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Flu shot facts & side effects

Here's a look at the science of flu shots. Shown here, a vial and syringe.
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The seasonal flu shot is a yearly vaccine administered to protect against the flu, or influenza. In the United States, flu shots are recommended for everyone ages 6 months and older, according to the Centers for Disease Control and Prevention (CDC). 

The flu can be a very serious illness, especially in young children, adults ages 65 and over, those with underlying health conditions, and pregnant women. The flu shot is the best way to protect yourself and family from the flu, the CDC says.

Strains of the flu virus are constantly changing, so a new flu vaccine is made each year. Scientists make the vaccine before the flu season starts by predicting which flu strains are likely to be the most common during the upcoming season.

"Since the flu virus frequently drifts in its genetic composition, you have to reformulate the vaccine, and this is one of the reasons that people have to [get a flu shot] on an annual basis," said Dr. William Schaffner, a preventive medicine and infectious disease expert at Vanderbilt University School of Medicine.

When should you get a flu shot?

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Exactly when the flu season starts and ends is unpredictable, so health officials recommend that people get their flu shot in early fall, preferably by the end of October, the CDC says. The same recommendation applies this year as the COVID-19 pandemic continues. Flu activity typically peaks in January or February.

"We'd like to get as many people protected against influenza before influenza becomes active in communities across the country," Schaffner said.

Most flu vaccines are given before Thanksgiving, Schaffner said, but people can still get their shot throughout the winter months. Each season's flu shot expires in June of that year, but Schaffner said that he would consider it "too late" to get a flu vaccine after March, unless a person is traveling to the Southern Hemisphere (where the flu season will then be starting).

After vaccination, it takes a person about two weeks to build up immunity against the flu.

People can visit the CDC's to find flu shot locations.

Who should NOT get a flu vaccine?

Children younger than 6 months cannot get a flu shot. Those who have had a severe allergic reaction to a flu vaccine in the past should not get that type of flu shot again, and they should speak with their health care provider about whether they can receive another type of flu shot, the CDC says. 

Similarly, people who have had a life-threatening reaction to ingredients in flu vaccines besides egg proteins shouldn't get flu vaccines with those ingredients; and they should speak with their health care provider about whether there is a flu vaccine that's right for them, the CDC says. These other ingredients might include gelatin or antibiotics, which are added to some flu shots to prevent bacterial contamination during manufacturing. (Not all flu shots contain antibiotics.)

People with egg allergies can still receive any type of flu shot that's recommended for their age group, even if the flu shot is made with egg-based technology (and thus contains a small amount of egg protein), the CDC says. Studies have found that people with egg allergies are very unlikely to experience a severe reaction to flu vaccines. People who've had a severe allergic reaction to eggs should get their flu shot under the supervision of a health care provider who can treat severe allergic reactions, the CDC says. In addition, several types of flu shots are egg-free, including recombinant flu vaccines and cell-based flu vaccines. 

You should not get the flu vaccine if you have a high fever; you should wait until the fever is gone. However, if you have minor illness, like a mild cold or a headache, you can still get a flu shot, Schaffner said. "The vaccine does perfectly well in those folks."

But people who have COVID-19 should not get a flu shot until they have met the criteria to discontinue isolation, according to the CDC. Although people generally can get a flu shot if they have a mild illness, they should not go get a flu shot while they could be contagious with COVID-19 to avoid exposing health care workers and other patients to the virus, the CDC says.

Flu shot side effects

A person holding their arm after a flu shot.

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According to the CDC, mild side effects from the flu shot include soreness, redness or swelling at the injection site, low-grade fever and aches. Only about 1% to 2% of people who get a flu shot will have fever as a side effect, Schaffner said. These mild effects should go away within a few days.

Rare but serious side effects can occur, including allergic reactions. Symptoms of serious side effects include difficulty breathing, swelling around the eyes or lips, hives, racing heart, dizziness and high fever. If you experience serious side effects, you should seek medical care immediately, the CDC says. In addition, it's important to ask your health care provider to file what a Vaccine Adverse Event Reporting System report either online or by calling VAERS at 1-800-822-7967, the CDC says.

For children, side effects from the flu nasal spray can include runny nose, wheezing, headache, vomiting, muscle aches and fever. For adults, side effects from the nasal spray version of the vaccine include runny nose, headache, sore throat and cough. These side effects last a short time compared with the actual flu illness, the CDC says.

How effective is the flu vaccine?

