Flu season has reached its onset threshold early in the Bay Area, according to Stanford University’s WastewaterSCAN project. In 2025, the region did not reach that mark until November.

WastewaterSCAN’s Amanda Bidwell said influenza appeared in more than 90% of samples from monitored Bay Area sites in September. Flu concentrations in local wastewater rose nearly 350% from August to September. Dr. Peter Chin-Hong, an infectious disease specialist at UCSF, said its lab was diagnosing up to five times as many flu cases as a month earlier.

The San Francisco Department of Public Health announced the city’s first flu death of the season on October 1; last winter, the city recorded no flu deaths until January. Several Bay Area counties are urging residents to get vaccinated sooner than usual. Doctors generally recommend a flu shot in October to build immunity as cases rise.

Chin-Hong said the early start does not necessarily mean a more severe flu season. The Bay Area’s summer COVID-19 wave, meanwhile, is tapering off.