The US is having its most active flu season in 15 years

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.