Mutated H1N1 Flu Strain Drives Unusually Early 2026–27 Season Across the United States

The United States 2026–27 influenza season is getting off to an unusually early start. By 21 October, flu activity was growing or likely growing in 44 states, with Louisiana and Hawaii already reporting moderate levels — well ahead of a typical winter onset. Cases began climbing as early as late August, months sooner than normal. For the week ending 17 October, the flu test positivity rate hit 5.7%, compared with less than 1% during the same period last year.

The dominant strain so far is influenza A H1N1, carrying a newly identified mutation called G155E, first spotted in Seattle in September. Johns Hopkins virologist Dr Andrew Pekosz says the mutation occurs at a position critical for generating protective antibody responses, meaning antibodies from prior infection or vaccination may not recognise the strain as well. Experts suggest this reduced cross-protection could be one reason people are getting sick sooner — though it remains unclear exactly why the season started this early. "Maybe that's why people are getting sick sooner," said Dr Bernard Camins of the Icahn School of Medicine at Mount Sinai.

Despite the mutation, health authorities say the 2026–27 flu vaccine — which targets H1N1, H3N2 and influenza B — still offers meaningful protection, and experts urge anyone not yet vaccinated to act before the end of October. Hospitalisations and deaths are also rising, though an early start does not necessarily signal a more severe overall season.