle in March of last year, and since then, nearly 960 herds across the nation have been affected. That's according to the latest numbers available on the Centers for Disease Control and Prevention (CDC) website.

The H5N1 virus has a complex family tree, and cows in the U.S. have so far been infected by a branch of that tree known as the B3.13 genotype of clade 2.3.4.4b. Meanwhile, another genotype, called the D1.1 genotype, has been spreading in wild birds and poultry in the U.S. and Canada.

Like B3.13, the D1.1 genotype has also infected people. For example, it recently infected poultry workers in Washington state, who had mild cases, and a teen in British Columbia, who had a severe case that required intensive care. Additionally, a person in Louisiana contracted a severe case of D1.1 that was ultimately fatal.

Now, in a first, this D1.1 genotype has been detected in dairy cows in Nevada, APHIS said in a statement released Wednesday (Feb. 5).

Related: How to avoid bird flu

"This is the first detection of this virus genotype in dairy cattle," APHIS representatives said in the statement. "Genotype D1.1 represents the predominant genotype in the North American flyways this past fall and winter and has been identified in wild birds, mammals, and spillovers into domestic poultry." (Flyways are routes regularly used by large numbers of migrating birds.)

The statement noted that "This confirmation was a result of State tracing and investigation, following an initial detection on silo testing under the USDA's National Milk Testing Strategy (NMTS) in Nevada."

The USDA has been testing samples of raw milk from dairy farms to help track the spread of bird flu. The testing is conducted on milk that hasn't yet been pasteurized because pasteurization inactivates the virus. As of mid-January, 36 states were enrolled in the program, covering about two-thirds of the nation's commercial milk supply.

The USDA is now working with the Nevada Department of Agriculture to conduct additional tests on the affected farm and to take measures to prevent further spread of the virus.

"We shouldn't be surprised about a new spillover to cattle, given the very significant activity in waterfowl across much of the United States," Michael Osterholm, a University of Minnesota epidemiologist and director of the Center for Infectious Disease Research and Policy (CIDRAP), said in an article published by the center's news arm. The virus is not going away, he added.

A strain of bird flu never seen before in the United States has been detected among poultry at a California farm.

The virus, called highly pathogenic H5N9, is a type of avian influenza, otherwise known as "bird flu." This is not the same type of bird flu that's already been spreading on dairy cow and poultry farms in the U.S., causing at least several dozen infections in humans. That virus is called H5N1.

So what is H5N9? How does it differ from H5N1, and should we be equally concerned about it?

Related: Bird flu strikes 1st child in the US — CDC says infection source unknown

What is H5N9?

H5N9 is a highly pathogenic avian flu (or bird flu) virus that bears some structural similarities to H5N1.

"Low pathogenic" H5N9 viruses have previously been detected in birds in the U.S. and elsewhere in the world, dating back to the 1960s. However, these infections are usually associated with less-severe symptoms than "highly pathogenic" strains are. This is the first time highly pathogenic H5N9 has been seen in the U.S., and these deadlier strains are also fairly uncommon worldwide.

How did H5N9 emerge?

New subtypes and strains of influenza can emerge if viruses pick up random genetic mutations as they multiply, as well as if multiple viruses swap genetic information with one another. This latter process is known as genetic reassortment, and it usually happens when an organism is infected with at least two different viruses at the same time.

Some experts suspect that H5N9 may have emerged among the ducks in California due to genetic reassortment occurring between circulating H5N1 viruses and other flu viruses containing the N9 protein. ("H5" and "N9" are two proteins found on the surface of the virus that give the bird flu subtype its name.)

H5N1 was also detected in the birds at the farm, so this could be plausible, according to a Jan. 24 report from the World Organisation for Animal Health (WOAH), which monitors disease outbreaks in animals worldwide.

Such reassortment can lead to "unpredictable new viruses," Angela Rasmussen, a virologist at the University of Saskatchewan in Canada, wrote on X.

She added that ducks may be great host animals for genetic reassortment to take place because, unlike chickens, they don't get very sick from many avian flu viruses. Therefore, while an infected chicken would quickly die, ducks can survive and continue to spread the disease to new hosts.

Related: How to avoid bird flu

What's happening at the farm where H5N9 was detected?

In the Jan. 24 report, the WOAH announced that H5N9 had been identified at a duck farm in Merced County, California. The current outbreak, which the report says was confirmed on Jan. 13, is being closely monitored by officials.

"The USDA Animal and Plant Health Inspection Service (APHIS), in conjunction with State Animal Health and Wildlife Officials, are conducting comprehensive epidemiological investigations and enhanced surveillance in response to the HPAI [highly pathogenic avian influenza] related events," the WOAH wrote in the report.

To prevent further spread of the infection, all of the nearly 119,000 birds at the farm have now been culled. These bird flu viruses are known as "highly pathogenic" because they are extremely deadly to certain poultry — like chickens — and culling an exposed flock is typically considered the best way of preventing additional flocks from getting sick.

Why is bird flu concerning?

Scientists have recently expressed increasing concern that a strain of bird flu could mutate into a form that easily spreads among humans, sparking a pandemic. This happened during historic pandemics, such as the "Spanish flu" in 1918 and the "Asian flu" in 1957.

Humans can become infected with avian influenza if the virus gets into their eyes, nose or mouth, or if it is inhaled in the form of droplets in the air. However, not all bird flu viruses are equipped to efficiently infect people and then spread between them. When given more chances to spread to new animals, the viruses have a higher chance of picking up that ability and harming humans.

Notably, over the past couple of years, H5N1 has spread to new countries, including the U.S. It's been widely infecting wild birds and poultry and has also jumped into various mammals, including cattle and domestic cats. Since its discovery, H5N1 has infected more than 900 people globally, and in the past year, at least 67 people in the U.S. have been infected. In December 2024, a woman in Louisiana became the first person in the U.S. to die as a result of an H5N1 infection.

Related: Bird flu could become deadlier if it mixes with seasonal flu viruses, experts warn

How does H5N9 differ from H5N1?

Avian influenza viruses come in different subtypes, which are named according to subtle variations in the structure of two key proteins on the surface of the virus: hemagglutinin (H) and neuraminidase (N). So H5N9 and H5N1 have the same hemagglutinin protein, but a different neuraminidase protein.

A virus's specific combination of H and N proteins determines a range of important traits, such as how good it is at spreading from one animal to another, how deadly it is, and whether it is resistant to antiviral treatment.

Besides the difference in their neuraminidase proteins, H5N9 differs from H5N1 in several other ways.

For example, H5N1 infections are more common among humans and are believed to be the deadliest of all the highly pathogenic avian influenza strains in humans and chickens.

Does H5N9 pose a risk to humans?

As of yet, little is known about whether H5N9 could spread to humans and potentially cause a pandemic.

On its website, the Centers for Disease Control and Prevention (CDC) currently states that the public health risk from H5 bird flu is "low" but that the agency is "watching the situation carefully and working with states to monitor people with animal exposures." The CDC's primary focus on that webpage is H5N1, but the agency is monitoring H5 viruses broadly, using its flu surveillance systems to watch for any cases in humans.

As of Jan. 18, there has been no evidence of H5 bird flu spreading from one human to another in the U.S.

"The combination H5N9 is not new and other versions, like H5N5 also exist," Florian Krammer, a professor of vaccinology at the Icahn School of Medicine at Mount Sinai in New York, told CBS News. "Just because this was now detected, doesn't mean trouble necessarily," he emphasized.

This article is for informational purposes only and is not meant to offer medical advice.

A person in Louisiana who was hospitalized with a severe case of bird flu has died.

The hospitalization, first announced Dec. 13, marked the first severe case of H5N1 bird flu in the United States. H5N1 is a subtype of bird flu that's been circulating among wild birds and poultry in the U.S. since 2021, and in 2024, it was detected in dairy cows for the first time. Since then, the virus has infected at least 66 people in the country, primarily those who were directly exposed to infected cows or poultry.

In the case of the patient in Louisiana, the exact source of the infection was initially unknown. However, an investigation uncovered that the person fell ill after exposure to a noncommercial backyard flock of birds, as well as wild birds, according to a statement from the Louisiana Department of Health (LDH).

"LDH's extensive public health investigation has identified no additional H5N1 cases nor evidence of person-to-person transmission," the department's statement added. "This patient remains the only human case of H5N1 in Louisiana."

Related: How to avoid bird flu

The patient was over the age of 65 and had underlying medical conditions, which may have made them more likely to develop a severe bird flu case. To preserve patient confidentiality and show respect for the patient's family, the department will not be sharing further updates about the case.

In its initial announcement about the case, the Centers for Disease Control and Prevention (CDC) noted that the virus that sickened the Louisiana patient matched a strain circulating in birds — called the D1.1 genotype — not the one spreading among cows, called the B3.13 genotype.

Separately, a teen girl in Canada also recently caught the D1.1 genotype and required intensive care in a hospital. She's since recovered from the infection, but a recent study published in The New England Journal of Medicine pinpointed several concerning mutations in the virus that infected her. These mutations may have boosted the germ's ability to infect humans and to cause severe illness.

A commentary published in the same journal noted that it's unclear whether these mutations were already present when the teen became infected or if they emerged as the virus replicated in her body. Nonetheless, the case highlighted the continued need to monitor the virus's spread to see if such mutations are becoming more commonplace, the commentary authors wrote.

"While the current public health risk for the general public remains low, people who work with birds, poultry or cows, or have recreational exposure to them, are at higher risk [of catching H5N1]," LDH officials noted in the statement.

"The best way to protect yourself and your family from H5N1 is to avoid sources of exposure," they added. "That means avoiding direct contact with wild birds and other animals infected with or suspected to be infected with bird flu viruses." Consuming products made with raw milk could also pose a risk of infection, health officials have previously warned.

If you've been exposed to potentially infected animals and develop respiratory symptoms or eye redness within 10 days, the department recommends informing your health care providers so they can provide the appropriate testing and treatment. Antiviral drugs used to treat seasonal flu — such as oseltamivir (Tamiflu) — have been used to treat bird flu.

Historically, of the more than 900 people known to have caught H5N1 globally, more than half have died. These previous deaths affected people outside the U.S. and involved H5N1 strains with different genetic traits that might help explain their severity.

This article is for informational purposes only and is not meant to offer medical advice.

Residents of Los Angeles county have been warned against feeding their cats Monarch Raw Pet Food after the company's products tested positive for bird flu.

The pet food brand is sold at several farmers markets in California, the locations of which can be found on the company's website.

Recently, a pet cat that ate Monarch Raw Pet Food tested positive for H5 bird flu, according to a Dec. 31, 2024 statement from the Los Angeles (LA) County Department of Public Health. "H5" refers to a specific protein found on the surface of a related group of avian influenza, or bird flu viruses.

One virus in this group — H5N1 — has been spreading among poultry and wild birds in the U.S. since 2021, and in 2024, it was detected in dairy cows for the first time in history. In the past year, H5N1 has infected at least 66 people in the U.S., mostly individuals who were exposed to either infected dairy cows or poultry.

Related: How to avoid bird flu

The statement from LA County does not note if the affected cat was infected with H5N1, but this is the most likely culprit. Four additional cats in the same household are "presumed to have also been positive" for an H5 virus after eating Monarch Raw Pet Food.

The pet food company sells a range of products, including ground chicken, chicken feet, jerky made from dehydrated chicken hearts, and dehydrated ground beef liver. The statement doesn't note if a specific product has been implicated in these cases or if a recall has been initiated. The company's website does not make note of the recent bird flu cases.

"Residents who fed their pets Monarch raw pet food products or other raw meat or dairy products and notice their pet is experiencing symptoms should immediately contact their veterinarian," the health department advised. LA County residents with questions can call the Public Health InfoLine at 833-540-0473 between 8 a.m. to 8 p.m. local time.

Symptoms of H5 infection in cats can be serious — they range from loss of appetite and fatigue to liver problems, respiratory issues and potentially deadly neurological symptoms. Vets working with potentially infected cats should wear personal protective equipment and be aware that the virus can be spread in bodily fluids, such as saliva, feces and mucus.

The cats who ate Monarch Raw Pet Food aren't the first to be infected with bird flu in LA.

Earlier in December, the health department confirmed four deadly bird flu cases in cats fed products from Raw Farm, LLC., a company whose products had recently been recalled due to H5N1 contamination. Among the company's products are a "raw milk pet food topper" and "pet food kefir." Officials are investigating whether additional pets have been sickened by the raw-milk products.

"The risk of H5 bird flu remains low in Los Angeles County, but these confirmed cases of the virus in pet cats are a reminder that consuming raw dairy and meat products can lead to severe illness in cats," Barbara Ferrer, director of the Los Angeles County Department of Public Health, said in a statement released in mid-December.

"To avoid the spread of disease, including H5 bird flu, we strongly encourage residents and their pets to avoid raw dairy and undercooked meat products, limit contact with sick or dead animals, report sick or dead birds and keep pets or poultry away from wild animals and birds," she said.

Separately, the Oregon Department of Agriculture (ODA) recently alerted pet owners that products from another pet food brand — Northwest Naturals — also tested positive for H5N1. The company has voluntarily recalled a product called "2lb Feline Turkey Recipe raw & frozen pet food" due to the contamination. According to the ODA statement, a house cat in Washington County had died after contracting H5N1 from the food.

As of yet, there are no known cases of people catching H5 viruses from cats, the LA County statements note.

This article is for informational purposes only and is not meant to offer medical advice.

You may have been told to bundle up before venturing outside in the winter, otherwise you'll "catch a cold." But are you actually more likely to get a cold when it's chilly out?

Put simply, yes: The incidence of respiratory infections, such as the common cold, peaks in temperate regions during the winter. While frostier weather doesn't directly cause infections, research suggests that it can make you more likely to catch them.

A common explanation for this comes down to how respiratory infections are transmitted from person to person. Viruses that cause the common cold and the flu, or influenza, spread from infected people to others via droplets in the air, which are released as infected people sneeze, talk or cough. Individuals can also become infected with these viruses when they touch contaminated surfaces or objects and then touch their mouth, nose or eyes.

During the winter, we typically spend more time indoors, meaning we're spending more time closer to other people, potentially in places with poor air ventilation. This makes viruses more likely to spread within the population.

Related: How to get better faster when you have the flu, according to science

But beyond these behavioral changes, scientists have shown that there are distinct biological mechanisms that explain why we get more colds in cold weather — and it's related to the temperature and humidity of the air.

For instance, "epithelial" cells that line the nose are a first line of defense against viruses breathed in from the air. Normally, in response to viral infection, these cells increase the secretion of tiny, fluid-filled sacs — called extracellular vesicles — which help mop up viral particles before they can invade the rest of the body, Dr. Benjamin Bleier, a director of endoscopic skull base surgery at Massachusetts Eye and Ear center, told Live Science.

These vesicles also contain molecules called microRNA that prevent viruses from making copies of their genetic material. This stops the germs from replicating and getting into our tissues, he said.

However, Bleier and colleagues have shown that this system gets derailed in colder weather. For example, this happens when the epithelial cells in the nose are exposed to temperature drops similar to what may happen when people go outside in cold weather — when the temperature inside the nose plunges from 98.6 to 89.6 degrees Fahrenheit (37 to 32 degrees Celsius). As a result, respiratory viruses effectively double their capacity to replicate, the team found.

These findings imply that wearing facial coverings, such as masks, may be beneficial for two main reasons, Bleier said. Firstly, these coverings lower the amount of virus particles that can enter the nose, and secondly, they retain a cushion of warm air in front of the face that helps keep nasal epithelial cells working their best, he said.

Another thing that can affect how the body responds to viruses in the winter is humidity, as the air inside our homes tends to be drier than usual during this season.

"When you import the cold air from the outside and then heat it inside the home, that air contains very little water vapor," Akiko Iwasaki, a professor of immunobiology at Yale University in Connecticut, told Live Science. Previous research has shown that deaths from the flu increase in the U.S. as the humidity of the air falls.

In a study published in 2019, Iwasaki and team found that when mice live in dry air conditions — between 10% and 20% relative humidity — their ability to fight influenza infections declines, compared with mice living in 50% relative humidity.

That's because dry air, like cold air, also messes with epithelial cells. In this case, it stops tiny fingerlike projections, called cilia, that stick out from the cells from wiggling around and clearing away viral particles. When the cilia malfunction, viruses can enter cells in the body much more readily, Iwasaki said.

For unknown reasons, in dry conditions, immune cells within the respiratory tract also secrete fewer chemicals, called interferons, which help to prevent viruses from replicating.

Although this 2019 study was conducted in mice, similar research in humans has also shown that cilia are less able to clear the respiratory tract of germs in low-humidity conditions, which may facilitate the spread of disease.

Therefore, the team believes that keeping homes, offices, hospitals and schools humidified during the winter may be crucial for keeping the immune defenses of the respiratory tract intact, Iwasaki said. The ideal level is between 40% and 60% indoor relative humidity, she added.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

California officials have declared a state of emergency amid growing fears over avian influenza. The virus has swept through dairy herds across the state and caused sporadic cases in humans, with the first severe human bird flu case in the U.S. reported in Louisiana on Wednesday (Dec. 18).

In a statement released Dec. 18, Governor Gavin Newsom's office said that the action came "as cases were detected in dairy cows on farms in Southern California, signaling the need to further expand monitoring and build on the coordinated statewide approach to contain and mitigate the spread of the virus."

To date, no person-to-person spread of the virus has been reported in the U.S. and the majority of infected individuals have had known exposure to infected cattle or poultry.

Why has California declared a state of emergency?

The virus H5N1 is a subtype of avian influenza, or bird flu. The virus is spread primarily through wild and domestic birds. However, it can also jump to mammals, such as dairy cows and occasionally humans.

Since the beginning of 2024, a multistate outbreak of the virus has affected 866 dairy herds across the U.S., according to the U.S. Centers for Disease Control and Prevention (CDC). Roughly 75% of these have been in California, largely in the state's Central Valley.

Related: How to avoid bird flu

Thirty-four of the nation's 61 confirmed human bird flu cases have also been reported in California, including the first infected child.

Newsom said that the emergency proclamation was a proactive measure to ensure state and local agencies have the resources and flexibility they need to respond rapidly to the outbreak.

