The flu season is off to an unusually early and aggressive start in the Bay Area, with positive tests already exceeding last winter’s peak and hospitals seeing patient volumes typically associated with January and February.
Flu test positivity in San Francisco began climbing in September, roughly two months ahead of its usual seasonal pattern, and reached 23%, compared with last winter's peak of 20% much later in the season, as the Chronicle reports. The flu typically arrives on the West Coast later than it does in the eastern and southern United States, but this year, California is reportedly experiencing an unusually early surge.
According to Bay Area News Group, the numbers are also showing up in hospitals. Bay Area patients are being admitted with influenza at rates comparable to the height of last winter's flu season in January 2026. Rakesh Chaudhary, physician-in-chief at Kaiser Permanente Santa Clara, said hospital activity already feels more like the middle of winter.
“Our hospital feels as busy as it does usually in January and February,” he said, speaking to the News Group.
Across the Bay Area, 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,” per the News Group. California has also recorded 120 influenza-related deaths so far this season.
The Chronicle reports that doctors aren't 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.
UCSF infectious diseases expert Dr. Peter Chin-Hong told the paper 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 is causing more severe illness. The updated seasonal vaccine also 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, according to the Chronicle. 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 told the Chronicle that people who have been putting off their vaccinations should get them now. The vaccine can reduce the risk of medical visits by 40% to 60% when it is well matched to circulating strains, according to the CDC.
People who already received their flu shot earlier in the fall reportedly do not need another one if the season stretches into the new year.
Related: Bay Area Wastewater Shows Rising Levels of COVID, Flu, and RSV After Holidays
