Flu test positivity in San Francisco began rising in September—roughly two months ahead of the usual seasonal pattern—and reached 23%, compared with last winter's peak of 20% much later in the season. While the flu typically arrives on the West Coast later than in the eastern and southern United States, California is now experiencing an unusually early surge.

Hospitals are feeling the impact. Bay Area patients are being admitted with influenza at rates comparable to the height of last winter's flu season in January 2026. Dr. Rakesh Chaudhary, physician-in-chief at Kaiser Permanente Santa Clara, said the hospital's activity already resembles mid-winter.

"Our hospital feels as busy as it does usually in January and February," Chaudhary said.

Wastewater surveillance has also detected high levels of influenza in Santa Clara County, San Francisco, and Marin County. Statewide, more than one-fifth of flu tests came back positive during the last week of October—a rate the California Department of Public Health classified as "very high." California has recorded 120 influenza-related deaths so far this season.

Doctors are not certain why the West Coast is seeing such an early wave, but one possibility is the region's close ties and travel connections with Asia. That region experienced a summer outbreak of H1N1, the same influenza A subtype now circulating in the United States. Dr. Peter Chin-Hong, an infectious diseases expert at UCSF, told the Chronicle that the West Coast may have encountered the strain before other parts of the country because of those connections.

The current strain is a slightly mutated version of H1N1, the virus responsible for the 2009 swine flu pandemic. However, doctors say there is no indication that the circulating version causes more severe illness, and the updated seasonal vaccine appears to be a good match.

Dr. Brad Hare, chief of infectious diseases at Kaiser Permanente San Francisco, expects the current wave could peak within a few weeks, followed by another increase later in the winter as a different strain—potentially influenza B—begins circulating. That could mean a longer season with multiple waves rather than one major peak.

The flu vaccine is recommended for everyone over 6 months old, according to the California Department of Public Health. It is particularly important for young children, older adults, and people with compromised immune systems or underlying conditions such as asthma and heart disease, who face greater risks of complications and hospitalization.

Hare said people who have been putting off their vaccinations should get them now. When well matched to circulating strains, the vaccine can reduce the risk of medical visits by 40% to 60%, according to the CDC. People who already received their flu shot earlier in the fall do not need another one if the season stretches into the new year.