"While the risk to the public remains low, we will continue to take all necessary steps to prevent the spread of this virus," Newsom said in the statement.

What are bird flu symptoms?

With the exception of the patient in Louisiana, the majority of human bird flu infections so far have been mild.

According to the CDC, bird flu may have the following symptoms:

• Eye redness
• Fever
• Cough
• Sore throat
• Runny or stuffy nose
• Muscle and body aches
• Headaches
• Fatigue
• Shortness of breath

Flu experts have said that it is too early to say whether this could become a more severe outbreak, and flu viruses are constantly changing and mutating. A recent study focused on an H5N1 strain from a cow suggested that a single gene mutation may enable the virus to spread between people, and the infection could become more dangerous if it starts to mix with other seasonal flu viruses. However, the CDC says that the risk to the general public is currently still low.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Editor's note: The patient described in this case has died, officials announced Jan. 6.

A person in Louisiana has been hospitalized with H5N1 bird flu, marking the United States' first severe case of the illness.

In a statement released Wednesday (Dec. 18), the Centers for Disease Control and Prevention (CDC) noted that, in total, 61 people in the U.S. have had a confirmed case of H5N1 this year. Of the previous 60 cases, 37 caught the virus from dairy cows, 21 from poultry, and two from unknown sources. The presumptive case was first announced by Louisiana Friday and was confirmed by CDC tests the same day.

Notably, however, the virus in the Louisiana patient differs genetically from the virus spreading among dairy cows.

The strain circulating in dairy cows is known as the "B3.13 genotype," while the one that sickened the Louisiana patient is the "D1.1 genotype." The latter is known to be circulating among wild birds and poultry in the United States and Canada. Recently, D1.1 infected poultry workers in Washington state, who had mild cases, and a teen in British Columbia, who had a severe case that required intensive care.

The exact source of the teen's infection has not been identified, but genetic testing suggested the virus is genetically similar to a strain detected in wild birds in the Fraser Valley area in southwestern British Columbia.

Related: How to avoid bird flu

The source of the Louisiana patient's infection is also unknown, but birds are suspected.

"While an investigation into the source of the infection in Louisiana is ongoing, it has been determined that the patient had exposure to sick and dead birds in backyard flocks," the CDC noted. "This is the first case of H5N1 bird flu in the U.S. that has been linked to exposure to a backyard flock."

The CDC added that seeing a severe case of H5N1 is not totally unexpected. Historically, of the over 900 people known to have caught the virus worldwide, more than half have died. But these deaths have affected people outside the U.S. and have involved H5N1 strains with different genetic traits that might help explain their virulence.

The CDC still considers the risk of H5N1 infection among the general public to be low. That's partly because there is still no evidence of the virus spreading from one person to another.

A recent study — albeit one that focused on an H5N1 strain from a cow — suggested that the virus may not need to mutate very much to start spreading between people. So officials will continue to monitor the virus's spread from all animals while also encouraging people at the highest risk of exposure to take precautions.

Following the Louisiana case, the CDC emphasized that "people with work or recreational exposures to infected animals are at higher risk of infection and should follow CDC's recommended precautions when around animals that are infected or potentially infected with H5N1 avian influenza virus.

"This means that backyard flock owners, hunters and other bird enthusiasts should also take precautions," the agency statement noted.

Dr. Demetre Daskalakis, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters at a news conference that the Louisiana patient was hospitalized with flu symptoms, according to the University of Minnesota's Center for Infectious Disease Research & Policy.

Daskalakis deferred to Louisiana health officials to share further details about the patient, should they choose. The health department has not yet shared specifics of the patient's condition due to privacy concerns and the fact that the investigation into the case is ongoing.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

U.S. dairies must now send samples of raw milk to the U.S. Department of Agriculture (USDA) so the agency can test them for bird flu, per a new federal order.

The move, announced Friday (Dec. 6), builds upon existing efforts by the USDA and other authorities to track the spread of bird flu in dairy herds around the country. As of Dec. 5, H5N1 — a subtype of bird flu — has been detected in 720 dairy herds in the U.S. this year.

At least 35 people have contracted bird flu as a result of working around infected cows. That's in addition to 21 people who became infected through exposure to sick birds, and an additional two cases whose infection sources are unknown.

Earlier in the outbreak, the USDA mandated that any lactating cows being shipped across state lines first be tested for H5N1 and that any positive tests be reported to the agency. It also launched a voluntary Dairy Herd Status Program, which dairy producers could join to monitor for infections via weekly bulk milk samples. But as of December, only 75 herds across 17 states were being tracked through the program.

Now, with the federal order, bulk milk testing will be mandated nationwide.

Related: Bird flu could become deadlier if it mixes with seasonal flu viruses, experts warn

"This new milk testing strategy will build on those steps to date and will provide a roadmap for states to protect the health of their dairy herds," Agriculture Secretary Tom Vilsack said in the USDA announcement.

"Among many outcomes, this will give farmers and farmworkers better confidence in the safety of their animals and ability to protect themselves," he said, "and it will put us on a path to quickly controlling and stopping the virus' spread nationwide."

Specifically, the order mandates that, upon request, any entity that sends or holds milk intended for pasteurization submit samples to the USDA. It also requires that herd owners with cows that test positive provide information to help the USDA track the spread of disease among the cattle. Finally, it requires private laboratories and state veterinarians to report to the USDA any raw milk samples that test positive for bird flu.

The main motivation behind the order is to help authorities determine the scope of the outbreak and track its spread. Where needed, "enhanced biosecurity measures" can then be implemented to help prevent the spread to additional cows, the USDA noted. The enhanced surveillance could also inform strategies to help guard farmworkers from the infection.

As of yet, there's no evidence that H5N1 is spreading from person to person; so far, people have only caught the virus from animals. However, a study released Thursday (Dec. 5) highlighted just how little the virus would have to evolve to start spreading between people.

A single mutation affecting one of the virus's key proteins could make it a "match" for human cells, setting up a key domino whose toppling could lead to a pandemic.

"The more people that get infected, the more likelihood is that … that mutation will get selected for," Ian Wilson, a structural biologist at the Scripps Research Institute and one of the study's authors, previously told Live Science.

The new testing plan rollout will start Dec. 16, initially in California, Colorado, Michigan, Mississippi, Pennsylvania and Oregon, according to Reuters.

Tracking bird flu will help authorities ensure that the commercial milk supply remains safe, the USDA statement notes. Importantly, though, the commercial milk supply is already pasteurized — heated to kill germs — so any H5N1 that might be in the milk is neutralized before it is sold. Several studies have confirmed that pasteurization inactivates the virus.

However, health authorities do warn against consuming products made with unpasteurized, or "raw," milk, which could pose a risk of infection if they're contaminated with H5N1. Recently, the California-based dairy company Raw Farm issued a recall after its products tested positive for the virus. Consuming raw milk also carries a risk of contracting other serious infections.

This article is for informational purposes only and is not meant to offer medical advice.

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Dozens of people in the United States have caught bird flu from animals this year, but there's no evidence that the viral disease has spread from one person to another. However, a single mutation in the virus could make human-to-human spread possible, a new study finds.

This genetic change would make the virus a much better "match" for cells in humans' airways; it would enable a protein on the surface of the virus to fit snugly into a receptor found on human cells. That would allow the virus to infect those cells more easily — and make it more likely to spark a pandemic.

"In order to get sufficient infection, the virus needs to very efficiently attach to the cells in the airway," said Jim Paulson, a biochemist at The Scripps Research Institute in La Jolla, California, and co-senior author of the study. "In fact, it's believed that transmission [between people] cannot occur until the virus has acquired the human-type receptor specificity."

Currently, the circulating bird flu virus — called H5N1 — is a much better match for bird receptors. The new study, published Thursday (Dec. 5) in the journal Science, essentially explored what it would take for the virus to switch its preference to people.

Related: How to avoid bird flu

Pandemic potential

As of Dec. 4, the Centers for Disease Control and Prevention has confirmed 58 H5N1 infections among people in the U.S. The virus has also been circulating among wild birds, poultry and cattle in the country. Most of the confirmed human infections — about 60% — have been linked to exposure to infected cattle, while 36% have been associated with birds. The two remaining infections have no known source, but they're also suspected to have originated in animals.

So far, the human cases in the U.S. have been mild, triggering eye redness or a cough, at most. However, a teen infected in Canada has had more severe symptoms, and historically, H5N1 infections have been deadly in hundreds of cases. So there's a concern that the strain circulating in the U.S. could evolve to become deadlier, more transmissible, or both.

But how is H5N1 infecting people now, if it's not a good match for humans? "When the virus is coming from an infected animal, like birds or from cows, it's a very high concentration of virus," Paulson told Live Science. So, even if the virus is not a perfect match, the sheer number of viral particles entering the person's body can still lead to infection, he said.

To then jump to another person, though, the virus would need to be a better match. That's because, when a respiratory virus spreads between people, it's typically passed in tiny droplets expelled from the infected person's mouth. These droplets carry a relatively low concentration of virus.

Related: Bird flu strikes 1st child in the US — CDC says infection source unknown

To probe how H5N1 might become a good match, the researchers looked at the genetic code of the virus that infected the first person ever known to catch bird flu from a cow. They zeroed in on the code for hemagglutinin (HA), a protein the virus needs to infect cells.

"What we're looking at is a protein that comes from a virus, but it's isolated," said co-senior author Ian Wilson, a structural biologist at Scripps. The team didn't work with whole viruses in the lab, he clarified.

When the researchers looked at how well the isolated HA protein plugged into bird receptors compared with human ones, they found that the existing version had "strong avian-type specificity." They then introduced mutations, triggering changes in the part of HA that directly interacts with receptors.

They found that a single mutation — called the Gln226Leu substitution — can "completely switch" the virus's preference, making it a match for humans instead of birds. The mutant virus still didn't bind to human cells quite as well as it had to bird cells, but the switch in its preference was "nevertheless clear and pronounced" across different tests. Adding a second mutation — Asn224Lys — did tighten the virus's grip, though.

That Gln226Leu mutation had been flagged in previous studies of H5N1, which also hinted that it could boost the virus's ability to infect humans. However, most previous studies found that the HA would need multiple mutations to completely switch its preference, the researchers noted in their report. In this case, it seems just one mutation is sufficient.

The switch to human receptors is a major factor that could give an animal virus the potential to spark a human pandemic, the authors noted. For that reason, scientists should look out for the Gln226Leu mutation as they continue to track the spread of H5N1, the study suggests.

Related: H5N1 bird flu is evolving to better infect mammals, CDC study suggests

For now, "the particular mutation that we're reporting in this paper has not yet been reported in a database," Wilson said. The recent Canadian case may have some notable mutations in the HA protein, at least according to informal discussions among scientists on social media, Paulson said. For now, though, "it's a little premature to talk about that particular case," he added.

Ultimately, more human H5N1 infections would make the Gln226Leu mutation more likely to crop up. "The more people that get infected, the more likelihood is that … that mutation will get selected for," Wilson told Live Science. "When there's very few people infected, there's less likelihood of that mutation coming up."

The study didn't consider all of the factors that could give H5N1 pandemic potential. A second viral protein, called neuraminidase, also plays an important role, as does the pH that the virus needs to fuse to and get inside cells. With the Northern Hemisphere's flu season ramping up, there's a possibility that H5N1 could infect a person who is already infected with seasonal flu. From there, those two viruses could swap genes, thus opening the door for H5N1 to pick up genes that might help it adapt to humans.

"Genes from the previous human virus — they're already adapted to humans," Paulson said. "Therefore, the mutation of the hemagglutinin becomes a key factor in the success of the virus."

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Colds and the flu are two incredibly common respiratory infections. Each calendar year, the average American adult gets around two to three colds, while approximately 8% of the population catches the flu as it circulates between about October and May.

In some ways, it is easy to confuse the common cold with the flu. After all, they're both contagious respiratory illnesses, and they share many symptoms, such as headache, sore throat and cough. However, there are important distinctions between these two illnesses, and the differences affect how each is diagnosed and treated.

So how is the common cold different from the flu?

First, colds and the flu are caused by different viruses. More than 200 respiratory viruses can cause colds. These include rhinoviruses, adenoviruses and non-rhinovirus enteroviruses, as well as some types of coronaviruses. The flu, in contrast, is caused only by influenza viruses. Most commonly, it's caused by the two main types of viruses that cause seasonal flu epidemics in humans: influenza A and B.

The commonly used nickname for the infection — "the flu" — is short for "influenza."

Related: How do people die of the flu?

The viruses that cause colds and the flu also vary slightly in the regions of the body they infect. A cold is a type of upper respiratory infection, Dr. Angela Branche, an associate professor of medicine at the University of Rochester, told Live Science. This means it affects the top portion of the respiratory system: the nose, mouth, sinuses and throat.

When you get a cold, you'll most likely experience a blocked or runny nose, a cough and sneezing. These symptoms are generally mild, and they come on gradually and often go away on their own within a week, without the need for medication. During that time, you may feel unwell but you can usually continue with daily activities. That said, the Centers for Disease Control and Prevention advises avoiding other people while your symptoms persist, to avoid spreading your cold to them.

Flu, on the other hand, usually affects both the upper respiratory tract and the lower respiratory tract, which includes the windpipe and the lungs, and it can also impact other organs. Symptoms of the flu are generally more intense than those associated with colds, and they come on more abruptly.

Patients with the flu may experience additional symptoms, such as fever, chills and body aches, and in severe cases, the infection can lead to serious health problems, including pneumonia and other complications that require hospitalization. Such complications can also happen with colds, but they are much less likely.

Colds and the flu also differ in the way they're diagnosed. Typically, doctors only need to conduct a physical exam and gather the medical history of a patient to determine if they have a cold. A specific test, such as one that requires a nasal swab, is needed to definitively diagnose the flu. In this case, samples of fluid from the nose are analyzed to see if they are laden with influenza viruses.

What's more, some products can now test for the flu and the virus behind COVID-19 at the same time. This is useful because COVID-19 and the flu can also cause some of the same symptoms, but they require different treatments.

Neither the flu nor a cold can be cured with medicine. However, specific drugs can help alleviate a patient's symptoms and shorten the length of time a person has them.

People with a cold may take nasal decongestants or pain relievers, such as ibuprofen (Advil or Motrin) or acetaminophen (Tylenol), Branche said. For the flu, antivirals like oseltamivir (Tamiflu) can reduce the duration of infection by about a day. For people with very severe flu infections, Tamiflu can also lower the risk of death if provided early in the infection.

There is no vaccine against colds, but there is a seasonal vaccine against the flu. With rare exceptions, the CDC recommends that everyone ages 6 months and older get vaccinated every flu season. This vaccination reduces the risk of developing serious complications or dying from the disease, especially for groups vulnerable to the illness, such as very young children, older adults and pregnant people.

This article is for informational purposes only and is not meant to offer medical advice.

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Editor's note: This is a developing story and will be updated as new information emerges.

An unidentified, flu-like illness has killed dozens of people in the Democratic Republic of the Congo (DRC), according to news reports.

The deaths were reported in the Kwango province, which is located in the southwestern portion of the country and shares a border with Angola to the south. Between Nov. 10 and Nov. 25, between 67 and 143 people died of the mystery illness in what's known as the Panzi health zone within the province, Rémy Saki, the deputy provincial governor, told The Associated Press (AP).

"A team of epidemiological experts is expected in the region to take samples and identify the problem," Saki said.

According to reports, symptoms of the mystery illness are similar to the flu and include high fever, headaches and a cough.

The number of affected people is continuing to rise, but in general, treatments in the region are lacking, civil society leader Cephorien Manzanza told Reuters. "Panzi is a rural health zone, so there is a problem with the supply of medicines," Manzanza said. Reports don't note what kinds of treatments might be helpful for treating those affected.

Related: How do people die of the flu?

A spokesperson for the World Health Organization (WHO) told Reuters that the agency was alerted to the illnesses last week and that it's working with local authorities to investigate. WHO currently has a team on the ground collecting samples for analysis, an anonymous WHO employee told the AP.

Apollinaire Yumba, the provincial health minister, told the AP that officials are advising the public to exercise caution and refrain from contact with dead bodies to avoid potential infection.

Meanwhile, the DRC is also contending with a major mpox outbreak, which was declared a public health emergency of international concern in August. In early November, the total number of suspected mpox cases across the continent of Africa topped 50,000, with the DRC bearing a high percentage of those cases.

As of Dec. 1, 20 African countries have reported 13,081 mpox cases confirmed by laboratory tests, including 55 deaths from the disease, the WHO reported. The DRC has the most confirmed cases, at 9,513, followed by Burundi with 2,334 and Uganda with 716. A significant number of cases that are "clinically compatible with mpox" have not been officially confirmed due to a lack of tests, the WHO noted.

This article is for informational purposes only and is not meant to offer medical advice.

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H5N1 bird flu is spreading through poultry and cattle in the United States, and some cases have been reported in people.

No human-to-human spread of the virus has been documented, and the vast majority of infections stem from contact with farm animals. Because of this, the Centers for Disease Control and Prevention (CDC) considers the risk to the general public very low. However, a recent CDC study found that the respiratory virus is evolving to better infect mammals.

The CDC and other health agencies are keeping an eye on the virus because, if it starts spreading between people, there is the potential for a human epidemic or pandemic.

Fortunately, there are steps people can take to minimize their risk of contracting H5N1. Here's what public health agencies know about preventing bird flu infections.

Related: Bird flu could become deadlier if it mixes with seasonal flu viruses, experts warn

How do people catch H5N1?

People almost exclusively catch H5N1 through direct contact with sickened animals. The virus can travel in tiny particles, called aerosols, through the air, as well as through slightly larger "respiratory droplets" and possibly on bits of dust, according to the CDC. These particles are shed when animals breathe, excrete waste, or — for cows — during milking.

The viral particles can cause infection when they enter a person's eyes, nose or mouth, either when the person breathes or touches their face after their hands have been contaminated.

There have been a handful of cases this year — two in the U.S. and one in Canada — in which someone has contracted H5N1 without known contact with livestock or wild birds. Public health officials aren't sure how the virus was transmitted in these cases. Avian flu can also be transmitted through wild bird droppings, so it's possible these individuals encountered the virus outdoors without actually getting close to a sickened animal.

