Health officials in California and Washington reported an uncharacteristically early rise in seasonal influenza cases in September, pointing to a mutated virus strain as a possible driver and urging residents to receive updated flu shots.
Fall has barely begun, but seasonal influenza is already taking root across parts of the United States. While the virus usually waits until October or November to ramp up before peaking between December and March, public health tracking indicates that activity started climbing as early as September (as reported by NPR).
West Coast Sees Unusually Early Surge
That early momentum is concentrated heavily in Pacific states. In Los Angeles County, local health reporting shows flu activity began rising in September rather than waiting for its usual early-November arrival, according to Dr. Dawn Terashita (noted by the Los Angeles Times).
Helen Chu, a respiratory infections specialist at the University of Washington School of Medicine, noted that flu in the United States traditionally starts in the South and Southeast before migrating up and west, making the Pacific Northwest and the West Coast regions that typically get the flu last.
Caitlin Rivers, an American epidemiologist at the Johns Hopkins Center for Health Security, noted in her newsletter Force of Infection that something afoot was occurring. Meanwhile, the atypical timing has been pointed out by both Thomas Russo, M.D., professor and chief of infectious disease at the University at Buffalo in New York, and Adalja, senior scholar at the Johns Hopkins Center for Health Security. Dr. Russo expressed surprise that the activity was happening earlier this year. Kaylan Christianer, a primary care doctor and assistant professor of medicine at Columbia University Vagelos College of Physicians and Surgeons in New York, concurred with the evaluation.
Public health guidelines from agencies such as Berkeley Public Health advise residents to protect themselves, their families, and their communities from respiratory illnesses by getting vaccinated, masking in higher-risk settings, and staying home when sick. Free flu vaccines are available through Berkeley Public Health by making an appointment at the West Berkeley Immunization Clinic, where no ID or insurance is required.
Virologists Track Circulating Strain
Virologists and epidemiologists investigating the early surge point to a shifting viral environment. