Flu season arrives early with a mutated H1N1 strain — and younger people may bear the brunt

The 2026-2027 flu season is arriving weeks ahead of schedule, driven by a mutated version of H1N1 that experts say could be better at evading the immunity people built up from past infections and older vaccines — a combination that has doctors urging everyone six months and older to get a flu shot now, before the season peaks.

Flu activity has been climbing since late August and early September, according to CDC surveillance data. The nationwide percent positivity rate — the share of all influenza tests coming back positive — stood at 3.6 percent as of the CDC's October 9 FluView report, according to Scientific American. That is a sixfold increase compared with the same point in 2025, when the rate was 0.6 percent. Clinical labs have reported positive tests climbing to a 10-year high for this time of year at 5 percent nationally, according to LiveNow Fox, with the West Coast far outpacing the rest of the country: positivity there reached 10.7 percent, versus 0.5 percent at the same point in 2025. CDC data show Region 9 — Arizona, California, Hawaii, and Nevada — recorded 9.5 percent positivity, which Scientific American describes as exceptionally high for this time of year.

It's not unheard of to see things rising at this stage. But it's also something that we probably shouldn't be seeing until maybe November or December at the earliest.— Andrew Pekosz, immunologist, Johns Hopkins Bloomberg School of Public Health

The strain doing most of the work is influenza A(H1N1)pdm09 — the same lineage that caused the 2009 swine flu pandemic. CDC data show it accounted for 98.4 percent of subtyped positive tests in the most recent weekly reporting period, with H3N2 making up the remaining 1.6 percent. But this year's circulating version carries a mutation called G155E, according to Reuters as cited by LiveNow Fox, which may reduce the protection people have from prior exposures or older vaccines.

The version [of H1N1] that's circulating seems to be a little bit different from the vaccine strain that we have and from H1N1 strains that have circulated before. It's what we call an antigenic drift. That could be why the virus is emerging so early. If it can evade prior immunity, the pathogen will be easier to transmit.— Andrew Pekosz, immunologist, Johns Hopkins Bloomberg School of Public Health

The distinction between H1N1 and H3N2 matters for predicting who gets hit hardest. H3N2-dominant seasons have historically been associated with particularly severe illness among older adults. H1N1, by contrast, has had a greater effect on younger populations — though both subtypes can cause serious disease at any age, according to Scientific American. That pattern, combined with the G155E mutation potentially undermining existing immunity, is why doctors are specifically flagging risk for younger people this season.

The 2026-2027 flu vaccines do target H1N1, H3N2, and one influenza B virus, according to Reuters as cited by LiveNow Fox. But how well the H1N1 component matches the mutated circulating strain — and how that affects vaccine effectiveness — is still unknown. Experts say it is too early to predict whether the early spike signals a particularly severe winter overall.

Still, the timing of vaccination matters regardless of the strain. Immunity from the flu vaccine is strongest for roughly four to six months, and flu typically peaks between December and February, making September and October the practical window for protection through the worst of the season.

Immunity from the flu vaccine is strongest for about four to six months, and we usually expect flu to peak in December or January. So a September or October vaccination will get you through the worst part of the flu season.— Andrew Pekosz, immunologist, Johns Hopkins Bloomberg School of Public Health

The best time to get vaccinated is before you see influenza in your area.— Andrew Pekosz, immunologist, Johns Hopkins Bloomberg School of Public Health

Trish Perl, a professor of internal medicine and infectious disease specialist at UT Southwestern Medical Center, tells her patients to get vaccinated before Thanksgiving at the latest — because after that, she says, people have unlimited excuses for why they can't find the time, according to Scientific American. With parts of the country already seeing spikes, getting the shot earlier than usual may be advisable.

Vaccination rates have declined significantly since the COVID-19 pandemic reshaped habits. Approximately 58 million adults received a flu shot in 2025, a drop of more than 20 percent from the nearly 75 million who got one in the 2019-20 season, according to data from the firm IQVIA cited by LiveNow Fox. About half of U.S. children received flu shots in 2025, compared with more than 60 percent in 2019-20, according to CDC data. The American Medical Association recommends flu shots for all people six months and older.

A new option exists for some adults this season. On August 5, 2026, the FDA approved Moderna's mFLUSIVA — the first mRNA seasonal influenza vaccine — for adults aged 50 and older. In a phase 3 clinical trial, it was found to be 26.6 percent more effective at preventing flu than a standard flu shot, according to Scientific American. Pekosz and Perl say that people who qualify and can access it should consider it, though access appears limited so far. The disruption to the CDC's Advisory Committee on Immunization Practices under the Trump administration has complicated the process of incorporating newly approved vaccines into federal guidance, according to Scientific American.

The data would suggest that if you can get the mRNA vaccine, you probably will get a stronger response to most of the components in the vaccine than if you get the standard one. But again, the standard one is pretty good as well.— Andrew Pekosz, immunologist, Johns Hopkins Bloomberg School of Public Health

The CDC notes that all current surveillance data are preliminary and may change as more reports are received. No new confirmed human infections with novel influenza viruses were reported to the CDC as of October 14, 2026.

Why it matters — An unusually early flu season dominated by a mutated H1N1 strain — with positivity rates at a 10-year high in some regions — means the window to get vaccinated before peak season is narrowing now, particularly for younger people who historically bear more risk from H1N1.

Reported by reuters.com, cdc.gov, scientificamerican.com, livenowfox.com, prevention.com