The flu season in California is off to an aggressive start as cases climb earlier than usual and doctors recommend getting vaccinated.
“If people are considering getting the flu vaccine, and they haven’t yet, they should get it like now,” said Rakesh Chaudhary, physician-in-chief at Kaiser Permanente Santa Clara. “Because there is so much flu out there.”
Levels of flu strains in wastewater were measured at high levels in Santa Clara County, San Francisco and Marin County. Regionally, more than one-fifth of tests for the flu came back positive the last week of October, a “very high” rate, according to the California Department of Public Health. That rate, 21.3%, is higher than any point during the 2025-2026 flu season, the data show.
For most people, a case of influenza, known as the flu, is similar to the common cold. It poses greater health risks to children under 2 years old and older adults, and can lead to hospitalization. People with compromised immune systems or other conditions, such as asthma and heart disease, are at higher risk too.
In the Bay Area, patients are being admitted to the hospital due to the flu at a rate on par with the peak of last year’s flu season in January 2026.
“Our hospital feels as busy as it does usually in January and February,” he said.
Statewide, 120 deaths have been attributed to influenza so far this season.
Based on guidance from national medical associations, the California Department of Public Health recommends a flu shot for anyone older than 6 months. A chorus of physicians in the Bay Area recommends it every season, as well.
When a flu vaccine is a good match for the strains circulating, it can reduce trips to the doctor’s office by between 40% and 60%, according to research cited by the CDC. During the 2024-25 season, vaccination prevented 180,000 influenza-related hospitalizations nationally and 12,000 deaths.
Influenza strains mutate quickly, posing a challenge for scientists who design new vaccines to debut every fall. On the recommendation of the U.S. Food and Drug Administration, vaccine developers updated formulas this year to protect against strains expected to circulate in the Northern Hemisphere this season.
Among the updates, the FDA said the new vaccine is designed to better protect against subclade K, a mutation of an influenza virus that spread quickly last virus season and was described as a “super bug” by some media outlets.
The new vaccine covers three strains of the seasonal flu, according to the newsletter Your Local Epidemiologist, which tracks infectious disease trends. That “should mean good protection,” it said.
“Flu is rising quickly, and it takes about two weeks after a shot to build full protection,” wrote Matt Willis, the former health officer of Marin County, in the newsletter. “This week is the right time if you want to have that added security through the Thanksgiving holidays and beyond.”