Has H5N1 infected people before?

The H5N1 subtype of avian flu was first detected in wild waterfowl in southern China in 1996, and it caused scattered outbreaks among domestic poultry in the region in 1997, according to the CDC. Since then, there have been more than 900 sporadic human cases around the world. Half of the people infected died, raising fears about the seriousness of the disease.

In 2005, the disease spread to Africa, the Middle East and Europe, although it was still mostly affecting animals. In 2021, a new genetic subtype of the virus emerged and quickly spread to the Americas. There, it sparked deadly outbreaks among commercial poultry, and recently, it's infected goats, dairy cows and the occasional person.

As of Nov. 26, there have been 55 confirmed cases of bird flu among people in the United States in 2024, according to the CDC. All but two of these cases are known to have involved direct contact with cattle or poultry. In California, the state with the most reported cases, 28 out of 29 infected people caught the flu by working with cattle.

The two remaining U.S. cases had no known animal source: one in a child in California and one in an adult in Missouri. A teenager in British Columbia also recently fell ill with no known animal contacts. In none of these cases has the disease spread to other people, according to the CDC. Officials suspect animals were also the source of these individuals' infections, although they haven't been able to pinpoint the exact route of exposure.

Who's at risk of catching bird flu?

Farmworkers and other people whose work brings them in close contact with poultry and cattle face the highest risk of bird flu. A recent study found that the virus can remain infectious in raw milk and on milking equipment for more than an hour, meaning milk could be a source of exposure for dairy workers. Workers may also become ill while cleaning droppings from poultry pens.

There are concerns that raw milk consumption could raise the risk of transmission to people beyond the agricultural industry. Additionally, because wild animals can carry H5N1, there's some risk of people becoming exposed through interactions with infected animals or via exposure to their feces, respiratory fluids or contaminated fur.

How can you avoid catching bird flu?

The CDC has issued guidance for the use of personal protective equipment (PPE) for farmworkers. This PPE includes respirators, a type of face mask, to prevent people from breathing in the virus and safety goggles to keep the virus out of their eyes.

Members of the general public can protect themselves by avoiding close contact with sickly or dead animals, according to the CDC. Unusual or suspicious cases of animal death can be reported to the local health department or state wildlife agency.

There have been no known cases of transmission of avian flu from the commercial food supply in the U.S., but raw poultry products have previously caused infections in southeast Asia, the CDC says. As a general food safety measure, the CDC recommends cooking poultry and eggs to an internal temperature of 165 degrees Fahrenheit (73.9 degrees Celsius). Ground beef should be cooked to 160 F (71.1 C) and whole cuts of beef should reach 145 degrees F (62.8 C).

Raw milk and products made from raw milk, such as yogurt, should be avoided, the CDC cautions. Raw milk not only poses potential risks due to avian flu but also carries known risks of other contamination, such as Listeria bacteria.

Is there a vaccine for bird flu?

There is no vaccine against H5N1 avian flu currently available, given that the disease still does not easily spread between people.

However, given the possibility that the virus could one day evolve to enable human-to-human transmission, the U.S. government has developed five candidate virus proteins as the basis for future vaccines. Federal health officials have also ordered the production of 4.8 million doses of an experimental bird flu vaccine to have on hand in case of human-to-human transmission or any other concerning change in how the virus operates.

Recent research also suggests that vaccines based on earlier genetic subtypes of H5N1 can still protect against the most recently emerged version of the virus.

In addition, earlier this year, the Argentinian vaccine manufacturer Sinergium Biotech launched a project with World Health Organization support to use mRNA — the genetic molecule used in some COVID-19 vaccines — to develop vaccines against H5N1. The goal is to ensure that low- and middle-income countries will be prepared for potential pandemics, despite lacking resources for vaccine programs of their own.

This article is for informational purposes only and is not meant to offer medical advice.

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A teenager in Canada is critically ill with the first human case of H5N1 bird flu contracted in the country, and public health officials aren't sure how the youth was exposed.

The teen had no known exposure to livestock such as cattle or poultry, where the virus is known to be circulating. The strain that sickened the teen, however, is different from the version that is infecting cows and has spread to dozens of dairy workers in the U.S.

Instead, the variant in question, a 2.3.4.4b virus of the D1.1 genotype, typically circulates in wild birds and poultry, according to StatNews.

Canada has not yet detected H5N1 in its cattle herds, though it has experienced poultry outbreaks.

The avian flu H5N1 lineage that is causing dairy outbreaks emerged in 1996 and has been circulating in wild birds and poultry ever since. A particularly aggressive variant emerged in 2020 and quickly spread throughout Asia, Africa and Europe, reaching North America in 2021 and Central and South America in 2022, according to the World Health Organization.

Since 2022, the virus has been popping up in mammals, causing devastating outbreaks in sea lions and elephant seals and spreading in dairy farms. At least 46 people in the U.S. have contracted the virus in 2024, according to the Centers for Disease Control and Prevention (CDC).

These illnesses have been mild so far, but H5N1 can cause severe disease; more than half of cases detected worldwide since 2003 were fatal, according to the WHO. This statistic does not account for mild, undetected cases that were likely circulating in this time, but does suggest a concerning mortality rate for H5N1 should it become better at spreading between people.

The unidentified teenager in British Columbia is too ill to answer questions about possible exposures, health officials said in a news conference on Wednesday (Nov. 13). It may not be possible to find the source, they said. In August, a person from Missouri fell ill with H5N1 with no known exposure to cows, poultry, or wild birds, and health officials were never able to find out how they caught the virus. (The virus was closer to the version circulating in cows than in wild birds, however. The patient ultimately recovered.)

A recent CDC study has found that H5N1 is evolving to better spread between mammals. It still does not spread as easily as the seasonal flu, however, researchers reported Oct. 28 in the journal Nature.

The case could be a one-off or a signal that H5N1 is spreading undetected, according to StatNews. Canadian health officials said that they were alert to both possibilities.

"We’ve not found anybody else who is sick. And we’ve been testing,” British Columbia provincial health officer Bonnie Henry told StatNews. She said that people close to the teen have not fallen sick. "So I’m more comfortable that it was a single exposure and a more rare event. … More cases would have come to light by now if this was an exposure event that exposed multiple numbers of people, or there had been person-to-person transmission, which we know is rare with H5N1."

A hospitalized teen in British Columbia is likely Canada's first case of H5 bird flu contracted within the country, officials reported Saturday (Nov. 9).

H5 bird flu viruses are a broad group of related viruses that carry a protein called hemagglutinin 5. The group includes H5N1, the virus that has been detected in at least 46 people in the U.S. this year, mostly farmworkers.

For now, H5N1 poses little threat to the general public, officials say, but emerging evidence suggests that the virus is evolving to be better at infecting mammals. What's more, the spread of seasonal flu may offer H5N1 a chance to swap DNA with other influenza viruses and potentially get better at infecting humans or causing more-severe disease. With these concerns in mind, officials in both the U.S. and Canada are keeping an eye on the spread and evolution of bird flu.

Now, a teenager from the Fraser Valley in southern British Columbia has tested positive for an H5 virus. The presumptive positive test was performed by the B.C. Centre for Disease Control's Public Health Laboratory, and now, samples are being sent to the National Microbiology Laboratory in Winnipeg, Manitoba, to officially confirm the diagnosis. The U.S. has been using a similar process to confirm suspected bird flu cases in the country.

Related: Source of person's recent bird flu case remains a mystery

An investigation is now underway to determine how the teen was exposed to the virus and whether any of their contacts were exposed or are showing potential symptoms of bird flu. At the moment, it's thought that the likely source of the teen's illness was an infected bird or another animal. The statement did not note whether the patient had recently been around farm animals.

To date, there isn't evidence of H5 viruses spreading from one person to another.

"Our thoughts are with this young person and their family during this difficult time," Dr. Bonnie Henry, British Columbia's provincial health officer, said in Saturday's statement. The statement doesn't note what kinds of symptoms the patient is experiencing. Historically, human H5 infections have ranged from mild to deadly, although the severity of the illness may vary with different virus strains.

"This is a rare event," Henry said. "While it is the first detected case of H5 in a person in B.C. or in Canada, there have been a small number of human cases in the U.S. and elsewhere, which is why we are conducting a thorough investigation to fully understand the source of exposure here in B.C." Canada has reported one H5N1 case previously, in 2014, but the infected person had caught the virus while traveling in China.

No additional human cases of H5 have been detected in Canada. As the public health investigation progresses, the Canadian Food Inspection Agency is also working with various partners to respond to an uptick in H5N1 that's been affecting both farmed and wild birds in British Columbia since October. At least 22 outbreaks have been reported among poultry in the province since then.

For instance, in early November, a farm on British Columbia's Vancouver Island learned that its chickens and ducks had been exposed to bird flu. After contacting authorities, the farm's owners were instructed to cull all 50 affected birds.

Because H5N1 is very deadly and spreads very quickly in poultry, culling the birds is the best way to stop the spread to additional flocks. U.S. farmers culled tens of millions of birds when bird flu ripped through poultry farms across the nation in 2022.

"In B.C., H5N1 has been detected in wild birds, on poultry farms and among small wild mammals, including skunks and foxes," the statement noted. However, unlike in the U.S., "in B.C. and Canada, there have been no cases reported in dairy cattle and no evidence of avian influenza in samples of milk." H5N1 can linger in raw milk but is neutralized through the pasteurization process.

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H5N1 bird flu is getting better at spreading between mammals but does not yet spread as easily as seasonal flu, a Centers for Disease Control and Prevention (CDC) study suggests.

H5N1 has been detected in at least 46 people in the U.S. this year, causing only mild illnesses so far. The CDC maintains that the virus still poses little risk to the general public, but as a precaution, scientists have explored whether the avian virus has adapted to infect mammals.

H5N1 has been detected in about 50 mammal species, including cattle. The question is how adept the avian virus is at infecting mammals, and how easily it can spread when it jumps into a new species, like humans. In the ongoing outbreak, researchers haven't found any examples of human-to-human transmission, but they're watching out for signs.

In the new study, published Oct. 28 in the journal Nature, the CDC used ferrets because the animals are susceptible to human influenza and display similar symptoms.

Related: 'Playing Russian roulette with your health': Officials warn that social media trend of consuming raw milk will not protect you from bird flu

"Because they expel virus into the air, they have been used as a model system to study airborne transmission," said Seema Lakdawala, an influenza virologist at Emory University who was not involved with the study but collaborates with the CDC on other projects. The lungs of humans and ferrets bear a similar distribution of receptors that the virus can use to enter cells, Lakdawala noted.

The study showed H5N1 transmits easily between ferrets under certain circumstances, suggesting it could spread between other mammals.

"It doesn't mean that because the virus transmits in ferrets, it will transmit in humans," said Troy Sutton, a veterinary researcher at Penn State who was not involved with the study. Rather, it shows that the virus may be gaining in its ability to spread between mammals, he clarified.

Deadly disease

The CDC researchers used an H5N1 virus sampled from a dairy farmworker in Texas, who caught one of the first human cases this year. This virus bore a mutation called E627K, which has been linked to the flu pandemics that took place in 1918, 1957 and 1968, Sutton told Live Science.

E627K changes a protein that helps the virus replicate, enabling it to do so more efficiently at colder temperatures, Sutton explained. Human body temperature tends to be a few degrees Celsius lower than birds', so this mutation plays a role in helping bird flu viruses infect humans.

Surveillance has not found any further cases in humans or mammals involving this mutation, Lakdawala said. But the CDC has studied its effects on ferrets in case it reappears.

When scientists deposited H5N1 directly into ferrets' noses, the animals developed severe symptoms — such as diarrhea, difficulty breathing and fever — and, in some cases, they died. By contrast, the infections among people in the U.S. have been quite mild, with minor symptoms, like eye redness.

Related: 21-year-old student dies of H5N1 bird flu in Vietnam

One possible explanation is that researchers administered millions of virus particles to the ferrets, which Sutton said is standard practice for flu studies in ferrets.

"It is probably higher than what a person would receive if they were exposed in a room to somebody with human influenza," he said. However, he added that a milliliter of unpasteurized cow milk can carry 100 times more virus than the ferrets received, so it's possible that farmworkers may be exposed to higher doses.

Humans also have preexisting immunity to related flu strains that afford them some protection, whereas the ferrets in this study grew up on flu-free farms. In a paper that hasn't yet been peer-reviewed, Lakdawala found that ferrets with immunity to the 2009 H1N1 virus — better known as "swine flu" — show some immunity to H5N1.

Efficient transmission

Scientists aren't certain how farmworkers catch the virus. It could be through the direct handling of animals, airborne transmission, or contact with contaminated surfaces, like milking equipment. The CDC studied all three possibilities in ferrets.

By pairing an infected ferret with a healthy ferret in the same cage, they studied direct contact. "Ferrets are very social animals. They nuzzle each other and snuggle," Lakdawala said. They tested direct contact for three ferret pairs and found that transmission and severe disease occurred in each case.

By moving a healthy ferret to a cage previously occupied by an infected one, the researchers explored transmission from contaminated surfaces, such as cage walls, bedding, food and water. To test airborne spread, they placed infected and healthy ferrets in adjacent cages separated by a perforated wall that allowed airborne viruses through. Neither of these routes of transmission were as efficient as direct contact, with a fraction of the ferrets remaining uninfected.

This study provides insight into the potential severity and transmission of H5N1. However, Lakdawala noted that it doesn't account for the complexities of the human immune system or human behavior, nor do these types of cage experiments assess the possibility of spread over both long and short distances.

She said it will be important to study H5N1 viruses collected from other human patients to see if the behavior of the virus changes as its genetics do.

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When flu season is in full swing, many of us will end up coughing, aching and hiding under the covers as we fight off the seasonal infection. Fortunately, most people with the flu feel better within about a week of their symptoms beginning.

Nonetheless, the average influenza infection can disrupt daily life and also pose the risk of spreading the flu. That raises a question: Are there any scientifically proven ways to shorten a flu infection?

Live Science spoke with two experts to find out.

Related: How do people die of the flu?

Can antiviral drugs shorten a flu infection?

The main way to reduce the duration of a flu infection is to take antiviral medications, meaning drugs specifically designed to combat viral infections, Dr. Timothy Brewer, a professor of medicine and epidemiology at UCLA, told Live Science. Four antivirals are currently approved by the U.S. Food and Drug Administration (FDA) to treat influenza: Tamiflu (generic name oseltamivir), Relenza (zanamivir), Rapivab (peramivir) and Xofluza (baloxavir).

Tamiflu is likely the best known example of a flu antiviral. This prescription drug, taken by mouth, is approved for use by people who are at least 2 weeks old and who have had flu symptoms for no more than two days. The drug works by blocking the activity of an enzyme that typically enables flu viruses to replicate inside the body. This enzyme is relevant to influenza A and B viruses, the two main types of seasonal flu that sicken people. Thus, Tamiflu helps the immune system thwart infections.

There's good evidence that, if you start taking Tamiflu within the first two days of your symptoms starting, the drug will shorten the duration of your illness, Brewer said. Flu symptoms typically last for around a week, and clinical trials have shown that Tamiflu can accelerate recovery by about a day, compared with a placebo.

The Centers for Disease Control and Prevention (CDC) also recommends that patients who are hospitalized for an influenza infection be given an antiviral like Tamiflu as soon as they are admitted. Research suggests that taking Tamiflu upon admission can slash death rates by 40%, compared with delaying the treatment by just a few days.

Relenza, Rapivab and Xofluza differ from Tamiflu in the way they are administered. For example, Relenza is inhaled, while Rapivab is injected. The drugs are also approved for different groups than Tamiflu; Xofluza is only available for patients who are at least 5 years old, for instance. There's also evidence that these drugs can shorten the duration of flu symptoms by about a day.

Related: Flu shot lowers hospitalization risk by 35% in vulnerable groups, data hint

Are there any other ways to shorten a flu infection?

Some people may take non-pharmaceutical products — such as vitamin C, zinc or echinacea — in the hope of alleviating their flu symptoms and getting better faster. However, there's currently not enough clinical evidence to suggest that these strategies accomplish either of those goals.

A lot of research has focused on how these products specifically target symptoms of the common cold, rather than the flu. Colds are also respiratory illnesses, but they can be caused by a number of different viruses, whereas the flu is only caused by influenza viruses.

"If people want to take vitamin C, that's certainly a fine thing to do, but there's no evidence that I'm aware of that it has any benefit [for the flu]," Brewer said. "Zinc is another thing that people have tried — again with no clear evidence that it impacts the duration of flu infection."

There are a few drugs on the market that can treat flu symptoms, but they don't target the root cause of the infection to shorten its duration. For example, pain relievers and fever reducers like acetaminophen (Tylenol) and ibuprofen can help alleviate flu symptoms, Dr. Richard Zimmerman, a professor of family medicine and clinical epidemiology at the University of Pittsburgh, told Live Science.

However, there is a very small risk that children and teenagers who take aspirin while they have a viral illness can develop a deadly disease called Reye's syndrome, which can cause brain swelling and liver damage. While the exact cause of the condition is unknown, because of this connection between the drug and viral infections, aspirin should not be given to people younger than 19 unless a doctor advises it.

Honey has a pretty good track record for treating coughs, Zimmerman noted. However, it should never be given to children younger than 1 year old because there is a risk that they could develop infant botulism. That's because honey can be contaminated with the spores of Clostridium botulinum, the bacterium behind botulism, and babies' immune systems are not mature enough to subdue the infection.

Getting a flu shot won't necessarily make your infection any shorter if you do end up catching influenza. However, the vaccine can reduce the severity of your symptoms if you get infected. This puts you at lower risk of developing serious complications of the illness that can lead to hospitalization or death, such as severe pneumonia.

Vaccination is a preventive measure for flu, not a treatment, in part because it takes about 14 days after vaccination for an immune response to fully develop. By that time, your body would have likely already fought off the infection if you caught the flu within the same time window, Zimmerman said. That's why experts recommend you get your flu shot in September or October, before flu season starts ramping up in earnest.

