If you\u2019re surprised to hear about people getting sick with the flu before Halloween, you\u2019re not alone. The 2026-2027 flu season appears to be off to an early and strong start, experts say.<\/p>\n
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.<\/p>\n
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<\/a>. <\/p>\n That\u2019s not surprising, given that flu activity typically ticks up around this time of year. However, this year\u2019s flu season started way ahead of schedule, with infections rising as early as August. <\/p>\n \u201cEarly-season influenza activity is currently higher than typically observed at this time of year,\u201d the CDC said in its FluView surveillance report<\/a> for the week ending Oct. 17. <\/p>\n Influenza A H1N1 is fueling the vast majority of cases in the U.S. so far this season, per CDC data. That\u2019s a shift from last winter, when H3N2 subclade K \u2014\u00a0aka, the \u201csuperflu\u201d \u2014\u00a0was the dominant strain. <\/p>\n What\u2019s more, it appears that the H1N1 strain circulating has picked up a new mutation which may affect its ability to evade pre-existing immunity. <\/p>\n If H1N1 sounds familiar, that\u2019s because it\u2019s the infamous \u201cswine flu\u201d strain that caused a pandemic in 2009. <\/p>\n As the mutated H1N1 strain spreads, doctors are urging people to get the 2026-2027 flu vaccine<\/a>. <\/p>\n Why Is Flu Season Starting So Early?<\/p>\n No two flu seasons look the same because the timing and severity can vary from year to year. It\u2019s 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.<\/p>\n However, there are multiple indicators that flu activity is higher than usual for this time of year. <\/p>\n Cases started increasing in late August and early September and have been rising steadily since, per the CDC.<\/p>\n 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<\/a>.<\/p>\n Outpatient visits to doctor\u2019s 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.<\/p>\n \u201cCases are still relatively low, so we\u2019re still in the early phases of a flu surge,\u201d Andrew Pekosz, Ph.D., a virologist at the Johns Hopkins Bloomberg School of Public Health, tells TODAY.com.<\/p>\n It\u2019s unclear why exactly cases are rising earlier than normal. <\/p>\n One potential reason is that people may have less pre-existing immunity to the H1N1 strain with a new mutation.<\/p>\n \u201cMaybe that\u2019s why people are getting sick sooner,\u201d Dr. Bernard Camins, an infectious disease physician and professor at the Icahn School of Medicine at Mount Sinai, tells TODAY.com.<\/p>\n The H1N1 strain spreading is different from the H3N2 strain that dominated last winter. \u201cEven people who got the flu last year are vulnerable,\u201d says Camins.<\/p>\n When H1N1 started ramping up in late August, the updated flu shots had just started rolling out. \u201cThe pattern we oftentimes see with early season flu is that the first strain that gets here is the one that escapes immunity the most,\u201d Pekosz adds.<\/p>\n Does an early flu season mean a more severe flu season? Not necessarily, experts say. <\/p>\n It\u2019s too early to tell how the 2026-2027 flu season will pan out, or whether it\u2019ll be milder compared to last year\u2019s. The U.S. saw a harsh flu season last winter fueled by the highly mutated H3N2 subclade K variant<\/a>, aka the \u201csuper flu.\u201d <\/p>\n \u201cWe still have to see how bad it will be this year, but I think it is just starting earlier,\u201d says Camins.<\/p>\n \u201cThe only thing predictable about the flu is that it\u2019s unpredictable,\u201d Dr. Natalie Azar, NBC News medical contributor, said on TODAY<\/a>.<\/p>\n What Is the Mutated H1N1 Strain?<\/p>\n Influenza viruses mutate constantly, which is why we get an updated flu shot every year. <\/p>\n Last summer, H3N2 picked up a host of mutations, which led to the emergence the subclade K variant.<\/p>\n 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. <\/p>\n The mutation was first identified in September in Seattle, Washington, according in a preprint paper published to bioRxiv.<\/a><\/p>\n \u201cIt is different from last year\u2019s H1N1 virus, and that mutation position (G155E) we know is important for generating protective antibody responses,\u201d Pekosz explains. <\/p>\n 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. <\/p>\n \u201cHow that (mutation) is going to affect the disease spread and severity is not clear,\u201d says Pekosz.<\/p>\n 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. <\/p>\n \u201cA mismatch is not a black-or-white situation. We may have less immunity, not no immunity,\u201d Pekosz adds.<\/p>\n Historically, H1N1 has hit young people particularly hard. The new mutation may make children more susceptible, especially if they haven\u2019t been exposed to H1N1 yet, Azar said.<\/p>\n \u201cIt is too early to know what viruses may predominate or to say whether there might be implications on flu vaccine effectiveness this season,\u201d the CDC said in its latest FluView report.<\/p>\n H1N1 Flu Symptoms <\/p>\n The flu often hits suddenly and intensely, says Camins. <\/p>\n