A mutated H1N1 flu strain turned up in 13 of 17 Seattle samples as scientists eye an early season

Alex Greninger’s laboratory at the University of Washington in Seattle has been sequencing H1N1 influenza samples from hospitals and clinics across the city for the past few months, and one mutation keeps appearing. The change, called G155E, showed up in 13 of 17 samples whose sequences were complete enough to identify individual mutations. The count is small and comes from one metropolitan area, but it lands while flu indicators are climbing early on the U.S. West Coast, in the United Kingdom and in Japan.

Greninger, a virologist, gave the tally to Nature’s news team, which published it on 6 October 2026. The denominator deserves attention: the 17 are the samples with readable-enough sequence data, not every H1N1 specimen the lab handled, so “13 of 17” describes a slice of Seattle’s circulating virus rather than the whole of it. Nobody has yet reported how the share looks in other cities, and the Nature report offers no comparison with samples from earlier months, so the trend inside Seattle itself is not described either.

G155E in the Seattle sequences

Jesse Bloom, a virologist at the Fred Hutchinson Cancer Center in Seattle, led an earlier analysis that flagged the mutation, according to Nature’s report. That work was posted last month on the bioRxiv preprint server and has not been through peer review. Bloom’s team tested blood samples from 260 people in Australia and the United States against 148 influenza A viruses that were circulating by mid-2026, and some of the strains carrying G155E were recognized less well by pre-existing antibodies.

The weakest recognition came from teenagers and young adults. Nature’s account says this is probably because many people in those age groups have never been exposed to H1N1 viruses with similar characteristics, so their immune memory has little to match against. That pattern is why the age split in Bloom’s data drew attention: the groups least protected by past infections are also the ones whose antibodies fared worst against the mutated strains in the lab tests.

The same report does not say whether the 2026-27 seasonal vaccine lines up with G155E-bearing viruses, and the Seattle sequencing measures how common the mutation is, not how sick it makes people.

H1N1’s 96 percent share in CDC subtyping

The mutation matters partly because H1N1 is nearly the only influenza A subtype in the subtyped U.S. results right now. In the week ending 26 September, CDC’s weekly surveillance report showed H1N1 accounting for 96% of positive cases that were subtyped, with H3N2 at almost 4% and influenza B not detected, according to Nature. Last year ran the other way: H3N2 dominated, and a variant called subclade K became globally dominant because it carried many mutations that left people more vulnerable.

Washington State’s own numbers fit the national picture. Public Health Seattle & King County data cited by KUOW showed 233 emergency-room visits with confirmed influenza in the week ending 19 September, against 31 in the same week a year earlier. At Seattle Children’s, about 150 children tested positive across the system in two weeks, up from about 10 in the same stretch last fall. Danielle Zerr, who heads pediatric infectious diseases there, said the season there is arriving “about two months early.”

Early signals and the timing of the shot

Scott Hensley, a virologist at the University of Pennsylvania, told Nature the United States would see “an incredibly early season” if West Coast cases keep rising at their current pace. A graphic in the report shows the 2026-27 Northern Hemisphere rise starting about a month ahead of previous years, when it did not begin until November. In Japan, outbreaks in August and early September closed some schools shortly after summer break.

Caution comes from the same sources. The signals rest on a relatively small number of positive tests, and James Hay, an epidemiologist at the University of Oxford, said an early start does not necessarily mean a more severe season.

Vaccination timing is where the early signals turn practical. Caitlin Rivers, an epidemiologist at Johns Hopkins University, wrote in her Force of Infection newsletter on 27 September that “there is something afoot,” and said she usually waits until mid-to-late October for her flu shot but is now considering getting it as soon as possible. Helen Chu, an infectious diseases professor at the University of Washington, told KUOW that people should get flu shots now rather than waiting until Halloween. CDC’s vaccine guidance recommends a flu vaccine every season for everyone 6 months and older, and says the vaccine lowers the risk of a doctor’s visit for flu by 40% to 60% when vaccine viruses are similar to the circulating ones.

That last condition is the one G155E puts in play. Bloom’s 260-person antibody panel shows that some mutated strains slip past existing immunity, and Greninger’s 13 of 17 shows how often Seattle’s sequenced H1N1 now carries the change.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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