At baseline, everything you can do to maintain a healthy immune system — such as eating a nutritious diet, getting enough sleep, not smoking, and staying hydrated — will also likely reduce your risk of getting seriously ill from influenza infection, Brewer said. But antivirals are your best bet for shortening the illness.

This article is for informational purposes only and is not meant to offer medical advice.

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Each flu season, about 8% of people in the United States get sick with influenza. Most recover from the sickness, but thousands die annually of seasonal flu. In the last decade, there have been between 21,000 and 51,000 deaths from flu each season, with the exception of 2020-2021 when flu activity was unusually low.

Certain groups are most likely to develop severe flu infections that could lead to death: children under 5, adults 65 and older, people with chronic health conditions and pregnant people.

But how do people actually die of the flu?

There are many ways people can die of the flu. Influenza viruses can inflict serious damage on a range of organs and systems throughout the body, including the lungs, heart, brain and overall immune system, said Dr. Akiko Iwasaki, a professor of immunobiology and molecular, cellular, and developmental biology at Yale University and investigator of the Howard Hughes Medical Institute.

Related: How long is the flu contagious?

These life-threatening complications include pneumonia, which inflames the air sacs of the lungs and can cause them to fill with pus. Flu viruses can directly invade the lungs, causing what's called viral pneumonia. In addition, by damaging cells that line the respiratory tract, the viruses can open the door for bacteria in the body to overgrow, driving inflammation and possibly bacterial pneumonia.

"These are the bacteria that commonly colonize the upper respiratory tract of people without causing any disease," Iwasaki said. After someone gets the flu, these bacteria can gain a foothold in the lungs and multiply.

That's partly because flu viruses make the cells lining the respiratory tract more vulnerable to bacterial penetration and infection, and then "bacteria can proliferate and cause damage more easily," Dr. Octavio Ramilo, chair of the Department of Infectious Diseases at St. Jude Children's Research Hospital, told Live Science.

Severe pneumonia can also cause acute respiratory distress syndrome (ARDS), in which the lungs accumulate fluid, become stiff and can't expand well.

Flu complications don't only affect the lungs, but also the heart. Myocarditis and acute pericarditis respectively inflame the heart's muscles and the sac that surrounds the organ. These conditions can affect the heart's ability to pump blood and disrupt its rhythm, sometimes with deadly consequences.

Flu infections can spark myocarditis because flu viruses inflame and damage the cells that line blood vessels, Ramilo said. The exact cause for pericarditis is unclear, but the tissue typically becomes inflamed following a viral infection like the flu.

Related: Could we ever eradicate the flu?

In rare cases, the flu can also lead to encephalitis, or swelling of the brain. This condition, which is estimated to occur in about 4 in 100,000 children in the U.S. each year, can lead to serious complications including convulsions, coma and death.

Scientists don't know exactly how the flu leads to encephalitis, but a leading hypothesis suggests the infection causes the body to release a flood of cytokines, which are molecules that cause inflammation. These molecules can then enter the brain because the organ's protective barrier has been compromised, potentially by the cytokines themselves. However, more studies are needed to confirm that theory.

Influenza can also lead to sepsis, a life-threatening, body-wide immune response. Flu viruses can trigger this immune reaction on their own, but they can also enable bacteria to enter the bloodstream by breaking down tissues in the body, Ramilo noted. Those bloodborne bacteria can then trigger sepsis. Some studies suggest influenza-associated pneumonia may raise the risk of sepsis up to sixfold, compared to a flu infection without pneumonia; that said, this complication is relatively uncommon overall.

Who is at highest risk of dying from the flu?

A major reason why older adults, young children, people with chronic medical conditions and pregnant people are vulnerable to deadly flu complications is the state of their immune systems.

Older adults experience senescence — essentially biological aging that leads to a decline in cell function that hinders the immune system, Ramilo said. In pregnant people, the immune system lowers its defenses so as not to attack the growing fetus. Thus, people become more vulnerable to flu-related complications during pregnancy, he explained.

Young children's immune systems haven't encountered many germs before. "The immune responses to the [flu] virus have to be developed from scratch," Iwasaki said. Because a child's immune system has no memory of flu viruses, "the virus has the chance to multiply before the immune responses can catch up," she said.

Chronic conditions — such as asthma, kidney disease, HIV/AIDS and cancer — raise the risk of flu complications by compromising either the immune or respiratory system, or both. The flu can also worsen chronic conditions, increasing the risk of death from other causes.

Thankfully, flu vaccines can help prevent severe flu infections and their potentially deadly knock-on effects. For people who do develop severe flu infections, treatments like Tamiflu can dramatically lower the risk of death if provided quickly.

This article is for informational purposes only and is not meant to offer medical advice.

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For the average healthy adult, the flu can be disruptive — long days of sore throat, fever and coughing cause people to miss school and work, and also just feel miserable. But for very young children, older adults and people with certain health conditions, seasonal flu can lead to serious complications, a hospital stay or even death — tens of thousands of people die of seasonal flu each year in the U.S.

To cut down on flu hospitalizations and deaths, people who catch the flu should do their best not to pass the infection to others. But how long is someone with the flu contagious?

A good rule of thumb is to wait at least 24 hours after your flu symptoms have subsided to interact with other people, said Dr. Donald Milton, a professor of environmental health at the University of Maryland School of Public Health.

"If you're a day without symptoms after the flu, you should be pretty safe at that point," Milton told Live Science. But there's some nuance to how long someone might be contagious, and there's ongoing debate about exactly how the flu spreads.

Related: Flu shot lowers hospitalization risk by 35% in vulnerable groups, data hint

Respiratory viruses such as influenza spread through three main pathways: viruses left on surfaces that people accidentally touch, larger droplets expelled onto a person through coughing or sneezing, or in tiny airborne particles released through breathing. Experts think people spread the flu primarily through the latter two routes — large and small droplets — but contaminated surfaces can still be a source of transmission.

The specifics and relative importance of each transmission pathway remain big topics of research in the field, Milton said. Virus transmission is tricky to study in a controlled environment — the research that yields the most useful information requires finding volunteers willing to catch the flu for science. (And yes, there are willing volunteers.)

According to the U.S. Centers for Disease Control and Prevention, people with the flu tend to be most contagious during the first three days of their symptoms, and studies have found that they "shed" the largest volume of virus into their snot and saliva during this period. The chances of spreading the illness fall after this peak, but adults can still remain contagious five to seven days after their symptoms appear.

The picture is more complicated for adults with certain health conditions, such as those that compromise immune function. People with weakened immune systems can take longer to fight off an influenza infection, so they may remain contagious longer than is typical. Conditions such as diabetes, which also affects immune function, may also influence how long someone sheds the virus, Milton said.

For children, the picture is also complicated — kids are prolific flu spreaders for a reason. Like adults, they're most likely to pass on the flu during the first few days of illness, but children can remain contagious for about one or two weeks after their symptoms start. Milton said this is likely because children's immune systems don't have as much experience overcoming the virus as adults' do, because on average, they've been exposed to flu viruses and flu vaccines fewer times.

Notably, the varieties of flu that spread change year to year, and this can lead the disease to be more or less transmissible in a given season, Milton noted. Circulating flu viruses mutate slightly every year, which is why the annual flu shot must be updated routinely. Some years, the predominant viruses do a better job of bypassing the immune system's learned defenses, so infected people may remain contagious longer, he said.

Related: At-home flu vaccine approved by FDA — what to know

The flu vaccine is a great way to lower your chances of getting severely ill if you get the flu. But it likely won't do much to change how contagious you are.

"It's not clear that the current vaccines do very much for that," Milton said. "They mainly keep you from dying of it and keep you out of the hospital, but it doesn't appear that they do a very good job of preventing you from spreading it." However, he added this is still a big question that influenza researchers are striving to answer.

The most difficult aspect of preventing flu transmission is stopping the spread that happens before a person develops symptoms. Similar to what happens in COVID-19 infections, people start shedding enough flu virus to infect others about one day before symptoms start.

That's why respiratory viruses like the flu and coronaviruses are so hard to control, Milton said — you're dealing with discreet spread that happens when virus levels are still pretty low and people's symptoms are mild or nonexistent. But new research could one day counter that spread.

Scientists are investigating how a protein on the surface of flu viruses, called neuraminidase (NA), contributes to transmission. The body makes some antibodies against NA after a flu infection, and evidence suggests people with higher levels of NA antibodies may be less likely to pass the virus on.

Flu vaccines have traditionally focused on a different flu virus protein, called hemagglutinin (HA), rather than NA. If future research confirms the importance of NA, finding a way to boost people's NA antibody levels could be one way to slow the chain of flu transmission, Milton said.

This article is for informational purposes only and is not meant to offer medical advice.

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A scratchy, sore throat, a relentless fever, a pounding head and a nasty cough — these symptoms all scream upper respiratory illness. But which one?

Many of the viruses that cause upper respiratory infections such as influenza A or B and the virus that causes COVID-19 all employ similar tactics. They target the same areas in your body — primarily the upper and lower airways — and this shared battleground triggers a similar response from your immune system. Overlapping symptoms — fever, cough, fatigue, aches and pains — make it difficult to determine what may be the underlying cause.

Now, at-home rapid tests can simultaneously determine whether someone has COVID-19 or the flu. Thanks in part to the National Institutes of Health's Rapid Acceleration of Diagnostics, or RADx, program, the Food and Drug Administration has provided emergency use authorization for seven at-home rapid tests that can distinguish between COVID-19, influenza A and influenza B.

Related: Flu shots have changed this year — here's why

Our team in Atlanta — composed of biomedical engineers, clinicians and researchers at Emory University, Children's Healthcare of Atlanta and Georgia Institute of Technology — is part of the RADx Test Verification Core. We closely collaborate with other institutions and agencies to determine whether and how well COVID-19 and influenza diagnostics work, effectively testing the tests. Our center has worked with almost every COVID and flu diagnostic on the market, and our data helped inform the instructions you might see in many of the home test kits on the market.

While no test is perfect, to now be able to test for certain viruses at home when symptoms begin can help patients and their doctors come up with appropriate care plans sooner.

A new era of at-home tests

Traditionally, identifying the virus causing upper respiratory illness symptoms required going to a clinic or hospital for a trained medical professional to collect a nasopharyngeal sample. This involves inserting a long, fiber-tipped swab that looks like a skinny Q-tip into one of your nostrils and all the way to the back of your nose and throat to collect virus-containing secretions. The sample is then typically sent to a lab for analysis, which could take hours to days for results.

Due to the COVID-19 pandemic, the possibility of using over-the-counter tests to diagnose respiratory illnesses at home became a reality. These tests used a much gentler and less invasive nasal swab and could also be done by anyone, anytime and in their own home. However, these tests were designed to diagnose only COVID-19 and could not distinguish between other types of illnesses.

Since then, researchers have developed over-the-counter multiplex tests that can screen for more than one respiratory infection at once. In 2023, Pfizer's Lucira test became the first at-home diagnostic test for both COVID-19 and influenza to gain emergency use authorization.

Related: Scientists find secret 'back door' flu viruses use to enter cells

What are multiplex rapid tests?

There are two primary forms of at-home COVID-19 and COVID-19/flu combination tests: molecular tests such as PCR that detect genetic material from the virus, and antigen tests — commonly referred to as rapid tests — that detect proteins called antigens from the virus.

The majority of over-the-counter COVID-19 and COVID-19/flu tests on the market are antigen tests. They detect the presence of antigens in your nasal secretions that act as a biological signature for a specific virus. If viral antigens are present, that means you're likely infected.

To detect these antigens, rapid tests have paper-like strips coated with specially engineered antibodies that function like a molecular Velcro, sticking only to a specific antigen. Scientists design and manufacture specialized strips to recognize specific viral antigens, like those belonging to influenza A, influenza B or the virus that causes COVID-19.

The antibodies for these viral targets are placed on the strip, and when someone's nasal sample has viral proteins that are applied to the test strip, a line will appear for that virus in particular.

Advancing rapid antigen tests

Like all technologies, rapid antigen tests have limitations.

Compared with lab-based PCR tests that can detect the presence of small amounts of pathogen by amplifying them, antigen tests are typically less sensitive than PCR and could miss an infection in some cases.

All at-home COVID-19 and COVID-19/flu antigen tests are authorized for repeat use. This means if someone is experiencing symptoms — or has been exposed to someone with COVID-19 but is not experiencing symptoms — and has a negative result for their first test, they should retest 48 hours later.

Another limitation to rapid antigen tests is that currently they are designed to test only for COVID-19, influenza A and influenza B. Currently available over-the-counter tests aren't able to detect illnesses from pathogens that look like these viruses and cause similar symptoms, such as adenovirus or strep.

Because multiplex texts can detect several different viruses, they can also produce findings that are more complex to interpret than tests for single viruses. This may increase the risk of a patient incorrectly interpreting their results, misreading one infection for another.

Researchers are actively developing even more sophisticated tests that are more sensitive and can simultaneously screen for a wider range of viruses or even bacterial infections. Scientists are also examining the potential of using saliva samples in tests for bacterial or viral infections.

Additionally, scientists are exploring integrating multiplex tests with smartphones for rapid at-home diagnosis and reporting to health care providers. This may increase the accessibility of these tests for people with vision impairment, low dexterity or other challenges with conducting and interpreting at-home tests.

Faster and more accurate diagnoses lead to more targeted and effective treatment plans, potentially reducing unnecessary antibiotic use and improving patient outcomes. The ability to rapidly identify and track outbreaks can also empower public health officials to better mitigate the spread of infectious diseases.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

This year's flu shot lowers the chance of being hospitalized for flu by 35% among high-risk groups, data released by the Centers for Disease Control and Prevention (CDC) suggests.

The new data, published Oct. 3 in the CDC's Morbidity and Mortality Weekly Report (MMWR), was pulled from five South American countries.

The Southern Hemisphere's flu season typically runs between April and September, while the Northern Hemisphere's runs from about October to May. Because of this alternating timing, the Northern Hemisphere looks to Southern Hemisphere data to see which strains of flu virus are circulating, to plan the design of flu shots for the upcoming season, and to determine how well those vaccines will likely work.

In the new MMWR, researchers looked at the flu shot's effectiveness in Argentina, Brazil, Chile, Paraguay and Uruguay. The report focuses on people at high risk of severe flu infections, including young children, older adults and people with conditions such as chronic lung disease, liver disorders, heart disease or immune-system problems.

Related: Flu shots have changed this year — here's why

Between mid-March and mid-July, these five countries reported that more than 11,700 people in these groups were hospitalized with severe respiratory illnesses. Of these, about 3,850 tested positive for the flu, with the vast majority testing positive for an influenza A virus. (Influenza A and influenza B are the two broad groups of flu viruses that circulate seasonally, and in the study, two subtypes of influenza A stood out as dominant: H3N2, followed by H1N1.)

The remaining 7,850 or so people in the study tested negative for both flu and COVID-19, and they acted as a point of comparison for the analysis. About 23% of these patients had been vaccinated for the flu, compared with only 18% of the group that ended up hospitalized for the viral infection.

Crunching the numbers, the researchers found that the vaccine likely lowered the risk of flu hospitalization by 34.5%. Breaking it down further, the shot lowered the risk of hospitalization by nearly 59% among adults with medical conditions, 39% for young children and 31% for older adults.

"While only one in five [severe acute respiratory infection] patients had received the 2024 influenza vaccine, those who were vaccinated were at significantly lower risk for hospitalization from any influenza virus infection," the study's authors concluded. These data suggest the flu shot was "effective in preventing approximately one third of influenza-related hospitalizations among groups prioritized for vaccination."

Trends in the Southern Hemisphere's flu season don't always accurately predict what will happen in the Northern Hemisphere. However, assuming the same flu virus strains circulate in the upcoming months, "health authorities might anticipate similar levels of protection from the 2024-25 vaccine," the authors wrote.

With a few exceptions, the CDC recommends that everyone 6 months and older get the annual flu vaccine. Additional measures, such as regular handwashing, can help reduce the spread of flu. For those who do catch the flu, getting antiviral treatments, such as Tamiflu, as soon as possible can significantly reduce the risk of flu-related complications.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

For more than a decade, the annual flu shot has protected against four types of the influenza virus — but this year, that has changed.

In the United States, vaccines for the 2024-2025 flu season provide protection against only three subtypes of influenza: two influenza A viruses, called H1N1 and H3N2; and one influenza B virus, known as the "Victoria lineage." A fourth kind of flu virus — the "Yamagata lineage" — has been dropped from the formulation. This will be the first time since 2012 that U.S. flu vaccines have been trivalent, meaning three-component, as opposed to quadrivalent, or four-component.

But why the change?

Based on all available evidence, the Yamagata lineage is now extinct. So, in March 2024, the U.S. Food and Drug Administration (FDA) announced that this year's flu vaccines would not include that subtype of virus, because it hasn't sickened anyone in years. We know this because there is a global consortium of scientists that regularly isolates flu viruses from sick patients and then analyzes the viruses' genetic material.

Related: At-home flu vaccine approved by FDA — what to know

The FDA made this decision following recommendations from its Vaccines and Related Biological Products Advisory Committee, and shortly after, the World Health Organization issued the same recommendation on a global scale. However, it may take time for global manufacturers to switch to the new formula, STAT News reported.

Flu vaccines are designed to protect people against the most common subtypes of influenza that sicken people each season. Within those subtypes, scientists predict which strains are most likely to circulate in the upcoming flu season by tracking infections in the opposite hemisphere, since the Northern and Southern hemispheres' flu seasons alternate.

Flu shots work by priming the immune system to make antibodies that are ready to attack the selected flu strains if a vaccinated person is exposed.

Scientists reported early hints that the Yamagata lineage had gone extinct in 2021, when they started to notice that no one in the world was getting sick with that subtype of flu. The most likely explanation, experts speculated at the time, is that Yamagata disappeared due to public health measures aimed at reducing the spread of COVID-19. Early in the pandemic, many people were masking, social distancing and staying at home when they were sick, which had a knock-on effect of curtailing other respiratory infections, like the flu.