The effectiveness of the seasonal flu vaccine depends upon several factors, including how well the flu strains in the vaccine match the strains in circulation. Some studies show that when strains in the vaccine are a good match with the ones that are circulating, vaccinated individuals are 60% less likely to catch the flu than people who aren't vaccinated, according to the CDC.

Flu vaccine effectiveness can also vary depending on the person being vaccinated — the vaccine tends to work best in healthy adults and older children, and less well in older adults.

For instance, a 2013 study from the CDC found that the year's flu vaccine was not very effective in adults ages 65 and over, Live Science previously reported. Older people who got the vaccine were just as likely to visit the doctor for flu symptoms as those who did not get the vaccine.

Related: Why is the flu shot less effective than other vaccines?

But other studies suggest that individuals who do get sick develop less severe symptoms if they are vaccinated. A 2013 study published in the journal Clinical Infectious Diseases found that people who got the flu shot were less likely to be hospitalized with the flu, Live Science previously reported. Studies published in 2018 and 2021 found similar results, in that vaccinated adults were less likely to be hospitalized or die from the flu, compared with unvaccinated adults, the CDC reported. And according to the 2018 study, among adults admitted to the ICU for flu, vaccinated patients on average spent four fewer days in the hospital than those who were not vaccinated.

There are some studies that suggest the high-dose flu vaccine — which contains four times the amount of inactivated virus as the regular shot and is given to individuals 65 and older — may provide better protection for older adults. A 2014 study in the New England Journal of Medicine found that the high-dose vaccine provides 24% more protection against the flu than the standard dose, Schaffner said.

What kinds of flu shots are there?

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Flu shots known as "quadrivalent vaccines" protect against four strains of flu virus. These include two influenza A strains — H1N1 and H3N2 — and two influenza B strains. 

Previously, some flu shots protected against three strains, and were known as trivalent flu vaccines, but starting in the 2021-2022 season, all flu shots are quadrivalent, according to the CDC.

In addition to the standard-dose flu vaccine given through a needle, flu shots are available in several different forms. These include a high-dose version for those ages 65 and older; a "cell-based" version that's grown in animal cells rather than hen's eggs and is approved for people ages 4 and older; a "recombinant" vaccine that does not use the full influenza virus or chicken eggs in the production process and is approved for people ages 18 and older; and a nasal spray, which is approved for healthy people ages 2 to 49, but not for pregnant women.

There is also a needle-free flu shot, delivered by a so-called jet injector, which uses a high-pressure stream of fluid to inject the vaccine, the CDC says. It is approved for adults ages 18 to 64. 

What's in this year's flu vaccines?

The composition of the 2021-2022 flu shot will be different from last season's flu shot. Specifically, the two influenza A components of the flu shot (H1N1 and H3N2) differ from those in last year's shot. In addition, all flu shots for the 2021-2022 season will be quadrivalent, meaning they will contain four strains of flu viruses.

According to the CDC, the 2021-2022 quadrivalent egg-based flu shot contains the following strains of the flu virus:

  • A/Victoria/2570/2019 (H1N1) pdm09-like virus — This H1N1 component differs from last year's flu shot. 
  • A/Cambodia/e0826360/2020 (H3N2)-like virus — This is the H3N2 component that is different from last year's flu shot. 
  • B/Washington/02/2019- like virus (B/Victoria lineage) — This influenza B strain component is the same as the one in last year's shot. 
  • B/Phuket/3073/2013-like virus (B/Yamagata lineage) — This influenza B strain component was also in last year's shot. 

Then, there are the two types of flu vaccines that don't involve eggs: cell-based and recombinant-based flu vaccines. In cell-based flu vaccines, the inactivated flu virus is grown in cultured cells from mammals rather than in hens' eggs. And recombinant flu vaccines are created synthetically. To make this type of vaccine, scientists combine a lab-made antigen specific to the flu virus with a baculovirus (this type infects only invertebrates, not humans).

For the 2021-2022 season, these two types of flu vaccines contain the same components as the egg-based vaccine except for the H1N1 component, which in the cell- or recombinant-based flu vaccines will be A/Wisconsin/588/2019 (H1N1) pdm09-like virus, according to the CDC.

Early evidence suggests that this year's flu shot may be a "bad match" for the dominant circulating strain of flu, due to a new mutation that's appeared in said strain, Live Science previously reported. Specifically, this year's shot may not be as effective against the H3N2 subtype of the influenza virus as originally hoped. That said, the vaccine should still protect against severe disease and death from the flu, and it's still possible that a better-matched strain, such as H1N1 or an influenza B virus, might become dominant later in the season. 

What's flu activity like this season?

