Flu activity is ramping up early this year. Is a mutated H1N1 strain to blame?

Vaccines Flu activity is ramping up early this year. Is a mutated H1N1 strain to blame? Flu activity is surging earlier than usual as an influenza A H1N1 strain with a new mutation spreads across the U.S. Here’s what to know. By Caroline Kee | TODAY • Published November 8, 2026 • Updated on November 8, 2026 at 11:33 am If you’re surprised to hear about people getting the flu in October, you’re not alone. The 2026-2027 flu season got off to an early and strong start in the United States, experts say. Influenza activity is still considered low nationally, but cases are rising quickly in many parts of the country, especially in the West. Stream NBC 5 for free, 24/7, wherever you are. WATCH HERE As of Nov. 5, flu infections are growing or likely growing in 42 states, according to the latest estimates from the U.S. Centers for Disease Control and Prevention . That’s not unusual, given that flu activity typically ramps 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. 31. So far this season, there have been at least 1,700,000 illnesses, 25,000 hospitalizations and 1,400 deaths from the flu in the U.S., per the CDC. H1N1, a type of influenza A virus, is currently fueling the vast majority of cases. That’s a shift from last winter, when influenza A H3N2 subclade K (aka, the “superflu”) was the dominant strain nationwide. U.S. & World Joe Biden 1 hour ago Joe Biden to scale back book tour as cancer ‘slows me down' Artificial Intelligence 17 hours ago Nvidia opens investigation after reports CEO Jensen Huang was targeted by Chinese intelligence What’s more, it appears that the H1N1 strain circulating in the U.S. has picked up a new mutation which may affect its ability to evade pre-existing immunity, experts say. If H1N1 sounds familiar, that’s because it’s the infamous “swine flu” virus that caused a pandemic in 2009. As the mutated H1N1 strain spreads, doctors are urging people to get the 2026-2027 flu vaccine . Here’s what to know about this year’s flu season and how to protect yourself. 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. It’s still too early to tell if this flu season will end up being more severe or what role the mutated H1N1 strain might play, the CDC tells TODAY.com in a statement. However, there are multiple indicators that flu activity is already 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 9.1% during the week ending Oct. 31. 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 healthcare providers for flu-like illness hit 2.8% 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. As of Oct. 31, seven states are reporting moderate to high levels of flu activity: California, Florida, Louisiana, Oregon, South Carolina, Texas and Washington. 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. “An earlier increase in activity does not necessarily lead to a higher seasonal peak,” the CDC said in its latest FluView report. 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 season. 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, flu A 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 latest 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 Influenza often hits suddenly and intensely, says Camins. Common symptoms of the flu include: Fever Chills Fatigue Body aches Runny nose Coughing It can be difficult to distinguish the signs of a flu infection from those caused by COVID-19, 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 shot There’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.