Of the influenza B viruses, Yamagata probably took the hit because it was circulating at much lower levels than Victoria before the pandemic began, Kevin McCarthy, an assistant professor of microbiology and molecular genetics at the University of Pittsburgh, told Live Science. Yamagata also might be inherently more vulnerable to extinction than other subtypes of the virus, he suggested. It has a comparatively poor ability to infect large numbers of people at once.

Because no one's been spotted with a Yamagata infection since March 2020, it makes sense to drop Yamagata from future flu vaccines.

"There's really no reason to include the strain if you're not expecting people to actually be exposed to it and get infected," Hanover Matz, a postdoctoral research associate at Washington University School of Medicine in St. Louis, told Live Science.

With Yamagata out of the equation, manufacturers might be able to make more doses of the flu vaccine, CNN reported in March. Plus, they wouldn't need to cultivate as many viruses in the lab, so the change could cut down on the potential risks associated with this production step. Currently, many flu vaccines require scientists to grow flu viruses in eggs or cells, which introduces a very tiny risk that the viruses may escape; scientists follow strict safety protocols to avoid such accidents.

Importantly, dropping Yamagata won't lower the effectiveness of future flu shots, because the vaccines are designed to provide protection only against the strains of the virus they contain, McCarthy said. Thus, removing Yamagata won't influence how well the vaccines work against H1N1 or the Victoria lineage.

Public health officials will continue to monitor for any traces of the Yamagata lineage, despite evidence of its extinction. If Yamagata were to return, scientists could just make the vaccine quadrivalent again, McCarthy said.

This article is for informational purposes only and is not meant to offer medical advice.

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A nasal spray version of the annual flu vaccine can now be taken at home, the U.S. Food and Drug Administration (FDA) says.

In a statement published Sept. 20, the FDA announced that it had approved the nasal spray influenza vaccine, called FluMist, for use at home. This allows people ages 18 to 49 to self-administer the vaccine — meaning they spray it up their own nose. The vaccine is also cleared for children and adolescents ages 2 to 17, whose caregivers can administer the spray.

FluMist was previously available for people ages 2 to 49 in the U.S., but it could be administered only by professionals in health care settings, such as pharmacies and doctors' offices. The FDA's new announcement comes after clinical trial data showed that FluMist was easy to self-administer and that it was just as safe and effective to vaccinate this way as it was by health care professionals.

The U.S. Centers for Disease Control and Prevention (CDC) recommends that, except in rare circumstances, everyone 6 months and older get a flu vaccine each season — ideally between September and October, before the flu starts circulating widely. The new approval could make vaccination more convenient, flexible and accessible, Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, said in the statement.

Related: 2-in-1 shot for flu and COVID shows promise in advanced trial

"Getting vaccinated each year is the best way to prevent influenza, which causes illness in a substantial proportion of the U.S. population every year and may result in serious complications, including hospitalization and death," Marks said. "This approval adds another option for vaccination against influenza disease and demonstrates the FDA's commitment to advancing public health."

Here's what we know so far about using FluMist at home.

What is FluMist?

FluMist is a nasal spray-based vaccine that helps prevent illness caused by two types of flu, known as influenza A and B. These virus types are responsible for most flu infections in humans.

FluMist is a live, attenuated vaccine, meaning it contains weakened versions of influenza A and B viruses. These weakened viruses cannot cause the flu; instead, they prime the immune system to produce antibodies to fight off future infections.

FluMist first gained FDA approval in 2003, for use in people ages 5 to 49. In 2007, this approval was expanded to include children as young as 2. Until now, FluMist could be administered only by health care providers in pharmacies and other health care facilities.

FluMist is the only non-injected flu vaccine available to people in the U.S. For the 2024-2025 flu season, FluMist and all of the other approved vaccines will guard against three subtypes of flu: two influenza A viruses and one influenza B virus.

How do you use FluMist?

FluMist is administered via one spray in each nostril. The vaccine is packaged in a small tube with a plunger on one end. The end of the tube opposite the plunger goes into the nose, and then you push the plunger to release the spray, emptying one half of the tube's contents into each nostril.

People ages 9 and older require only one dose of the vaccine per year — meaning one spritz in each nostril. However, children ages 2 to 8 may need an additional dose at least a month later. This depends on the child's vaccination history; children who haven't gotten at least two doses previously or whose vaccination history is unknown need two doses.

In these instances, your health care provider can help determine whether your child needs a second dose.

How do I order FluMist?

Although the FDA has approved FluMist for use at home, it will take some time for the spray to be available for home delivery.

The vaccine's maker, AstraZeneca, said it anticipates the vaccine will be available to order for home delivery through a third-party online pharmacy service in time for the 2025-2026 flu season. The FDA notes you still need a prescription for FluMist — in this case, that means a pre-approved prescription provided by an online pharmacist.

Is there anyone who shouldn't get FluMist?

There are several groups of people who should not get a FluMist vaccine, according to the CDC. They include individuals who are pregnant and those with weakened immune systems. These precautions are taken because the FluMist vaccine contains live (albeit weakened) viruses, while flu shot options do not.

Although not everyone can use FluMist, it is hoped that the new approval will make needle-free options for flu vaccines available to more people.

"It's very exciting to see the FDA take this step and find one more way to make it easy for people to get vaccines," said Dr. Andrew Handel, a pediatric infectious diseases expert at Stony Brook Children's Hospital in New York.

It's possible that the new approval may even inspire pharmaceutical companies to pursue the development of nasal vaccines for other respiratory infections, such as COVID-19, he told Live Science.

This article is for informational purposes only and is not meant to offer medical advice.

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Over a dozen cases of H5N1 have been reported among people in the United States this year. Current evidence suggests this bird flu virus spreads to humans only from infected animals, such as dairy cows and poultry, and not from person to person.

However, the latest case of human infection — reported Sept. 6 by the Centers for Disease Control and Prevention (CDC) — occurred in a person in Missouri who'd had no known exposure to relevant animals.

The CDC has previously stated that there is no evidence of the person passing the virus to close contacts, and says "the risk to the general public from H5N1 remains low." The agency maintained that the risk is low even after two health care workers got sick after interacting with the patient — one tested negative for flu and one has yet to be tested for H5N1 or antibodies.

In an update on Sept. 27, the CDC noted that four more health care workers tending to the Missouri case fell ill with mild respiratory symptoms. Antibody blood tests are currently underway to determine if any of them have H5N1, which would suggest transmission between humans.

"I agree with the CDC that it remains low-risk," said Dr. Nahid Bhadelia, founding director of the Center on Emerging Infectious Diseases at Boston University. "The good news is you haven't seen an uptick in influenza cases," as that would show up in surveillance for seasonal flu, she added. For the moment, that should help quell public concern that this could turn into an outbreak.

Nonetheless, Bhadelia and another expert told Live Science that the Missouri case raises other concerns — namely, that the U.S. is not tracking the spread of H5N1 closely enough to head off a potential epidemic.

Related: H5N1: What to know about the bird flu cases in cows, goats and people

The Missouri case

The Missouri patient was hospitalized in August, marking the country's 14th H5N1 case overall and its first without any connection to sick animals at the person's workplace. The patient, who had underlying medical conditions, experienced chest pain, diarrhea, vomiting and weakness. They were not severely ill during their hospital stay and have recovered.

The patient initially tested positive for influenza A, the broad group of viruses to which H5N1 belongs. This group also includes seasonal flu viruses, like H1N1, but the patient tested negative for those subtypes. Further testing then revealed H5N1.

In a Sept. 13 update, the CDC reported that it had partially analyzed the genome of the virus; there was not enough genetic material for a complete sequence. The results showed that the virus that infected the Missouri patient bears high similarity to those circulating in cattle and hasn't mutated enough to be well adapted for infecting human lungs. Such mutations could pave the way for human-to-human transmission.

Despite these reassuring findings, Bhadelia argues that stakeholders involved in preparing for possible pandemics should find the Missouri case alarming.

H5N1 has been circulating among birds in the U.S. since 2022, jumping into various mammals — including cows — since its introduction. The Missouri patient's infection likely came from infected cows but happened in a state that has not detected the virus in cattle, Bhadelia noted, even though Missouri has historically detected 600,000 infections in poultry. Infected herds may have slipped under the radar.

Seema Lakdawala, an associate professor of microbiology at Emory University who researches H5N1, said not enough cattle are undergoing tests to chart the virus's geographic distribution.

"The USDA [Department of Agriculture] is not doing enough testing — full stop," she told Live Science. "We do not have a good handle on how many cows are actually infected because there is no universal testing of dairy farms." As of July 12, Missouri had only tested 17 of their approximately 60,000 dairy cows, The Missouri Independent reported.

Related: 'Increased evidence that we should be alert': H5N1 bird flu is adapting to mammals in 'new ways'

Remaining unknowns

The CDC has not identified the source of transmission in the Missouri case. One possible source involves consuming raw (unpasteurized) cow milk, which the CDC warns against.

"An infected cow can have ten to hundreds of millions of particles of infectious virus per milliliter [of milk], and these animals are producing gallons of this," Lakdawala said. It's not known whether someone could catch bird flu from drinking raw milk, but the product can spread other germs. Raw milk left on milking equipment may be a way dairy workers get exposed.

Humans could be another theoretical source of H5N1 infections — but as far as experts know, the virus has never spread between people. "One way I think we can clarify it is to rule out the human component by doing serologies [antibody tests] on everybody around the patient," Bhadelia said.

One household contact of the Missouri patient developed "similar symptoms" the same day but was not tested for flu. Blood from that individual is now being tested for antibodies. Based on the timing of their illnesses, it's likely that the patient and household contact were simultaneously exposed to the same source of infection, rather than spreading the virus to one another, Lakdawala said. The CDC has said the same.

Public health laboratories mostly test for H5N1 in people exposed to animals suspected of carrying the virus. If they detect H5N1, they ship the samples to the CDC for confirmatory testing. The CDC said it's partnering with biotechnology companies to enable earlier and more widespread testing.

The CDC has also scaled up surveillance of the virus among people who interact with farm animals. Last week, it worked with the Ohio health officials to test veterinarians for antibodies while they'd congregated for a conference. Positive tests would help reveal which vets have previously been infected.

Lakdawala argues that the USDA still needs to do more to understand how the virus spreads among cattle. "What the USDA needs to be doing right now, in my opinion, is working with all the state veterinarians and state agricultural departments," she said. They should go into every farm and bulk-test milk tankers, she proposed.

Only 50 herds from 14 states have been enrolled in the USDA's voluntary Dairy Herd Status Program, which bulk tests milk tanks each week. Missouri has not participated.

Editor's Note: This story was updated on Monday, Sept. 30 at 3:00 p.m. EDT to note that four more health care workers showed respiratory symptoms after caring for the Missouri case, and that antibody testing will help determine if person-to-person transmission of H5N1 is occurring.

This article is for informational purposes only and is not meant to offer medical advice.

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Starting Tamiflu upon hospital admission can slash flu deaths by 40% compared with delaying treatment for two to five days, a new study from the Centers for Disease Control and Prevention (CDC) suggests.

Tamiflu (generic name oseltamivir) is one of four antiviral drugs that is currently FDA approved to treat the flu, or influenza. The drug targets two of the most common types of flu viruses — influenza A and B — and works by inhibiting a protein on the surface of the viruses that enables them to replicate within host cells.

The CDC and the Infectious Diseases Society of America recommend that patients who are hospitalized with a suspected or confirmed case of influenza start taking flu antivirals as soon as possible. However, until now, the extent to which earlier treatment improves patients' outcomes was not completely clear.

In a study published Aug. 22 in the journal Clinical Infectious Diseases, CDC researchers examined data from more than 26,000 adults in 13 U.S. states who'd been hospitalized with flu at some point between 2012 and 2019. Each patient's infection was confirmed with a laboratory test, and all had been diagnosed with pneumonia — a potentially deadly complication of influenza. The median age of those hospitalized was 71 years old, which aligns with the fact that people ages 65 and up have the highest rates of severe flu each season.

Related: Scientists reveal rare antibodies that target 'dark side' of flu virus

Nearly all of the patients — 99.7% — were treated with Tamiflu at some point during their visit. Approximately 60% of those patients started antiviral treatment on the day they were admitted to hospital, 30% were treated the day after, and about 10% were treated between the second and fifth day after admission. The study did not investigate why treatment was delayed in the latter cases.

Overall, the researchers found that patients in that last group were 40% more likely to die of any cause within a month of their hospital admission, compared with those who began treatment the first day.

Delaying by just one day was associated with a 14% increased risk of death, compared with first-day treatment, the team found.

For every additional day a patient didn't receive antiviral treatment, their risk of experiencing severe outcomes other than death — such as being admitted to the intensive care unit or requiring mechanical ventilation to breathe — also increased.

"These findings reinforce the importance of starting influenza antiviral treatment as soon as possible for all patients hospitalized with confirmed or suspected influenza," Dr. Mark Tenforde, lead study author and a medical officer at the CDC, told Live Science in an email.

The study authors acknowledged several limitations of their analysis that may have skewed the results. They didn't account for differences in the amount of time that patients were sick before they were admitted to the hospital, so it's possible that some arrived with much more severe disease than others. However, patients from different groups generally shared similar baseline characteristics, for example in terms of their age, race and underlying health conditions, including asthma and heart disease.

In addition, the study authors considered the risk of death from any cause, meaning it's possible the data captured some deaths that were not directly related to flu — say, cancer.

Nevertheless, these findings support previous research, published in 2015, that suggested that early antiviral treatment reduces the death rate among hospitalized patients with pneumonia from flu.

In an associated statement, the CDC cautioned that, although drugs like Tamiflu are important for treating and preventing flu after a likely exposure, they should not be used as a substitute for an annual vaccine. Flu shots are considered the best way to reduce the risk of flu-related medical visits, hospitalizations and death, as well as the potential worsening of other medical conditions after flu.

This article is for informational purposes only and is not meant to offer medical advice.

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Most flu viruses enter human cells through a single entryway — but new research has revealed a "back door" some germs can use to more easily infect cells and jump between species.

Seasonal influenza viruses infect about 1 billion people each year. Typically, these flu viruses enter our cells by binding to sugar molecules, called sialic acids, on the surfaces of cells. This triggers a process that moves the virus into the cell, where it can then replicate.

Like human flu viruses, many flu viruses that infect animals use sialic acids to enter host cells. However, some use a different entryway, called the major histocompatibility complex class II (MHC-II). Scientists had speculated that there might be flu viruses that could use both these routes, but none had ever been discovered — until now.

In a study published July 15 in the journal Nature Microbiology, researchers examined several subtypes of flu viruses to see whether any of them showed an ability to use both routes. They used two types of human cells: one with no detectable surface MHC-II expression and one with high levels of expression. "Expression" refers to the process by which cells make proteins.

Related: H5N1: What to know about the bird flu cases in cows, goats and people

The scientists exposed the cells to virus-like particles designed to resemble three different subtypes of flu viruses that have previously caused flu epidemics in humans: H1N1, H3N1 and H2N2.

The researchers saw that 10 times as many H2N2 lookalikes were able to enter cells with MHC-II than could enter MHC-II-deficient cells. That's despite both these cell types' having the same levels of sialic acids on their surfaces.

Next, the researchers wanted to double check that MHC-II complexes were actually an entry route for the viruses. They used gene editing to remove the cells' ability to make sialic acids, thus eliminating the route most flu viruses take. The H1N1 and H3N1 lookalikes were unable to infect the cells that lacked sialic acids. But the H2N2 particles could bind and enter those cells almost as efficiently as they could infect cells with normal sialic acid expression.

Finally, the researchers looked at different H2N2 strains to see which could use this alternate route. They identified three amino acids — building blocks of proteins — in the viral structure that are essential for binding to the "back door" receptor.

Interestingly, they also found that these viruses could replicate faster and reach higher levels in human lung cells than viruses that only use the "front door." In theory, this could make those viruses more likely to cause disease in and spread between humans.

A major concern is that not just human flu viruses enter our cells this way — it's that animal flu viruses can use the same route. Avian H2N2, for example, is a bird flu virus that caused a pandemic in the 1950s after it swapped genes with a human H1N1 virus.

"Today, H2N2 viruses no longer circulate in humans, but reside in the avian reservoir," study author Umut Karakus, a postdoctoral research scientist at the University of Zürich, told Live Science in an email. "Given that people born after 1968 were not exposed to H2N2 viruses and therefore do not have immunity against them, avian H2 viruses are of particular interest for pandemic preparedness."

H2 viruses can infect pigs, mice and ferrets, along with a variety of birds. This ability to move between species and exchange genes with other flu viruses increases the likelihood that, someday, H2N2 could transmit "zoonotically" from animals to humans. If such viruses pick up the ability to spread easily between people, they can spark pandemics.

So, how great is the threat of zoonotic infection from viruses that can use this "back door" into our cells?

"We need to understand how relevant that entry route is in jumping between species," said Richard Webby, director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds.

"Do viruses actually use that route? What is the relative contribution of MHC-II versus sialic acid?" said Webby, who was not involved in the study. "These questions are a good way to assess the risk that 'super viruses' circulating in animals might eventually jump over to humans."

With a better understanding of how these viruses behave in the wild, scientists can make more accurate judgements about where the greatest risks lie, and how we can best prevent the next flu pandemic.

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A fourth dairy worker in the U.S. has tested positive for bird flu following exposure to infected cows, the Centers for Disease Control and Prevention (CDC) reported Wednesday (July 3).

The case is the first human illness detected in Colorado in connection with an ongoing outbreak in cattle. It follows one human case in Texas and two in Michigan. The second case in Michigan caused mild respiratory symptoms, such as coughing, but the other three cases — including the most recent in Colorado — have only included eye symptoms, such as redness.

The culprit behind these infections is a bird flu virus called influenza A(H5N1). This subtype of flu was first spotted among U.S. dairy cows in March. Since then, dozens of herds across the country have tested positive for the virus.

Recent research suggests that, when it infects cows, the virus can end up in the animals' milk and remain infectious in unpasteurized milk for at least an hour after harvest. This poses a significant risk to dairy workers, who may come into contact with contaminated milk through contact with cows or with milking equipment. The pasteurized milk supply, on the other hand, is safe because the pasteurization process inactivates any viruses in the milk. 

