Flu season jumps the gun on the West Coast as an H1N1 mutation spreads

Flu season showed up to work early. Cases are climbing on the West Coast well ahead of the usual December start and February peak, with positive tests running hotter than normal for this time of year. California's influenza test positivity sat at 9.65% as of 10 October, according to state health department data — a steep jump from the roughly 1% seen in early October during each of the past two flu seasons.

Utah is feeling it too: 242 influenza A cases as of 11 October, up from just 10 at the same point last year, according to Intermountain Health's Germ Watch database. Parts of central Oregon have also logged an influenza A surge.

Experts are crediting a new mutation of the influenza A strain H1N1 — the same strain behind the 2009 swine flu pandemic. It accounts for 242 of the 300 influenza viruses that public health labs have reported to the CDC so far.

The 2026-27 vaccines, which the FDA approved in August, target H1N1, H3N2 (last season's dominant strain) and one influenza B virus. The American Medical Association recommends flu shots for everyone 6 months and older, noting they cut the risk of severe illness and hospitalization. Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three vaccine components look very well matched to circulating strains — though the H1N1 mutation might affect antibodies in younger people, making early vaccination a smart call, he said.

One wrinkle: promotion of flu shots has been dialed back under US health secretary Robert F Kennedy Jr, a longtime anti-vaccine activist who has driven an overhaul of longstanding childhood vaccine schedules. That leaves the early-season messaging to state data and experts urging people not to wait.