Flu activity is appearing unusually early across parts of the Northern Hemisphere, with influenza tests turning positive at higher-than-usual rates in the UK and on the US West Coast. Early sequencing points to H1N1 as the dominant influenza A subtype in US samples, with a mutation known as G155E attracting attention from researchers.

The findings do not yet show that the coming season will be more severe, but they have prompted questions about vaccination timing.

Early Signs Are Arriving Before the Usual Flu Window

A 6 October report said positive influenza tests were unusually high for this point in the year in the UK and on the West Coast of the US. Japan has also recorded outbreaks early, including school closures in August and September.

In the US, H1N1 has replaced H3N2 as the main subtype identified so far. For the week ending 26 September, 96% of subtyped influenza A samples were H1N1, compared with about 4% H3N2, according to data cited in the report. Researchers stress that the numbers are still small compared with testing at the height of flu season, so the early rise cannot yet predict how the wider season will develop.

The pattern is notable because influenza activity normally builds later in autumn. James Hay, an epidemiologist at the University of Oxford, said an early start does not necessarily mean a more severe season.

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G155E Mutation Raises Questions About Immune Recognition

The genetic finding adding to concern is G155E, a mutation in the haemagglutinin protein of H1N1. Alex Greninger, a virologist at the University of Washington, reported that his laboratory found the mutation in 13 of 17 H1N1 specimens from Seattle with enough genetic coverage for analysis.

Researchers are investigating whether the change affects how well existing antibodies recognise the virus. A preprint by Caroline Kikawa, Jesse Bloom and colleagues has also examined the mutation. Early laboratory findings suggest the change may reduce recognition by some antibodies, but that does not establish that the virus spreads more easily, causes more severe disease or makes vaccines ineffective.

The 2026-27 US flu vaccine includes an H1N1 component based on an A/Missouri/11/2025-like virus, while US seasonal vaccines are trivalent. A new mutation does not mean the vaccine should be abandoned; vaccination remains the main recommended protection against seasonal influenza.

October Vaccination May Still Be the Right Time

England's 2026-27 programme began vaccination for most eligible adults on 1 October, while pregnant women and children began from September. The policy reflects evidence that protection can wane in adults, with flu usually peaking in December or January.

That schedule could look awkward if flu activity rises earlier than expected, but health officials have not advised people to wait for a different vaccine. NHS England says the vast majority of vaccinations should be completed by the end of November. In the US, the CDC says September and October are generally good months for vaccination, while vaccination should continue after October while flu viruses circulate.

For now, the evidence points to an early start rather than a confirmed unusually severe season. Surveillance and sequencing over coming weeks should show whether the H1N1 signal expands and whether G155E becomes more widespread.

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