Related: 'Playing Russian roulette with your health': Officials warn that social media trend of consuming raw milk will not protect you from bird flu

Because of the risk posed by raw milk, the CDC recommends that dairy workers wear personal protective equipment.

In the recent Colorado case, the individual was being monitored because they'd been exposed to H5N1-infected cattle at work. Clinical samples from the person were sent to the CDC, who confirmed an infection with an influenza A(H5) virus. The agency is continuing to analyze the sample to confirm that the virus is in fact H5N1.

The infected person was treated with a common drug for seasonal flu, called oseltamivir; Tamiflu is a well-known, brand-name version of the drug. The person recovered following this treatment. 

All three people previously infected in the outbreak also recovered from their mild infections. In the past, though, H5N1 has been known to sometimes kill people.

Overall, the Colorado case doesn't change the CDC's overall assessment of the unfolding outbreak.

"CDC has been watching influenza surveillance systems closely, particularly in affected states, and there has been no sign of unusual influenza activity in people," the agency reported. "Based on the information available at this time, this infection does not change CDC's current H5N1 bird flu human health risk assessment for the U.S. general public, which the agency considers to be low."

Nonetheless, the CDC will continue to monitor for unusual flu activity. There's a possibility that, in time, H5N1 could pick up mutations that enable the virus to spread easily from person to person — something it hasn't yet been able to do. Such a change in the virus' genetics would be concerning because the pathogen could then potentially spark a widespread outbreak, or even a pandemic.

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Bird flu is infecting cows across the U.S. Now, scientists have discovered that the virus from infected dairy cows can remain infectious in unpasteurized milk and on the surface of milking equipment for at least an hour. 

This means that dairy workers potentially face an increased risk of infection during the milking process. While the risk of drinking unpasteurized, or raw, milk was not the focus of this study, the research does confirm that live viruses can persist in raw milk. 

Avian influenza A(H5N1) primarily infects birds but can sometimes jump across to mammals. Scientists first detected the virus in cows in the U.S. in March, and it has since been found in more than 60 cattle herds across the country.

Since then, three people in the U.S. have contracted the H5N1 virus following exposure to infected dairy cows. Two developed eye infections and one had some mild respiratory symptoms. However, scientists are still unsure exactly how the disease is being transmitted from cow to cow and from cow to human.

Related: H5N1: What to know about the bird flu cases in cows, goats and people

In addition, "how [H5N1] got into the cows in the first place is a question that is a little bit mind-boggling to scientists," study lead author Valerie Le Sage, a microbiologist at the University of Pittsburgh, told Live Science. The researchers published their findings June 24 in the journal Emerging Infectious Diseases. 

Researchers already knew that H5N1-infected cows could shed the virus in their milk. Earlier this year, the Food and Drug Administration (FDA) reported that about 1 in 5 retail milk products in a sample had tested positive for H5N1 genetic material. However, none of the viruses detected in these products were "live," or infectious, because the products were pasteurized. Pasteurization kills most viruses and microbes. 

Little was known about whether the virus could remain infectious in unpasteurized milk, however. Just in case, the FDA has warned consumers against drinking raw milk and urged state regulators to stop any sales of raw milk produced by infected cows. 

To confirm whether the H5N1 virus can remain infectious in raw milk, Le Sage and her colleagues collected raw milk samples from cows, added H5N1 and then monitored the samples for an hour. These contaminated milk samples were pipetted onto stainless steel and rubber materials found in common milking equipment.

After an hour, the scientists found that the virus showed very little sign of decay on either material. More concerningly, the viruses appeared infectious when exposed to cells in lab dishes.

"Because the virus remains stable on these surfaces, if they're not cleaned up pretty soon after [contamination], the milk is just there with the virus in it," Le Sage said. "It does pose a threat to the dairy workers that are in the milking parlors," she said.

For a comparison to the milk, the scientists put H5N1 viruses in a saline solution, but surprisingly the pathogens did not survive for an hour in this liquid.  

"There is something about the milk that is making it [H5N1] more stable, and allowing it to survive for longer periods of time," Le Sage said. "But we don't know what component is causing this."

The scientists have not yet tested the virus's infectiousness over a longer period of time due to laboratory safety concerns. Nonetheless, the study emphasizes the need for dairy farmers to wear appropriate personal protective equipment when handling infected cows, Le Sage said. 

"Masks and face shields do their job. If workers were provided these and wore them, they should be protected," she said. The Centers for Disease Control and Prevention (CDC) has issued similar recommendations for dairy workers, but the use of such safety equipment has been limited. 

While personal protective equipment would help minimize the risk of catching the virus, "it's not 100%," said Jaydee Hanson, the policy director at the nonprofit Center for Food Safety. 

But "the good news is that if we do a proper job of pasteurizing the milk from these cows, it's safe," Hanson told Live Science. 

In general, the CDC considers the general public's risk of H5N1 to be low — the risk is highest for those who work closely with infected dairy animals. There's no evidence of the virus spreading between people.  

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An experimental two-in-one vaccine triggered strong immune responses against both the flu and SARS-CoV-2, the virus behind COVID-19, in an advanced clinical trial that included adults ages 50 and older.  

The ongoing clinical trial pits this new combination shot, called mRNA-1083, against several licensed vaccines that protect against either COVID-19 or the flu. Compared with these separate vaccines, the combo shot produced "significantly higher immune responses" to three subtypes of flu and to the coronavirus, the pharmaceutical company Moderna, which developed the vaccine, announced Monday (June 10).

These results haven't yet been reviewed by scientists outside of Moderna. The company plans to present the data at an upcoming medical conference and to publish the results in a scientific journal. 

Moderna representatives told news outlets that the company aspires to have the new shot approved by fall 2025 — so it won't be available by this flu season, but it might be by the next. Pfizer-BioNTech and Novavax are also working on their own COVID-flu vaccines, but Moderna is the first to release late-stage clinical trial data. 

Related: A branch of the flu family tree has died and won't be included in future US vaccines

People currently have to get their annual flu shots and updated COVID-19 vaccines separately. One major benefit of a combination vaccine would be that it would streamline that vaccination process, boosting coverage through convenience, Dr. Francesca Ceddia, Moderna's chief medical affairs officer, wrote in a Moderna blog post. Such a shot could also free up doctors' and pharmacists' time by cutting the number of individual shots they'd have to give, she wrote.

Beyond convenience, though, it's possible that the new vaccine could be more protective than the current vaccines that are given individually, Moderna's early data hint.

"When we think about the combination vaccine, we often only think about the element of convenience, one shot instead of two," Ceddia told CNN. "But what is really, really breakthrough is the fact that you not only offer that advantage, you also offer the proof of clinical benefit."

The combination vaccine includes two components, both made of a genetic molecule called mRNA. Once inside the human body, mRNA instructs human cells to build specific proteins — in this case, proteins found on the surface of SARS-CoV-2 and influenza viruses. The new vaccine builds upon the Nobel Prize-winning technology used to develop the first COVID-19 vaccines. Since those shots were released, mRNA has been used to develop experimental treatments and vaccines for many diseases, including deadly brain cancers and ultra-rare genetic disorders.

The ongoing trial is testing the new vaccine in two groups of roughly 4,000 adults each: one group of people ages 65 and up, and one group of 50- to 64-year-olds. Participants in the older group received either the new shot alone or Moderna's old COVID-19 vaccine, Spikevax, along with a high-dose flu shot that's recommended to older adults. Participants in the younger group got either the new vaccine or Spikevax and a standard-dose flu shot.

In both age groups, the combo vaccine elicited a superior immune response against a variant of SARS-CoV-2 called omicron XBB.1.5; two types of influenza A virus called H1N1 and H3N2; and a type of influenza B virus known as the Victoria lineage. The researchers measured this immune protection in terms of the number of protective antibodies that appeared in people's blood.

(Notably, newer versions of omicron have now overtaken XBB.1.5, so the new vaccine's effectiveness against those variants isn't yet known.)

The most common side effects of the new shot included pain at the injection site, fatigue, muscle aches and headaches. These side effects also show up with existing COVID-19 and flu vaccines. However, Moderna hasn't released the full details of the trial so it's not yet known whether the rate of side effects differs between the old and new vaccines. 

Between October 2023 and June 2024, up to 810,000 people were hospitalized for influenza in the U.S., and up to 71,000 people died from the infection, the CDC estimates based on preliminary data; firmer figures will be released in coming months. In addition, data recently presented to the Food and Drug Administration suggest that more than 500,000 were hospitalized for COVID-19, and 40,000 died from it, during last autumn and winter, CNN reported.  

This past season, the rate of flu vaccination far outpaced that for COVID-19 vaccination among adults, so some experts hope that introducing combination vaccines could help pull that rate up. For now, Moderna is working with regulators to approve the new vaccine for adults but the company hasn't noted when the shot might be tested in younger groups.

This article is for informational purposes only and is not meant to offer medical advice.

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A person in Mexico recently died after catching the first known human case of H5N2 bird flu.

The case marks the first time someone in Mexico has been sickened by any type of influenza A(H5) virus, a broad group of related bird flu viruses, the World Health Organization (WHO) reported June 5. 

Initially, authorities reported that the infection was a "confirmed fatal case" of H5N2. However, in an update issued June 14, they said that, although the man had tested positive for the flu virus around his time of death, he likely actually died of complications related to his underlying medical conditions.  

Specifics about those conditions haven't been released, but the investigators said that the patient's medical records pointed to this conclusion. Notably, the man had reportedly been "bedridden" for other reasons three weeks prior to developing any bird flu symptoms.  

Related: 'Increased evidence that we should be alert': H5N1 bird flu is adapting to mammals in 'new ways'

The case is still notable, though, because it demonstrates that H5N2 can spread to humans and cause disease. 

The broad category of virus that H5N2 belongs to also includes the one currently circulating among U.S. dairy cows, called H5N1. H5N1 recently sickened three people who had close contact with cattle. When it's caused sporadic infections in people, H5N1 has led to severe pneumonia and death in at least 50% of cases. In general, bird flu-related deaths have been reported in people with and without underlying medical conditions, the WHO says. 

The spread of bird flu to people is concerning, in part, because these viruses can sometimes be deadly. Another reason for concern is that the more times bird flu jumps into people, the more chances it has to pick up mutations and gain the ability to spread easily between humans. As of yet, no A(H5) viruses can sustainably spread from person to person, the WHO reported — but that could change in the future. 

For now, though, "based on available information, WHO assesses the current risk to the general population posed by this virus [H5N2] as low," the agency reported June 14. 

The recent case in Mexico involved a 59-year-old who developed fever, shortness of breath, diarrhea, nausea and general malaise in mid-April. The person sought medical treatment at a Mexico City hospital on April 24 but died the same day.

A sample of the patient's respiratory fluids, taken April 24, revealed he'd had H5N2. The virus is similar to H5N1, in that both viruses carry the same type of protein — H5 — on their surfaces, but a second protein called neuraminidase (N) is a slightly different shape on each virus. 

A close genetic analysis of the man's sample revealed that the virus was 99% similar to a strain of H5N2 detected this year in birds in Texcoco, Mexico. The strain is known to have low pathogenicity in birds, meaning it causes few to no symptoms in poultry. Highly pathogenic avian influenza viruses, in contrast, cause severe disease and death in poultry. 

Both highly and low pathogenic H5N2 viruses have caused outbreaks among birds in several Mexican states this year. 

According to the WHO, the man had had no recent contact with poultry or other animals; typically, people catch bird flu through contact with sick or dead animals, or from contact with materials those animals contaminated with the virus. As of June 14, authorities are still investigating the potential source of the patient's infection.

So far, none of the person's close contacts have tested positive for bird flu, and there's no evidence the virus has spread between people. "This case and this update do not change the current WHO recommendations on public health measures and surveillance of influenza," the agency reiterated in its June 14 statement.

Since people generally catch A(H5) viruses from animals or from handling those animals' bodily fluids, you can reduce the risk of bird flu by avoiding sick and dead animals and regularly washing your hands with soap and water. In the U.S., dairy workers have been advised to wear personal protective equipment around potentially sick animals, as an additional safety measure, and authorities have also emphasized that drinking raw milk could pose a risk to consumers. 

Again, the risk of transmission to the general public is low and overall these bird flu infections are rare. Should you catch an A(H5) virus, existing drugs for seasonal flu can help treat the infection.

Editor's note: This story was updated on June 14, 2024, following an update from the WHO that clarified the man likely died of comorbidities. The original story was published on June 6. 

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A third human case of bird flu has been linked to the ongoing outbreak in cows on U.S. dairy farms — and this one came with respiratory symptoms, such as cough, the Centers for Disease Control and Prevention (CDC) reported May 30.

Officials first became aware of bird flu spreading among U.S. dairy cows in March, and since then, the viral disease has been detected among cattle on farms in nine states. The type of bird flu spreading is known as H5N1, which has sporadically infected individual humans in the past but has never spread widely from person to person. However, these rare human infections can sometimes be fatal, and there's concern about the virus picking up the necessary mutations to spread easily through the populace.

Prior to the outbreak in cows, only one person in the U.S. had ever been infected with H5N1, as far as we know. Now, since the outbreak began, three people have likely been infected via exposure to sick cows. The first person infected by a cow was in Texas, and the second was in Michigan; both are dairy-farm workers and developed only eye infections from the virus before recovering.

The third, newly reported case linked to cow exposure was also in Michigan but on a different farm. The person was given treatment and is now isolating at home as their symptoms resolve. This infection is notable because it is the first human H5N1 case in the U.S. associated with respiratory symptoms. 

Related: H5N1: What to know about the bird flu cases in cows, goats and people 

"This is the first human case of H5 in the United States to report more typical symptoms of acute respiratory illness associated with influenza virus infection, including A(H5N1) viruses," the CDC reported. "The patient reported upper respiratory tract symptoms, including cough without fever, and eye discomfort with watery discharge." 

In humans, bird flu viruses, including H5N1, can cause both mild symptoms — including pink eye, cough, sore throat, runny nose and body aches — as well as severe ones, such as pneumonia, difficulty breathing and seizures. If severe, the infections can be fatal, but antiviral drugs available for seasonal flu can help treat the disease.

Finding another case of human H5N1 infection is "not surprising," given the extent of the spread of this virus in dairy cows, the CDC says. H5N1 was also already known to cause respiratory symptoms — however, seeing this is still concerning because, generally, symptoms like coughing and sneezing can make it easier for a virus to spread between people, The New York Times reported.

Each time the virus jumps to people, it gains an opportunity to pick up mutations that make it capable of spreading sustainably between humans — a chain of events that, in the worst scenario, could precipitate a pandemic. 

With that possibility in mind, the CDC continues to closely monitor for unusual flu activity across the country. So far, "there has been no sign of unusual influenza activity in people, including no increase in emergency room visits for influenza and no increase in laboratory detection of human influenza cases," the agency reported. 

Scientists are also looking to see if the virus has picked up any mutations that would raise the risk of spread, although thus far, there's no evidence of person-to-person transmission. 

Based on the available information, the risk of spread to the general public remains low, the CDC noted. As always, health authorities still recommend against drinking raw milk. Dairy-farm workers currently face the highest risk of exposure, the agency emphasized. 

Some experts have argued that federal officials should be screening many more workers for H5N1 and offering optional experimental bird flu vaccines, The New York Times reported. At this point, only about 40 farm workers have consented to being tested for H5N1, while a few hundred are being monitored for possible symptoms, according to news reports.

This article is for informational purposes only and is not meant to offer medical advice.

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The COVID-19 pandemic drastically reduced people's in-person interactions with others as events were canceled and people limited their excursions, and many also practiced social distancing and masked up if they did go out. These moves were intended to control the spread of the disease and were shown to help flatten the curve.  

However, some concerns have been raised about the potential impact of these actions on children's immune systems, and namely, kids' ability to fight infections. But did COVID "lockdowns" and other pandemic-related restrictions actually stunt kids' immune systems? 

On that front, parents should not fear, experts told Live Science. Social distancing did not permanently stunt children's immune systems. Rather, it just delayed young children's exposure to various germs.

"During the COVID lockdown, our children were exposed to far fewer pathogens than they typically are in a normal year," Dr. Sarah Nicholas, an assistant professor of pediatrics-allergy and immunology at Baylor College of Medicine in Texas, told Live Science. 

Related: Master regulator of inflammation found — and it's in the brain stem

Examples of common respiratory viruses that children may be exposed to include respiratory syncytial virus (RSV); the flu, or influenza; and human metapneumovirus, as well as various viruses that cause the common cold. After such an infection, the body builds up its immune defenses so that it can tackle these pathogens if it encounters the same ones again. 

Social distancing reduced not only the spread of COVID-19 but also that of other respiratory viruses, which tend to spread in similar ways. When these restrictions were lifted, the viruses were free to circulate in the population again. In addition, viruses responsible for causing other types of disease, such as viral gastroenteritis or stomach flu, also began spreading again. 

As social distancing behavior waned, toddlers who had never caught these viruses were suddenly playing "catch-up" with their immune systems, Dr. James Antoon, an assistant professor of pediatrics at Monroe Carell Jr. Children's Hospital at Vanderbilt in Tennessee, told Live Science. Consequently, many children became sick at the same time, leading to a surge in hospitalizations, he said. 

In the spring and summer of 2021, there was an unusual surge in RSV cases in the U.S.; the virus's transmission normally peaks in the winter. There was also a particularly large spike in cases of RSV between 2022 and 2023 that inundated many hospitals in the country. 

Although this resurgence was notable, several studies suggest that the severity of children's illnesses did not rise as COVID-19 restrictions relaxed. In other words, although case numbers rose, on a whole, children were no less able to combat these infections than kids their age had been before the pandemic. 

And while social distancing reduced children's exposure to germs, most children likely still underwent routine vaccination regimens during the pandemic, Dr. Janko Nikolich, a professor of immunobiology at the University of Arizona, told Live Science. These vaccines help prime children's immune systems against many major pathogens that are harmful to their age group. Thus, their immunity would be ready for action as social distancing eased. 

Notably, though, there was a dip in children's routine vaccinations during the pandemic, and many are still catching up to this day. 

"We know that with COVID and with flu that vaccination is absolutely the best way to prevent poor outcomes, hospitalization and complications from these viruses," Antoon said. "So if your child is eligible for some of these vaccines, it's advantageous to get them."