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In general, flu activity in the U.S. began to trend upward in late October and through December, the CDC reported

That's exactly what scientists had predicted — that this year's flu activity would eventually spike.After the country experienced historically low levels of flu during the 2020-2021 season (which marked the beginning of the COVID-19 pandemic), several studies have predicted that the flu could come roaring back this fall and winter, Live Science previously reported.

That's because the U.S. population "missed the opportunity to establish or boost their immunity [to the flu]" last season, which raises the concern that the flu could make a comeback as preventive measures for COVID-19 are lifted, according to the authors of a recent study on predicting flu activity, posted Aug. 30 to the preprint database medRxiv.

In October, the CDC began an investigation at the University of Michigan's Ann Arbor campus, where there was a sudden spike in flu cases, Live Science previously reported. Most of these infections were caused by an H3N2 flu virus. This trend has since cropped up in data from across the U.S., as H3N2 appears to be the dominant strain circulating countrywide. 

As of Jan. 5, 2022, "Overall, influenza (flu) activity is still low nationally, but CDC surveillance systems continue to detect slow but steady increases in flu," the CDC website reads. In its Dec. 25 Weekly U.S. Influenza Surveillance Report, the agency noted that "Influenza activity is increasing, with the eastern and central parts of the country seeing the majority of viruses reported and the western part of the country reporting lower levels of influenza virus circulation." (Official estimates of the total number of flu cases nationwide will be released sometime in the future.)

Even if flu activity is currently low in your community, you shouldn't wait to get your flu shot. Flu activity could begin to increase at any time, and it takes about two weeks for people to develop antibodies against flu viruses after receiving their shot, the CDC says.

"The flu season is just getting started. There's still time to get vaccinated," the CDC weekly report states. 

Flu shot and COVID-19: Your questions answered

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How do I know if I have the flu or COVID-19?

Symptoms of COVID-19 and the flu can be similar, but your doctor can order a test to determine if you have COVID-19 or the flu. There are even tests that check for both flu viruses and SARS-CoV-2, the virus that causes COVID-19, at the same time.

Should I get a flu shot during the COVID-19 pandemic?

Yes, getting a flu shot every year is the best way to protect yourself and others from the flu. You can take precautions to protect yourself from COVID-19 while getting a flu shot, such as getting vaccinated against the coronavirus and wearing a mask.

Will a flu vaccine protect against COVID-19?

No, flu shots do not protect against SARS-CoV-2. (Separate vaccines against SARS-CoV-2 have been approved or authorized for emergency use in the U.S.) But flu shots do protect against strains of seasonal flu viruses and can reduce your risk of getting sick from the flu, as well as the risk of hospitalization and death from flu, according to the CDC.

Can I get a COVID-19 shot and flu shot at the same time?

Yes, people can get a flu vaccine and a COVID-19 vaccine — including COVID-19 booster shots — at the same time, according to the CDC. People tend to experience a similar set of side effects, and receive the same protection from diseases, whether vaccines are given alone or with other vaccines, according to the CDC. If you haven't been vaccinated against COVID-19, you should get vaccinated as soon as possible to protect yourself from the disease; and you should aim to get a flu shot by the end of October. Flu shots are available throughout the winter season, but it's best to get them in early fall.

Myth #1: You get the flu from the flu shot.

"It's a myth that you can get flu from the flu vaccine," Schaffner said.

The viruses in the flu shot are killed, so people cannot get the flu from a flu vaccine. However, because it takes about two weeks for people to build up immunity after they get the flu vaccine, some people may catch the flu shortly after they're vaccinated, if they are exposed to the flu during this time period. Some people may also mistakenly attribute symptoms of a cold to the vaccine, Schaffner said.

The nasal spray vaccine contains a "live attenuated" flu virus, but the virus is weakened so that it cannot cause the flu. The viruses in the nasal spray can't replicate in the warm temperatures of the lungs and other parts in the body. However, because temperatures in the nose are colder, the virus causes a small infection in the nose. This infection does not cause symptoms in most people, but in some people, it causes symptoms such as runny nose and sore throat, Schaffner said. These symptoms are typically short-lived, the CDC says.

This local infection will prompt the body to make antibodies against the flu virus, Schaffner said. "That provides better protection against the real flu, which is of course, is a virus that can make you seriously ill," Schaffner said.

Myth #2: The flu vaccine isn't safe for pregnant women.

Studies show flu vaccines are safe for women in any stage of pregnancy, the CDC says. There are several reasons why it's important for pregnant women to get a flu shot, Schaffner said.

