Flu Activity is Ramping Up Early This Year. Is a Mutated H1N1 Strain to Blame?

If you’re surprised to hear about people getting sick with the flu before Halloween, you’re not alone. The 2026-2027 flu season appears to be off to an early and strong start, experts say.Influenza activity is still low nationally, but cases are rising quickly in many parts of the country, especially the West, and two states, Louisiana and Hawaii, are already reporting moderate levels.As of Oct. 21, flu infections are growing or likely growing in 44 states, according to the latest estimates from the U.S. Centers for Disease Control and Prevention. That’s not surprising, given that flu activity typically ticks up around this time of year. However, this year’s flu season started way ahead of schedule, with infections rising as early as August. “Early-season influenza activity is currently higher than typically observed at this time of year,” the CDC said in its FluView surveillance report for the week ending Oct. 17. Influenza A H1N1 is fueling the vast majority of cases in the U.S. so far this season, per CDC data. That’s a shift from last winter, when H3N2 subclade K — aka, the “superflu” — was the dominant strain. What’s more, it appears that the H1N1 strain circulating has picked up a new mutation which may affect its ability to evade pre-existing immunity. If H1N1 sounds familiar, that’s because it’s the infamous “swine flu” strain that caused a pandemic in 2009. As the mutated H1N1 strain spreads, doctors are urging people to get the 2026-2027 flu vaccine. Why Is Flu Season Starting So Early?No two flu seasons look the same because the timing and severity can vary from year to year. However, there are multiple indicators that flu activity is higher than usual for this time of year. Cases started increasing in late August and early September and have been rising steadily since, per the CDC.The percent of flu tests coming back positive was 5.7% during the week ending Oct. 17. Last year during the same time period, the weekly test positivity rate was less than 1%, per the CDC National Respiratory and Enteric Virus Surveillance System dashboard.Outpatient visits to doctor’s offices or urgent cares for flu-like illness hit 2.2% during the week ending Oct. 17, which is higher than the previous two seasons, CDC data show. Hospitalizations and deaths from the flu are also rising.“Cases are still relatively low, so we’re still in the early phases of a flu surge,” Andrew Pekosz, Ph.D., a virologist at the Johns Hopkins Bloomberg School of Public Health, tells TODAY.com.It’s unclear why exactly cases are rising earlier than normal. One potential reason is that people may have less pre-existing immunity to the H1N1 strain with a new mutation.“Maybe that’s why people are getting sick sooner,” Dr. Bernard Camins, an infectious disease physician and professor at the Icahn School of Medicine at Mount Sinai, tells TODAY.com.The H1N1 strain spreading is different from the H3N2 strain that dominated last winter. “Even people who got the flu last year are vulnerable,” says Camins.When H1N1 started ramping up in late August, the updated flu shots had just started rolling out. “The pattern we oftentimes see with early season flu is that the first strain that gets here is the one that escapes immunity the most,” Pekosz adds.Does an early flu season mean a more severe flu season? Not necessarily, experts say. It’s too early to tell how the 2026-2027 flu season will pan out, or whether it’ll be milder compared to last year’s. The U.S. saw a harsh flu season last winter fueled by the highly mutated H3N2 subclade K variant, aka the “super flu.” “We still have to see how bad it will be this year, but I think it is just starting earlier,” says Camins.“The only thing predictable about the flu is that it’s unpredictable,” Dr. Natalie Azar, NBC News medical contributor, said on TODAY.What Is the Mutated H1N1 Strain?Influenza viruses mutate constantly, which is why we get an updated flu shot every year. Last summer, H3N2 picked up a host of mutations, which led to the emergence the subclade K variant.H1N1 viruses typically change less often than H3N2 viruses, the CDC says. However, this year, H1N1 (pdm09), subclade D.3.1.1, acquired one new mutation, G155E, says Pekosz. The mutation was first identified in September in Seattle, Washington, according in a preprint paper published to bioRxiv.“It is different from last year’s H1N1 virus, and that mutation position (G155E) we know is important for generating protective antibody responses,” Pekosz explains. This means the H1N1 strain may be better able to escape immunity because the antibodies people have from previous infection or vaccination may not recognize it as well. “How that (mutation) is going to affect the disease spread and severity is not clear,” says Pekosz.Even if the H1N1 strain circulating does not match the one included in the 2026-2027 flu vaccines, the shot is still expected to provide protection. “A mismatch is not a black-or-white situation. We may have less immunity, not no immunity,” Pekosz adds.Historically, H1N1 has hit young people particularly hard. The new mutation may make children more susceptible, especially if they haven’t been exposed to H1N1 yet, Azar said.“It is too early to know what viruses may predominate or to say whether there might be implications on flu vaccine effectiveness this season,” the CDC said in its latest FluView report.H1N1 Flu Symptoms The flu often hits suddenly and intensely, says Camins. Common symptoms of the flu include:FeverChillsFatigueBody achesRunny noseCoughingIt can be difficult to distinguish the signs of a flu infection from those caused by COVID, which is why testing is important, Camins says.If you test positive for influenza, you can seek treatment with antivirals, which can lessen symptom severity and duration. Antivirals are an especially important tool for high-risk groups, including people over the age of 65, children under the age of 5, and immunocompromised individuals. These medications aren’t available over-the-counter, so you’ll need to see a doctor first. “You have to get that prescription fast ... within the first 48 hours after symptom onset, (or else) they don’t work as well,” says Pekosz.If you experience severe flu symptoms, contact your doctor.When to Get the Flu ShotThere’s no reason to panic over the mutated H1N1 strain, the experts emphasize, but it is a good reminder to protect yourself.“View (this) as a a sign to go out and get the flu vaccine,” says Pekosz.Getting a flu shot is the most effective way to prevent against severe illness, complications and death from the flu.The 2026-2027 flu vaccines are available at most pharmacies, doctors’ offices and clinics. The updated shots protect against three viruses: influenza A H1N1 (pdm09), influenza A H3N2 and influenza B.There are multiple options available, including standard flu shots, high-dose shots, a nasal mist and the new mRNA flu vaccine.“Even if there’s a mismatched strain, there’s two other strains that the vaccine will provide better protection against,” says Pekosz. Different flu strains can circulate later in the season, so the shot can protect you throughout the winter and early spring.The flu shot is never 100% effective, Camins adds, and you can still get infected if you’ve been vaccinated — but you won’t get nearly as sick or end up in the hospital. Vaccination is especially important if you’re considered high-risk, but everyone can benefit. The best time to get the flu shot is before the end of October, but you can still benefit if you get it later, the experts note.“If you haven’t gotten it yet for whatever reason, now’s the time to get vaccinated because the flu season is starting a little bit sooner and you need at least two weeks for the vaccine immunity to (reach) protective levels,” Pekosz adds.If you aren’t sure which flu vaccine you should get, talk to your doctor or pharmacist.