Parents should know that children's immune systems are generally robust, regardless of whether they lived through pandemic lockdowns, Dr. Dean Blumberg, chief of the Division of Pediatric Infectious Diseases at UC Davis Health, told Live Science. "The difference is that there were atypical patterns of exposure and infection" during and immediately after the pandemic, he said. 

"We're hopeful that kids are caught up now and going forward we shouldn't see these atypical patterns of infection," he said. 

Importantly, the pandemic highlighted the impact that basic public health measures can have in preventing the spread of harmful viruses, Antoon said. In daily practice, such measures can include staying home, social distancing and masking when you're sick. 

"Those are measures that are really easy to do and we can continue moving forward in a reasonable way," he said. 

This article is for informational purposes only and is not meant to offer medical advice.

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A second human case of bird flu has been linked to the ongoing outbreak in cows on U.S. dairy farms, the Centers for Disease Control and Prevention (CDC) reported Wednesday (May 22). 

A subtype of avian influenza called H5N1 was initially discovered in U.S. cows in March and has since been detected among cattle in nine states. The commercial milk and meat supply is safe to consume, although raw milk samples from sick cows have been found to carry the virus, so unpasteurized milk might pose a risk of infection to humans. 

However, no human infections with H5N1 have yet been tied to consuming raw milk; the two human cases flagged so far affected dairy workers employed by farms with H5N1-infected cows. The first possible case of cow-to-human transmission, reported in April in Texas, involved an individual whose only symptom was eye redness. The second case, just identified in Michigan, also involved only eye-related symptoms. 

The person infected in Michigan tested negative for bird flu when a swab sample was taken from their nose, but an eye swab from the patient tested positive.

Related: 32 diseases you can catch from animals

"It's not known exactly how eye infections result from avian influenza exposures," the CDC report notes. They may arise from contamination of the eyes "with a splash of contaminated fluid," such as milk, or else touching the eyes with a hand contaminated with the virus. 

The CDC is now analyzing the genetic makeup of the virus that infected the Michigan patient and will release those data soon. In the meantime, the CDC still considers the risk of bird flu spread to the general public to be low.

"Given the high levels of A(H5N1) virus in raw milk from infected cows, and the extent of the spread of this virus in dairy cows, similar additional human cases could be identified," the CDC says. "Sporadic human infections with no ongoing spread will not change the CDC risk assessment for the U.S. general public, which CDC considers to be low."

The agency has issued guidance for people working with H5N1-infected animals and those potentially exposed to the virus. For instance, people should avoid close, long or unprotected exposures to sick or dead animals, including wild birds, poultry and cows. They should also avoid unprotected exposure to the animals' poop, bedding, unpasteurized milk and materials that have been in close contact with animals with suspected or confirmed H5N1 infections.

H5N1 is known to be deadly to poultry, but in humans, the symptoms triggered by the virus can vary widely, from being completely absent to being fatal. Only three cases of human H5N1 infection have ever been reported in the U.S. and all have been very mild. Available drugs for the seasonal flu can treat the infection in people.

Initial genetic testing of the virus on farms has not indicated that the virus has picked up any mutations that would make it more likely to spread easily between people. However, the CDC is closely monitoring the situation with that possibility in mind. 

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website! 

Editor's note: This is a developing story and will be updated.

A bird flu virus called H5N1 recently infected animals that it had never been seen in before: goats and cows. Now, it's suspected that four people have picked up the infection after being exposed to infected cows, or potentially their bodily fluids or bedding.

Here's what we know so far about these unusual cases of H5N1 in the U.S.

Related: 21-year-old student dies of H5N1 bird flu in Vietnam

What is H5N1 bird flu?

H5N1 is a subtype of avian influenza A, a broad group of related bird flu viruses. It's categorized as a form of "highly pathogenic" avian influenza, or HPAI, because it causes severe and fatal disease in poultry. For example, H5N1 and other "H5" bird flu viruses can rapidly cause organ damage in chickens and kill up to 90% to 100% of chickens they infect, often within 48 hours, according to the Centers for Disease Control and Prevention (CDC). H5N1 has been responsible for the major bird flu outbreaks seen on U.S. poultry farms and on farms in other countries since 2022.

Although HPAI viruses are known for devastating domestic bird populations, H5N1 also infects wild birds and occasionally infects various mammals, including people. In non-avian species, it can still cause fatal disease, but some cases are mild or asymptomatic.

H5N1 infections in mammals, in general, raise concern that the virus could evolve to infect humans more easily and perhaps spread widely between people, sparking a pandemic, according to the World Health Organization (WHO). But scientists haven't seen any changes in the virus that suggest that's happening at this point.

Which animals can H5N1 infect?

H5N1 most often infects domestic and wild birds, although some wild birds act as "reservoirs" for the virus, meaning they can carry and spread it without getting sick. These carriers include waterfowl, such as ducks, geese and swans; and shorebirds, such as plovers and sandpipers, the CDC says.

In the past 20 years or so, H5N1 has also been detected in at least 48 mammal species across 26 countries. These include foxes, bears, seals, sea lions and polar bears, as well as pet cats and dogs and farmed mink. The U.S. Department of Agriculture (USDA) maintains a map of where different mammals have been infected in the U.S.

This year, for the first time, H5N1 was detected in goats and cows. This marks the first time H5N1 has been found in ruminants, which include hooved, cud-chewing mammals like cows, goats and sheep.

The infections among goats were reported in mid-March on a Minnesota farm where ducks and chickens had previously tested positive for the virus. The first infections in cattle were reported a week later among dairy cows in Texas and Kansas, which also likely got the virus from birds.

Related: 'Novel' swine flu virus sickens Pennsylvania child in 1st case of the year

Where has H5N1 been detected in cows?

After being detected in cows in Texas and Kansas, H5N1 infections were also confirmed in cows in Idaho, Michigan and New Mexico, according to the USDA's Animal and Plant Health Inspection Service. As of June 14, the USDA has reported that 92 dairy cow herds in 12 U.S. states have confirmed cases of infection. 

Early data suggest that the specific strain of H5N1 infecting cows matches one introduced to the U.S. by wild birds — known as clade 2.3.4.4b — back in 2021. Before that, the strain had spread mostly in Europe, Asia and Africa.

"[H5N1] could have been infecting dairy cattle a year ago. We just never thought about looking … for it," Dr. David Swayne, a bird flu expert who is now a private consultant after having worked for the USDA, told STAT. Experimental data published in 2008 suggested that at least one H5N1 strain can infect baby cows, so it's not totally unexpected to see the virus in cattle.

The recently infected cows have been reported to produce less milk, show poor appetite, develop cold-like symptoms and become lethargic. There have been no reported deaths among the cows.  

Can H5N1 infect people, and how dangerous is it?

H5N1 can occasionally infect people and can lead to death, with about 50% to 60% of reported cases resulting in fatal disease. More than 880 human cases of H5N1 have been reported worldwide since the late 1990s, according to the CDC.

Most often, human H5N1 infections occur after a person had close or lengthy unprotected contact — without gloves, a face mask or eye protection — with infected birds or with objects contaminated with sick birds' saliva, mucus or poop. These cases have not been tied to sustained human-to-human spread of the virus, but there's been some evidence of very limited spread between people.

Although H5N1 infections can be fatal, some people show no symptoms or only mild symptoms. Mild symptoms might include eye infections, such as conjunctivitis (pink eye), and upper respiratory symptoms, like sneezing and coughing. Severe cases can lead to life-threatening pneumonia.

Bird flu can be treated with antiviral drugs used for seasonal flu. The CDC also has several candidate vaccines prepared for manufacturing, just in case this or a closely related virus were to suddenly spread between people; seasonal flu vaccines do not protect against H5N1.

There have been five human cases of H5N1 in the U.S. The first was in 2022, in a person in Colorado who'd had direct exposure to infected poultry. It's possible that this case wasn't a full-blown infection but rather remnants of the virus being detected in the person's nasal passages. 

The second case was in April 2024, when a person in Texas caught the virus after likely being exposed to infected cows; this person's only reported symptom was pink eye. The third was reported in Michigan in May 2024, also in connection with infected cows on a farm. This third individual also only showed eye-related symptoms. The fourth case was reported later in May 2024, when a worker at a different Michigan dairy farm developed mild respiratory symptoms, including cough. The fifth was reported in July 2024, in a dairy worker in Colorado who developed only eye symptoms.

Related: 1st-known 'highly pathogenic' bird flu cases in Antarctic could threaten penguins

Should you worry about H5N1 in cows?

Finding H5N1 in cows is interesting to scientists because it hasn't been seen before, but as of now, it poses a low risk to the general public.

As for cows, there's some early hints that the virus can spread among cattle — the cows in Idaho reportedly became sick after being exposed to cows shipped from Texas, Science reported, and the Michigan cases were also thought to be tied to cows from Texas, the CDC reported. However, H5N1's ability to spread among cows still needs to be confirmed.

In regards to people, "initial testing has not found changes to the [H5N1] virus that would make it more transmissible to humans," the USDA noted in an April statement. "While cases among humans in direct contact with infected animals are possible, this indicates that the current risk to the public remains low."

The CDC also reported that there have been no changes to the virus that would make it less vulnerable to Food and Drug Administration (FDA)-approved antiviral drugs for the flu.

Nevertheless, the CDC recommends that people avoid sick or dead animal carcasses, including wild birds, poultry and cattle. They should also avoid raw milk, poop, litter or other materials contaminated by animals with suspected H5N1 infections, and they should avoid uncooked and undercooked food products from such animals.

Is the milk and beef supply safe?

The discovery of H5N1 in cows does not threaten the country's milk supply, the USDA noted. Dairies are required to send only milk from healthy animals into processing for human consumption, and the commercial milk supply is pasteurized before going to market, which kills viruses. Unpasteurized milk always carries a risk of causing dangerous infections, but the likelihood of H5N1 infection from raw milk is not yet known.

Related: H5N1 bird flu can remain infectious in raw milk for at least an hour, study finds

Since March 25, shortly after news of the cow infections came to light, weekly sales of raw milk have increased by as much as 21% compared with previous years. But experts have warned that consuming raw milk could expose individuals to dangerous pathogens, alongside increasing the likelihood of H5N1 gaining the necessary mutations to jump to and spread between humans. 

Notably, there isn't yet enough evidence to know whether drinking raw milk contaminated with H5N1 could result in a human infection.

More information on milk safety given the current situation can be found on the FDA's website.

So far, H5N1 has been found in dairy cows, but the USDA also emphasized that the beef supply is safe and that cooking meat properly kills viruses.

Editor's note: This article was updated on July 5 to note an additional human case of H5N1. The original article was posted on April 4.

This article is for informational purposes only and is not meant to offer medical advice.

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A person in Texas has tested positive for bird flu after being in contact with dairy cows that were likely infected with the virus, health officials announced on Monday (April 1).

The individual's only reported symptom was eye redness, likely caused by an infection of the eye called conjunctivitis. The person is now isolating and being treated with an antiviral drug for influenza, according to the Centers for Disease Control and Prevention (CDC).

This is the second reported case of a human in the U.S. being infected with a type of "highly pathogenic" avian influenza virus called A(H5N1) and the first person to likely be infected by cows. The first human case in the U.S. was in 2022 and involved a prisoner in Colorado who had been culling infected poultry at a commercial farm as part of a pre-release employment program.

H5N1 is a subtype of highly pathogenic avian influenza (HPAI), a type of flu that can cause severe disease and death in birds. The virus causes outbreaks in both poultry and wild birds but can also spread to mammals. In the past few years, the virus has been reported in more than 48 mammal species, including foxes, skunks, raccoons, seals and polar bears.

Now, the virus has made its way to cows. The announcement of a person likely catching H5N1 from cattle comes one week after dairy cows in Texas and Kansas tested positive for the virus, and two weeks after the disease was found in goats for the first time. H5N1 has since been detected in cows in New Mexico, Michigan and Idaho.

Health officials say the risk of humans being infected remains low and that, as milk is pasteurized before being sold, dairy products remain safe. The meat supply is also safe, and cooking meat to safe internal temperatures kills viruses like influenza.

Nonetheless, the spread of H5N1 among mammal species is concerning scientists, especially if it's a sign that the virus is now capable of mammal-to-mammal transmission.

"Mammal-to-mammal transmission has been suspected for a while, but not confirmed. If sequencing confirms this, it would be a pretty worrying step in the evolution of the virus," Deepti Gurdasani, a clinical epidemiologist and statistical geneticist at the Queen Mary University of London, wrote on X, formerly known as Twitter. "If mammal-to-mammal transmission can occur, that raises the possibility of the potential of human to human transmission."

About 860 human cases of H5N1 have previously been reported worldwide. The mortality rate is around 60%, according to the World Health Organization (WHO), although the CDC notes some cases cause no or few symptoms. Symptoms can include a cold, cough and fever, and the infections are typically treated with the same antivirals used for seasonal flu.

This article is for informational purposes only and is not meant to offer medical advice.

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A student in Vietnam has died of H5N1 bird flu, according to the country's Department of Preventive Medicine.

The 21-year-old man developed symptoms of fever and a cough on March 11. A week later, he presented at Ninh Hoa Medical Center in Vietnam's eastern Khanh Hoa province, where he was diagnosed with pneumonia and transferred to Khanh Hoa General Hospital.

The student tested positive for highly pathogenic avian influenza on March 20, and further tests conducted two days later at the Nha Trang Pasteur Institute showed the patient was infected with the H5N1 subtype. 

This strain of bird flu is known to cause deadly outbreaks in wild and domesticated birds and to occasionally infect people. 

"Due to the serious progression of the disease, the patient died on March 23," representatives of the Department of Health of Tay Ninh Province wrote in a translated statement.

An investigation revealed the student, who was enrolled at Nha Trang University, had gone wild bird trapping before and after the Lunar New Year on Feb. 10. However, no sick or dead birds have been reported in the area where he went trapping, close to Ninh Hoa town.

Related: Bird flu wipes out over 95% of southern elephant seal pups in 'catastrophic' mass death

The student lived with his family in a village near Ninh Hoa. "Cases of close contact with patients are listed and their health monitored daily," representatives wrote in the statement. "To date, no new cases have been detected."

Since 2003, 128 people have been infected with H5N1 bird flu in Vietnam, according to the statement, and 65 of these cases resulted in death. Worldwide, there have been more than 880 sporadic human infections across 23 countries in the same time period, according to the Centers for Disease Control and Prevention (CDC). 

There have been six outbreaks of the virus in poultry flocks in Vietnam since the beginning of 2024, with the country's Ministry of Health warning of the risk of transmission from birds to humans in December 2023.

Avian influenza can jump from birds to mammals, including humans. The virus has been reported in close to 50 mammal species — including foxes, seals and polar bears — and has recently been detected in cattle for the first time. There are no known cases of bird flu being passed on from a mammal to a human or between humans, and the risk of infection is limited to people who are in direct contact with infected animals.

Dairy cows on farms in Texas and Kansas have tested positive for bird flu in the first known cases in cows in the U.S., and likely the world. Cows in New Mexico are also thought to have been infected, but they have not yet been tested.

Three weeks ago, the cows became sick with a cold-like illness, The Associated Press reported. The animals were producing less milk than usual, their appetites decreased and they appeared lethargic. 

Unpasteurized milk samples and throat and nose swabs showed that the cows were infected with the H5N1 strain of highly pathogenic avian influenza, the Texas Animal Health Commission announced Tuesday (March 26). This strain is known to cause outbreaks in wild and domesticated birds and to occasionally infect people. 

The announcement comes a week after the first U.S. cases of bird flu in goats (Capra hircus) were reported. The goats were sharing a pasture and water source with ducks and chickens that were infected with bird flu on a backyard farm in Stevens County, Minnesota, according to a report by the World Organisation for Animal Health.

The spread of bird flu to goats "highlights the possibility of the virus infecting other animals on farms with multiple species," Brian Hoefs, the Minnesota state veterinarian and executive director of the Minnesota Board of Animal Health, said in a statement released March 20.

Related: Bird flu wipes out over 95% of southern elephant seal pups in 'catastrophic' mass death

Based on the new results from Texas, officials think the dairy cows were likely infected by wild birds. The tests did not detect any changes to the bird flu virus that would make it more transmissible to humans, the AP reported.

In outbreaks of bird flu in poultry, officials have had to cull entire flocks to control the spread of the virus. By contrast, the cows that tested positive for bird flu appeared to recover without treatment within seven to 10 days, experts told the AP. 

The virus infected about 10% of lactating dairy cows in the affected herds. As such, the situation "doesn't look anything like the high-path influenza in bird flocks," Michael Payne, a food animal veterinarian and biosecurity expert at the University of California, Davis, told the news agency. 

Only milk from healthy animals on the affected dairy farms will enter the food supply. The U.S. Department of Agriculture considers the risk to people low and noted in a statement that pasteurization, which is required for milk sold through interstate commerce, kills viruses and bacteria. 

So far, H5N1 bird flu has been reported in 48 mammal species, including foxes, skunks, raccoons, seals and polar bears. "It was probably only a matter of time before avian influenza made its way to ruminants," which include hooved, cud-chewing mammals like cattle, goats and sheep, Payne said.

There are no reported cases of bird flu being transmitted from nonhuman mammals to humans in the U.S. to date, according to the Minnesota Board of Animal Health. There have been rare cases of the infection spreading from birds to people. Any risk of infection is limited to people who have direct contact with infected animals, experts told the AP.

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A type of flu virus that used to sicken people every year hasn't been spotted anywhere on Earth since March 2020. As such, experts have advised that the apparently extinct viruses be removed from next year's flu vaccines.

The now-extinct viruses were a branch of the influenza B family tree known as the Yamagata lineage. Scientists first reported the apparent disappearance of Yamagata viruses in 2021. At that time, experts speculated that precautions taken to stop the spread of COVID-19 — such as masking and social distancing — had not only driven the overall number of flu cases to historic lows but may have completely snuffed out this type of flu virus. 