"Pregnant women, when they get influenza, have a tendency to get a more severe disease," and are at increased risk for complications and hospitalization from the disease, Schaffner said.

In addition, flu vaccination in pregnancy helps to protect the baby against flu during the first six months of life, when the baby is too young to receive a flu shot, Schaffner said. The mother "passes that protection on to her newborn baby," Schaffner said.

Related: Am I pregnant? 12 early signs of pregnancy

Myth #3: Antibiotics can fight the flu if you get it.

Antibiotics only kill bacteria, but the flu is caused by a virus.

There are antiviral drugs that can fight flu infections, but they've only been shown to work when they're given within 48 hours of the start of symptoms. "Most people, by the time they go to the doctor, they're past the 48-hour mark," said Dr. Dennis Cunningham, an infectious disease specialist at Nationwide Children's Hospital in Columbus, Ohio.

For patients hospitalized with severe flu, the drugs may help, he said. But they aren't a cure, and for most people who aren't hospitalized, these drugs may only cut down on the duration of the flu by a day or two.

Myth #4: You don't need to get the flu vaccine every year.

There are two reasons why doctors recommend that people get the flu vaccine every year, Cunningham told Live Science.

For one, the strains of the flu virus that are circulating change from year to year. "It's like the common cold — there's more than one type of virus that causes the flu," and, in fact, there are hundreds of influenza viruses, he said. Each year, health officials identify the virus strains that are the most likely to cause illness during the upcoming flu season, according to the CDC.

Second, the immunity you develop after getting the shot wanes by the following year. "If you get your shot in August, you'll be safe through March, but those antibodies won't be for the next flu season," Cunningham said.

Myth #5: It contains thimerosal, which may be harmful.

Thimerosal — a preservative that contains mercury — has never been shown to be harmful, Cunningham said. The type of mercury linked with nervous system damage is called methyl mercury, he said. Concerns over levels of methyl mercury have led to recommendations that pregnant women avoid eating large amounts of certain types of fish, such as swordfish.

In contrast, thimerosal is an ethyl mercury compound that has antimicrobial properties. Still, because the preservative raised controversy, especially over a now-disproven link to autism, it was taken out of almost all U.S. vaccines starting in 2001, Cunningham said.

The injectable form of the flu vaccine is available to health care providers as large, multidose bottles and small vials carrying individual doses. A tiny amount of thimerosal is added to the multidose bottles to ensure that no bacteria will grow in the vaccine, Cunningham said. The individual-dose bottles contain no thimerosal. 

The nasal spray form of the flu vaccine also contains no thimerosal, he noted.

Myth #6: The flu isn't serious.

"The flu is certainly a very serious disease," Cunningham said.

Every year, between 15 million and 60 million cases of the flu are reported in the U.S., Cunningham said. More than 200,000 people with the flu are admitted to hospitals yearly. And between 3,000 and 50,000 people in the U.S. die of the flu yearly. During the 2019-2020 flu season, early estimates by the CDC suggested that 38 million Americans were infected with the flu and 22,000 people died from it.

One reason people may not perceive the flu as being serious is that cases of the "stomach flu" are mistaken for influenza virus infections. "True influenza is an infection of the lungs and respiratory tract," Cunningham said. Infected people may develop a high fever, body aches and nasal congestion, he said.

People with the stomach flu — which is commonly caused by a type of virus called a norovirus — have diarrhea, cramping and other gastrointestinal symptoms. Influenza does not cause such symptoms.

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

Editor's note: This article was last updated on Jan. 7, 2022. Live Science writer Nicoletta Lanese contributed to this report.

Originally published on Live Science.  

Rachael Rettner
Rachael Rettner

Rachael has been with Live Science since 2010. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.

  • Timmer
    Are the shots made in England. Seems useless to use eggs or other resources from another continent. Why not us local eggs and/or other local resources?
  • 4TimesAYear
    Seems to me that the flu vaccine can also enhance the flu, much like the COVID I vaccine did to the ferrets and mice. Whenever I got the vaccine, I'd get so sick (2-3 times/season) every time I got the vaccine, I'd have to go to the doctor for treatment - and at least once I ended up in the hospital with it. I was young and healthy. Does anyone else have a better explanation for why this happened? (I don't get the vaccine any more - and I don't get the flu any more) I found out later that people can even die from the flu they'd been vaccinated for. Everyone I know still gets the flu in spite of being vaccinated. They fortunately didn't end up with it 2-3 times/season like I did, but they still got it. Btw, I wouldn't call this "anecdotal" - it's EXPERIENTIAL. There are medical records to back it up.