Early H1N1 flu surge lifts West Coast test positivity to 9.65%
California flu test positivity reached 9.65% in early October versus about 1% in past seasons, as experts point to an H1N1 mutation and urge early shots.
The story in focus
Flu activity on the US West Coast has started months earlier than usual, the Guardian reports, with California test positivity at 9.65% in early October versus about 1% in prior seasons. Experts attribute the early start to a mutated H1N1 strain and recommend early vaccination.
- California influenza test positivity was 9.65% as of 10 October, against slightly above 1% in early October of the two prior seasons.
- The H1N1 strain accounted for 242 of 300 influenza viruses reported to the CDC to date.
- Utah recorded 242 influenza A cases as of 11 October, up from 10 a year earlier, per Intermountain Health's Germ Watch.
What the data shows
US flu activity typically builds in December and peaks in February. This season began earlier, and the Guardian reports higher-than-normal positive cases on the West Coast. California's department of health data put influenza test positivity at 9.65% as of 10 October; in the two previous seasons the rate hovered slightly above 1% in early October.
Utah recorded 242 influenza A cases as of 11 October, compared with 10 at the same point last year, according to Germ Watch, an Intermountain Health database. Parts of central Oregon have also seen an influenza A surge.
The strain and the vaccine
Several experts credited the early start to a new mutation of the influenza A H1N1 strain, which drove the 2009 swine flu pandemic. That strain accounted for 242 of 300 influenza viruses reported by public health laboratories to the CDC to date.
The 2026-27 vaccines, approved by the FDA in August, target H1N1, H3N2 and one influenza B virus. Scott Hensley, a University of Pennsylvania microbiologist, said two of the three components appear very well matched to circulating strains; the H1N1 mutation might affect antibodies in younger people, and he said via email that getting vaccinated early is probably a good idea.
Public health context
The American Medical Association says influenza vaccines can reduce severe illness and hospitalization and recommends shots for everyone 6 months and older. The Guardian reports that promotion of flu shots has been reduced under US health secretary Robert F Kennedy Jr, a longtime anti-vaccine activist who has overhauled long-standing childhood vaccine schedules.
How far above normal is California's early-season flu test positivity?
California's influenza test positivity was 9.65% as of 10 October, according to state health department data cited by the Guardian, compared with slightly above 1% in early October during the two previous flu seasons.
Sources & further reading
- Flu cases rise on west coast earlier than usual as US health experts encourage vaccine ↗The Guardian
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