Now, according to news reports, a panel of advisers to the Food and Drug Administration (FDA) has unanimously agreed that Yamagata viruses should be dropped from the flu shot formulation for the 2024-2025 flu season. For the past decade, U.S. flu vaccines have protected against four types of flu — two influenza A strains and two influenza B strains — but that number will now fall to three.

The advisory committee has been pushing for this change for some time, STAT reported, and in fall 2023, the World Health Organization raised the same recommendation on a global scale. 

Related: Is it too late to get a flu shot?

Dropping Yamagata from flu shot formulations could help boost manufacturers' production capacity, so they can make more doses. Plus, it would eliminate any potential risks associated with growing the virus in a lab — a process currently required to make flu vaccines, CNN reported. 

And at baseline, experts emphasized that people needn't be vaccinated for something that appears to be extinct, STAT reported. 

"We don't want to vaccinate you for a virus that's no longer in circulation for three, four years now," Dr. Hana El Sahly, a professor of molecular virology and microbiology at Baylor College of Medicine and chair of the FDA advisory committee, said at a meeting Tuesday (March 5). 

"We've been talking about this for four years," Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia and a member of the FDA advisory committee, told CNN. 

Despite the consensus among health officials, leaders in the pharmaceutical industry argued that manufacturers would need more time to switch to a trivalent formulation. Making the switch requires manufacturers to clear various regulatory hurdles. But with the FDA advisers pushing ahead, manufacturers are prepared to make trivalent vaccines for the U.S. starting this upcoming season; other countries will likely follow later, in accordance with their regulatory policies, STAT reported.

According to CNN, Jerry Weir, director of the FDA's Division of Viral Products, confirmed at the meeting Tuesday that all U.S. flu shot manufacturers have submitted the required regulatory paperwork and should be on track to make trivalent vaccines next season. 

As these changes take effect, scientists around the world will continue to watch out for Yamagata alongside other flu viruses that infect people.

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Scientists have uncovered antibodies that target the "dark side" of the flu virus.

Influenza viruses have a mushroom-shaped protein known as neuraminidase (NA) that is said to have a "dark side" because the structure beneath its mushroom cap has been largely unexplored by science. Antibodies that latch onto this dark side could help form the basis for antiviral drugs and vaccines that work against many flu viruses, researchers wrote in a paper published Friday (March 1) in the journal Immunity.

Currently, flu vaccines are designed to target a different structure on the surface of flu viruses: hemagglutinin (HA). This lollipop-shaped protein lets the viruses latch onto the outside of human cells and then infiltrate them. But it mutates rapidly, which is why the flu shot must be updated every year to match the HAs of circulating flu strains.

Related: Never-before-seen antibodies can target many flu viruses

By comparison, the dark side of NA doesn't mutate nearly as fast and it looks very similar in different strains of flu virus, according to a statement from the National Institutes of Health (NIH).

NA is thought to help flu viruses navigate to their preferred receptor on the outside of host cells. Then, once a virus has infected a cell and multiplied inside it, NA helps those new viruses exit the cell. Several antiviral drugs for the flu, including Tamiflu (generic name oseltamivir), actually work by inhibiting NA and thus preventing flu viruses from escaping cells they've infected. Mutations that tweak the structure of NA can therefore make viruses less vulnerable, or resistant to such drugs.

Scientists have only ever discovered a handful of human antibodies against NA, and these generally latch onto the top or the side of the protein's mushroom cap, the researchers noted in their report. These parts of the mushroom are more prone to mutating in ways that help flu viruses dodge the effects of antiviral drugs, they added.

In their new study, the NIH scientists analyzed blood drawn from two people who'd been infected with H3N2, a subtype of influenza A virus that spreads seasonally and mutates especially quickly. In the blood samples, the team identified six antibodies that latch onto the dark side of NA.

In laboratory tests, these antibodies latched onto a number of different H3N2 viruses and slowed their replication. The antibodies also worked against a different type of influenza A, called H2N2.

In experiments with mice, the antibodies saved many rodents from a lethal dose of H3N2 virus, which hints they could be useful for the prevention and treatment of influenza in people. The antibodies showed strong protection both when they were given to mice prior to infection and when they were given afterward. The team also tested how the antibodies fared against some drug-resistant strains of flu and found they still showed the same degree of protection.

To better understand how these antibodies work, the researchers used cryogenic electron microscopy (cryo-EM), a microscopy technique that uses beams of electrons to reveal the detailed 3D structures of molecules. The team used this to zoom in on two of the antibodies as they grabbed hold of NA. The experiment showed that the two antibodies target different parts of the dark side that don't overlap.

This suggests multiple features of the dark side could be targeted with future medicines or vaccines, and it opens up the possibility to take aim at multiple regions at once, for instance.

"Our findings help guide the development of effective countermeasures against ever-changing influenza viruses by identifying hidden conserved sites of vulnerability on the NA underside," the study authors wrote.

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A newly discovered class of antibodies in human blood can neutralize different types of the flu virus and could be key to the development of broadly protective vaccines against the seasonal viruses, scientists say.  

Circulating flu viruses constantly mutate, so "we need annual influenza virus vaccines to keep pace with continuing viral evolution," the researchers behind the discovery said in a statement. "Our work suggests that the barriers to eliciting more broadly protective immunity may be surprisingly low," they said. 

There are four types of flu virus, known as influenza A, B, C and D, with A and B being responsible for the seasonal flu epidemics in the U.S. every year. 

Influenza A comes in many subtypes whose differences lie in two proteins that the virus uses to infect our cells: hemagglutinin (H) and neuraminidase (N). For example, H1N1 and H3N2 are subtypes of influenza A that routinely infect people. 

Within each subtype are different "strains" that constantly tweak their genetic code. For example, a strain of H1N1 is currently the dominant virus causing flu in the U.S. Influenza B, meanwhile, is divided into two lineages — Yamagata and Victoria — and is typically responsible for a much smaller proportion of flu cases.

Making effective flu shots relies on harnessing the protective power of antibodies — immune proteins that attack invading pathogens — but the virus' ability to quickly mutate makes this challenging. Flu vaccines prime the immune system to produce specific antibodies that latch onto a flu virus and prevent it from infecting cells after it invades the body. However, these vaccines are formulated to target specific strains, and because those strains mutate year over year, people then need a new flu shot each year to keep up. 

Related: 'Long flu' is real, and we've likely 'ignored it for a long time'

In the new study, published Thursday (Dec. 21) in the journal PLOS Biology, scientists described a newfound class of antibodies in human blood samples that target multiple forms of the influenza A virus. 

The research was conducted only in the lab, so the scientists aren't sure exactly how these antibodies contribute to the body's flu shot response. However, one day, these antibodies could be used to develop vaccines that are more effective at protecting people from multiple strains of flu at the same time. 

To guard against influenza A, conventional flu vaccines usually prompt the immune system to produce antibodies against the H protein on the surface of the virus. Antibodies have previously been discovered that target two main types of hemagglutinin, called H1 and H3, at the same time. However, they can only do this if there is a specific mutation in H1, namely the insertion of an amino acid in the outer edge of the protein that binds to a receptor on the outside of our cells. This consequently limits the antibodies' efficacy against different flavors of flu virus. 

Through lab experiments, the study authors identified antibodies that are abundant in human blood and can bind to certain H1 and H3 strains of influenza A, whether or not this hemagglutinin mutation is present. This means that they'd theoretically be able to provide broad protection against both subtypes of virus, potentially even as circulating strains mutate over time. 

The authors also looked at how well these antibodies targeted strains of H1 and H3 that have circulated in the past. The antibodies reacted with H3 strains from the late 1980s to late 1990s and H1 strains from the early 2000s through to 2015. 

This suggests the patients whose blood was sampled originally made the antibodies in response to H3 strains of the virus. Then, after a later exposure to H1 strains at a later date through either infection or vaccination, the antibodies became primed to target H1 as well. 

These findings may have important applications for future vaccine design. 

"Given the right series of influenza virus exposures/vaccinations, it is possible for humans to mount robust antibody responses that neutralize divergent H1N1 and H3N2 viruses, opening new avenues to design improved vaccines," the authors said in the statement. 

In other words, there may be a way to ensure vaccines trigger the production of these broad-acting antibodies, to ensure the shots guard against both subtypes of the virus equally well.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

People hospitalized for either COVID-19 or the flu face a heightened risk of medical issues, hospital admission and death in the 18 months after they're discharged.

That's according to a new study, published Thursday (Dec. 14) in the journal The Lancet Infectious Diseases, that compared the long-term outcomes for patients sickened with either viral infection. 

Awareness around long COVID — long-term symptoms that persist or newly emerge months to years after an initial COVID-19 infection — has steadily grown since the coronavirus SARS-CoV-2 first appeared. But the new study highlights another significant public health burden: what the study authors informally call "long flu."

Historically, people have thought of viral illnesses like influenza as mostly transient, or "acute," sicknesses. But that idea doesn't account for the impacts that sometimes linger in the "post-acute" phase, said senior study author Dr. Ziyad Al-Aly, a clinical epidemiologist at Washington University in St. Louis and chief of the research and education service at the Veterans Affairs (VA) Saint Louis Health Care System. 

"I think we've ignored this for a long time, and the COVID-19 pandemic kind of put it in the spotlight," Al-Aly told Live Science. As seen with COVID-19, "flu can lead to long-term problems," he said.

Related: When should you get a flu shot? What to know for the 2023-2024 flu season

However, when you compare COVID-19 and the flu head-to-head, the long-term risks tied to a COVID-19 hospitalization are more numerous and more severe than those tied to flu, Al-Aly emphasized, although the flu has more profound effects on the respiratory system, specifically.

"What we saw is that people with COVID-19 had a higher risk of death, hospitalization and sequelae [knock-on effects of disease] than people with the flu," he said. Therefore, "COVID remains a much more serious problem than seasonal influenza." 

For the new study, Al-Aly and colleagues drew information from VA databases to identify people who'd been hospitalized for either COVID-19 or the flu. They included more than 81,000 people who'd been hospitalized for COVID-19 between March 2020 and June 2022, as well as nearly 11,000 people who'd been hospitalized for seasonal flu between October 2015 and February 2019. (The rate of flu hospitalizations was very low between 2020 and 2022, due to precautions taken to reduce COVID-19 spread, so those years weren't included.)

Previous studies have compared the lingering effects of COVID-19 and flu out to about six months post-hospitalization — but the researchers wanted to look further, out to 18 months. 

They looked at the rates of death, hospital admission and intensive care unit (ICU) admission among the study participants, as well as the rates of 94 specific health outcomes related to 10 organ systems. For example, chest pain caused by poor blood flow to the heart (angina) was related to the cardiovascular system, while shortness of breath was related to the respiratory system.

The analysis revealed that the rates of death, medical care and poor health outcomes were "high in both those admitted to hospital for COVID-19 and those admitted to hospital for seasonal influenza," the authors reported. But overall, the risks were substantially higher for those hospitalized with COVID-19.

For every 100 people, the COVID-19 group saw about eight to nine additional deaths within the follow-up period compared with the flu group. Similarly, they experienced about 20 more hospital admissions and eight more ICU admissions per 100 people, compared with the flu group. 

Both COVID-19 and flu came with a risk of various health problems, but COVID-19 affected far more organ systems. COVID-19 was linked to a risk of 64 of the 94 studied symptoms and health conditions, hitting the cardiovascular, neurological and gastrointestinal systems, and more. Influenza was tied to just six: angina, fast heart rate (tachycardia), type 1 diabetes, cough, low blood oxygen and shortness of breath. 

The latter three symptoms tied to flu are related to the respiratory system, and in fact, the flu carried a higher risk of respiratory effects than COVID-19 through the entire follow-up period. This highlights that "flu is more of a respiratory virus, but COVID is more of a multisystemic illness," Al-Aly said.

For all organ systems except gastrointestinal, more than 50% of the health effects occurred more than one month after hospital discharge. This was true for both flu and COVID-19 and highlights the lasting impact both infections can have in the post-acute phase.  

The new study has several limitations. A major one is that it focused only on people hospitalized for either disease. "This is really a major limitation, that we studied it in this context," so more work is needed to understand how the long-term impacts of milder cases of flu and COVID-19 compare to one another, Al-Aly said. In addition, the VA data were heavily skewed toward older male patients, so there are remaining questions as to how these health burdens affect people of other ages and sexes.

With the data at hand, Al-Aly said he hopes the work raises both patients' and health care providers' awareness of the lasting impacts of these diseases.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website! 

Respiratory infections like the common cold often start with a stuffy nose, and this minor ailment is sometimes followed by pain and tenderness around the cheeks, eyes and forehead — a telltale symptom of blocked sinuses.

But what do clogged sinuses actually look like?

To paint this mucus-filled picture, let's first discuss what the sinuses are and how they function in the body.

"It is easiest to think of the nasal passageway as a main highway, with the sinuses as rest stops," Dr. John Fornadley, an ear, nose and throat specialist and a clinical associate professor of surgery at Penn State, told Live Science by email.

Related: Why don't we breathe equally out of both nostrils?

The sinuses are four pairs of small, air-filled cavities lined with a layer of mucus. Two pairs of sinuses are located inside facial bones — one pair in the cheekbones, or "maxillary" bones, and another pair in the frontal bone just above the eyes. The other two pairs of sinuses are located within skull bones that sit just behind the nose, according to the medical resource StatPearls. All the sinus cavities are connected to the main nasal passageway through a network of narrow tunnels, and they also connect to one another, Fornadley said.

This unique structure helps sinuses fulfill a number of important roles in the body. For instance, the hollow cavities reduce the weight of the head and help cushion the brain from potential damage that could be caused by traumatic injuries to the face, according to StatPearls. They also vibrate as we speak, which boosts the resonance, or deep quality, of our voices; this is why people with clogged sinuses may speak with a flat, nasal voice.

Of course, one of the most important functions of sinuses relates to breathing. When you inhale air through your nose or mouth, it passes through the sinuses and begins to warm to body temperature. The tissues of the sinuses have an ample blood supply that helps warm up the air, and these tissues also produce mucus that helps to moisturize the air and clear it of pollutants, such as dust particles and bacteria, according to StatPearls. This prevents cold, dry air from harming the lungs and also helps oxygen pass from the lungs into the blood.

The sinuses drain any excess mucus through the passages that connect to the nasal cavity. Blocked sinus symptoms can start when these cavities don't drain properly. Nasal congestion, which causes the tissues in the nose to swell and leads to rapid mucus buildup, is one of the main culprits that can block these openings and stop the normal drainage of mucus from the sinus cavities.

"Nasal obstruction can cause sinus congestion the same way a clogged sewer pipe can make the bathtub slow to drain," Fornadley said. So when your sinuses feel clogged, you can imagine mucus pooling around these openings and trickling out very slowly.

When mucus gets trapped in the sinuses, it often leads to sinusitis, a condition in which the sinus lining gets swollen and inflamed, according to the American College of Allergy, Asthma & Immunology. Sinusitis causes pain and tenderness around the cheeks, eyes and forehead, as well as additional symptoms such as jaw pain, fever, coughing and fatigue.

Sinusitis brought on by respiratory viruses cannot be treated with antibiotics, but some symptoms can be managed with decongestants. This type of medication works by reducing swelling in the blood vessels and tissues of the nose, which helps reopen up the sinus passages, according to the Cleveland Clinic. (Be wary, though, as some popular decongestants have recently been found not to work.)

This article is for informational purposes only and is not meant to offer medical advice.

The common cold can leave you with a stuffy nose, sore throat and phlegmy cough, but along with these main symptoms, you may also struggle to hear properly.

So why does your sense of hearing falter when you have a cold or another respiratory infection?

The answer lies in the Eustachian tube, a narrow canal that connects your ear with the back of your nose, Dr. John Fornadley, a Pennsylvania-based ear, nose and throat specialist and a clinical associate professor of surgery at Penn State, told Live Science by email.

The hollow Eustachian tube is typically closed, but it opens when we swallow or yawn, according to Stanford Medicine. It helps to drain excess mucus made by the lining of the middle ear into the nose and throat, and it's also critical to our sense of hearing.

Related: How long do cold symptoms last?

"Our eardrums vibrate in response to sound, and they transmit this vibration to the inner ear," Fornadley said. "To do this effectively, the eardrum, like the musical instrument drum, requires air behind the drum to have the same air pressure as air on the outside of the eardrum for normal vibration." The Eustachian tube connects the space behind the eardrum, known as the middle ear, to the nose in order to equalize the air pressure in the middle ear with that of the outside world, he explained. 

When there is a significant difference in air pressure, the Eustachian tube opens rapidly — this is the sensation often referred to as "popping" the ears. And if the tube doesn't pop open on its own, you can force it to by pinching your nose and then blowing air into the ear. 

"We are more aware of the ears popping when traveling by air, or going up a mountain quickly in a vehicle," Fornadley said. "The process of opening the Eustachian tube remains important, just more subtle, when living at sea level."  

When cold or other respiratory viruses infect you, they trigger an immune response. To combat the infection, the nose and sinuses rapidly increase their production of mucus and swell as the tissues become inflamed, creating the sensation of a stuffy nose. This extra fluid can then spread to the ear through the Eustachian tube and blunt the vibration of the eardrum as a result. 

Related: How does water get stuck in your ear — and how do you get it out? 

"For this reason, nasal congestion will temporarily decrease hearing," Fornadley said. 

In addition, the resulting buildup of mucus in the Eustachian tube can sometimes cause a middle-ear infection, worsening the hearing problems. This common complication of colds is particularly prevalent in babies and toddlers. An estimated 5 in 6 children will get at least one ear infection by their third birthday, according to the National Institute of Deafness and Other Communication Disorders. Compared with those of adults, children's Eustachian tubes are smaller and more horizontal, rather than set on a steep angle, making it more difficult for them to drain fluids out of the ear.  

Middle-ear infections can temporarily decrease the perceived volume of sounds by up to 40 decibels, producing a similar sensation to wearing a pair of ear plugs, according to a 2022 review published in the journal Cureus. Recurring or untreated infections of the middle ear can eventually cause a perforation of the eardrum and, in the most severe cases, permanent hearing loss, the review notes.

But thankfully, the mild hearing loss that people experience during a cold usually goes away on its own once the illness resolves.

This article is for informational purposes only and is not meant to offer medical advice.