Flu season could start ‘incredibly’ early – what scientists are monitoring

Data shows the Northern Hemisphere’s flu season could start earlier, prompting a rethink about when people should get their annual flu virus shots. The percentage of positive flu tests is unusually high for this time of year in countries like the United Kingdom and on the U.S. West Coast (see “Surprising start”). In Japan, outbreaks in August and early September forced some schools to close as they resumed activities after summer vacation.

“It would be an incredibly early season” for the United States if cases seen on the West Coast continue to rise at the current rate, says Scott Hensley, a virologist at the University of Pennsylvania in Philadelphia.

The flu subtype responsible for most flu cases in the United States so far is H1N1, unlike last year when H3N2 dominated. And sequencing data from these first cases indicates an emerging mutation of the H1N1 virus that worries specialists.

These results come from a relatively low number of positive tests compared to those available during peak flu season. It is therefore too early to predict the overall trajectory of the epidemic, say the researchers. “An early start does not necessarily mean a more severe season,” says James Hay, an epidemiologist at the University of Oxford, UK. Still, “something is brewing,” epidemiologist Caitlin Rivers wrote in her Sept. 27 “Force of Infection” newsletter.

It’s enough to make Rivers, a researcher at Johns Hopkins University in Baltimore, Maryland, reconsider her personal protection strategy. In a typical year, when flu season peaks in December and January in the United States, she would wait until mid-October to get vaccinated to get the best protection. Now she plans to get vaccinated as soon as possible.

An emerging mutation

Seasonal flu is usually caused by a combination of three viruses: two subtypes of influenza A – H3N2 and H1N1 – and influenza B. Since influenza viruses mutate genetically over time and people’s immunity changes as they receive vaccines and infections, which of the three viruses emerges as a dominant change for each flu season. US data showed that in the week ending September 26, the H1N1 virus accounted for 96% of positive cases for which subtyping was carried out. H3N2 accounted for almost 4% of cases and influenza B was not detected.

This contrasts with last year, when a new variant of H3N2, called subclade K, emerged. It became globally dominant because it had many mutations that made people more vulnerable.

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This year, the H1N1 mutation that worries researchers is G155E. Alex Greninger, a virologist at the University of Washington in Seattle, says his lab detected G155E in 13 of 17 H1N1 flu samples taken from people whose genetic sequences were complete enough to identify specific mutations. The samples came from Seattle hospitals and clinics over the past several months.

This mutation was previously reported in an analysis led by Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, published on the preprint server bioRxiv last month, ahead of peer review.¹.

Bloom and colleagues tested how well antibodies in blood samples from 260 people in Australia and the United States were able to recognize and block infection by 148 circulating influenza A viruses by mid-2026. “We found that a number of strains carrying the G155E mutation are, in some cases, less well recognized by pre-existing human antibodies,” says Bloom. Antibodies in the serum of adolescents and young adults were particularly unable to recognize this variant, likely because many in these groups had not previously been exposed to H1N1 viruses with similar characteristics.