An early start to the season
The 2026-2027 flu season appears to be starting early and strongly, according to experts cited by TODAY.com. Flu activity is still low nationally, but cases are climbing quickly in many parts of the country, particularly the West. Louisiana and Hawaii are already reporting moderate levels.
As of Oct. 21, flu infections were growing or likely growing in 44 states, according to the latest estimates from the Centers for Disease Control and Prevention. Flu typically picks up around this time of year, but this season's rise began unusually soon, with infections increasing as early as August.
In its FluView report for the week ending Oct. 17, the CDC said, "Early-season influenza activity is currently higher than typically observed at this time of year."
What the data show
The CDC says cases began rising in late August and early September and have climbed steadily since. The share of flu tests coming back positive was 5.7% for the week ending Oct. 17. During the same period last year, weekly test positivity was below 1%, according to the CDC's respiratory virus surveillance dashboard.
Outpatient visits for flu-like illness reached 2.2% that week, higher than in the previous two seasons, CDC data show. Hospitalizations and deaths from flu are also rising.
"Cases are still relatively low, so we're still in the early phases of a flu surge," Andrew Pekosz, a virologist at the Johns Hopkins Bloomberg School of Public Health, told TODAY.com.
The H1N1 mutation
Influenza A H1N1 is driving the vast majority of U.S. cases so far, per CDC data. That is a change from last winter, when the H3N2 subclade K variant, nicknamed the "superflu," dominated and produced a harsh season. H1N1 is the strain behind the 2009 swine flu pandemic.
Pekosz said the circulating H1N1 (pdm09), subclade D.3.1.1, has acquired a new mutation, G155E. A preprint on bioRxiv reported it was first identified in September in Seattle. Pekosz said the position is known to be important for generating protective antibody responses, so antibodies from past infection or vaccination may not recognize the virus as well. The CDC says H1N1 viruses typically change less often than H3N2 viruses.
Why cases are rising early is unclear. Dr. Bernard Camins of the Icahn School of Medicine at Mount Sinai suggested that reduced immunity could be a factor. "Even people who got the flu last year are vulnerable," he said. Pekosz noted that the first strain to arrive in an early season is often the one that escapes immunity the most. Updated flu shots had only just begun rolling out when H1N1 started increasing in late August.
Severity, vaccines and treatment
An early season does not necessarily mean a severe one. The CDC told TODAY.com it is too early to say how severe the season will be or what role the mutated strain might play. Pekosz said how the mutation will affect spread and severity is not clear. Historically, H1N1 has hit young people particularly hard, and NBC News medical contributor Dr. Natalie Azar said the mutation may make children more susceptible, especially those not yet exposed to H1N1.
Experts say the vaccine should still help even if the strain is not a perfect match. "We may have less immunity, not no immunity," Pekosz said. The 2026-2027 vaccines protect against H1N1, H3N2 and influenza B, and options include standard and high-dose shots, a nasal mist and a new mRNA vaccine. They are available at most pharmacies, doctors' offices and clinics.
Camins said the shot is never 100% effective, but vaccinated people who get infected are far less likely to become very sick or be hospitalized. Pekosz said immunity takes at least two weeks to reach protective levels, so those who have not yet been vaccinated should do so now.
Flu can be hard to tell apart from COVID, so testing is important, Camins said. Antivirals, which require a prescription, can reduce symptom severity and duration, and are especially important for people over 65, children under 5 and immunocompromised people. Pekosz said they work best when started within 48 hours of symptom onset.
