As of Oct. 21, flu activity was growing or likely growing in 44 states. Louisiana and Hawaii were reporting moderate activity, while national levels remained low, according to the Centers for Disease Control and Prevention. The CDC said activity was higher than typically seen at this point in the season.

For the week ending Oct. 17, 5.7% of flu tests were positive, compared with less than 1% during the same period last year. The share of outpatient visits for flu-like illness reached 2.2%, also above levels recorded during the previous two seasons. Hospitalizations and deaths were increasing as well, though experts cautioned that the season is still in its early stages.

Influenza A H1N1 is driving most cases so far, a change from last winter, when H3N2 subclade K was dominant. The H1N1 strain, also known as the virus behind the 2009 swine flu pandemic, has acquired a mutation called G155E. Researchers first identified it in Seattle in September.

The mutation affects a part of the virus that can matter for protective antibody responses. That raises the possibility that antibodies from past infection or vaccination may recognize the strain less effectively. But researchers say it is not yet clear whether the change will significantly affect how widely the virus spreads, how severe illness will be, or how well this season’s vaccine performs.

“Cases are still relatively low,” said Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, describing the country as being in the early phase of a flu surge. Infectious-disease physician Bernard Camins said the H1N1 strain differs from the H3N2 virus that circulated last winter, so people who had flu then may still be vulnerable.

An early start does not necessarily mean a more severe season. The CDC said it is too soon to determine which viruses will predominate or what effect the mutation could have on vaccine effectiveness.

Health experts are urging people to get the 2026-27 flu vaccine. The shots are designed to protect against H1N1, H3N2 and influenza B. Even if protection against the circulating H1N1 strain is reduced, vaccination can still provide some protection against that virus and stronger protection against other strains, Pekosz said.

Flu vaccines are available at many pharmacies, doctors’ offices and clinics. Protection takes about two weeks to build, but getting vaccinated later in the season can still help. Vaccination is especially important for people at higher risk of complications, including adults 65 and older, children under 5 and people with weakened immune systems.

Flu symptoms often start suddenly and can include fever, chills, fatigue, body aches, a runny nose and cough. Because flu and COVID-19 symptoms can overlap, testing can help identify the illness. Antiviral medicines require a prescription and work best when started within 48 hours of symptoms beginning, Pekosz said.