Flu season did not wait for winter this year. It showed up in October, about a month ahead of schedule — and it picked the West Coast to make its entrance. The percentage of flu tests coming back positive has been climbing since early September, and flu-driven emergency room visits have been inching up for weeks. "There are enough signs pointing in the same direction," epidemiologist Caitlin Rivers of Johns Hopkins University told NPR on October 6. "This is the start of flu season."
Flu season in the United States normally begins sometime in October or November. This year it got going roughly four weeks ahead of that window. Stranger still is where it began: California, Washington, Hawaii and Alaska are seeing the first wave of activity. That is not how respiratory virus seasons usually play out — new waves typically start in the South and expand outward, Rivers noted, making this western ignition doubly unusual. In Seattle, doctors at the University of Washington are seeing as much flu right now as they would expect around Christmas, said Dr. Alex Greninger, who heads infectious disease diagnostics there.
California's numbers tell the sharpest story. The state public health department reported 1,788 flu hospitalizations statewide as of September 26 — already above this point last year — and 34 flu-related deaths so far this season. San Francisco announced its first flu death of the season. The Bay Area currently has the highest flu test positivity in California, rated "high" on the state respiratory virus dashboard, with elevated virus levels in sewage in San Francisco, San José, Sunnyvale and Palo Alto, according to WastewaterScan and county health data. In Los Angeles and Orange counties, test positivity, hospitalizations and emergency department visits are all rising. Analysts tracking wastewater data say flu concentrations in California are already near where they were last December — nearly two months ahead of schedule.
Behind the early start sits a souped-up virus: influenza A H3N2 subclade K, formerly known as J.2.4.1. It surfaced toward the end of summer, months after health officials had locked in this fall's vaccine formulation back in February. Subclade K carries multiple mutations at key antigenic sites on its hemagglutinin surface protein — including changes dubbed T135K and S144N — that make it better at dodging immune defenses built up against older strains. It dominated early-season samples in Japan and the United Kingdom, accounting for roughly nine in ten specimens in some analyses, and the California Department of Public Health says it is "causing an active, early flu season" across the Northern Hemisphere.
Doctors stress that this is not a reason to skip the shot. A University of Pennsylvania study found that antibodies triggered by the current vaccine still efficiently recognized subclade K in nearly 40 percent of people, and the shot remains the best defense against severe disease and hospitalization.
One of the most pointed warnings is aimed at parents. Physicians in Southern California expect this season may hit young children especially hard, the Los Angeles Times reported. Dr. Elizabeth Hudson, regional director of infectious diseases at Kaiser Permanente Southern California, said flu cases at Kaiser are concentrated in clinics for now — but hospitalizations typically surge after Christmas, and she expects the same pattern this year, just arriving sooner. The shape of the early curve, she added, looks much like what the Southern Hemisphere experienced during its winter flu season.
The expert advice is blunt: do not wait. It takes about two weeks for vaccine protection to build, and the shot is recommended for everyone six months and older. Public health agencies suggest getting it done by Halloween, while activity is still manageable. For those who dread needles, the nasal-spray vaccine FluMist is now available for home administration. And if flu symptoms strike, antivirals such as oseltamivir work best when started within a day or two of onset — a particular priority for older adults, young children and others at high risk. With Thanksgiving and Christmas travel approaching, the window to get ahead of this season is open now.