California’s rate was slightly above 1% at the same point in the previous two seasons. The early rise has prompted health experts to stress annual vaccination.
Flu activity is rising unusually early in the western US, with California and Utah recording much higher October figures than in earlier seasons, the Guardian reported on October 23. Flu activity typically picks up in December and peaks in February.
| Measure | October 2026 | Earlier comparison |
|---|---|---|
| California: share of flu tests returning positive | 9.65% as of October 10 | Slightly above 1% in early October in each of the previous two flu seasons |
| Utah: recorded influenza A cases | 242 as of October 11 | 10 at the same time in 2025 |
California’s figures come from the state health department; Utah’s come from Intermountain Health’s Germ Watch database. The California percentage measures positive results among flu tests, not the share of the population infected. The Guardian also reported a rise in influenza A in parts of central Oregon.
Several experts cited by the Guardian attributed the early start to a new mutation—a genetic change—in H1N1, a subtype of influenza A. H1N1 accounted for 242 of the 300 influenza viruses reported by public health laboratories to the US Centers for Disease Control and Prevention by the time of the report.
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Those laboratory totals count the H1N1 subtype, not specifically viruses carrying the reported mutation. The experts’ explanation for the early rise is distinct from what those counts measure.
The 2026–27 flu vaccines, approved by the US Food and Drug Administration in August, target H1N1, another influenza A subtype called H3N2, and one influenza B virus. H3N2 was the dominant strain last flu season.
Scott Hensley, a University of Pennsylvania microbiologist, said two of the vaccine’s three components appeared very well matched to circulating flu strains. He also said the H1N1 mutation might affect antibodies in younger people.
The American Medical Association says flu vaccination can reduce severe illness and hospitalization and recommends it for everyone aged six months and older. With flu activity already rising, Hensley said it was probably a good idea to get vaccinated early.
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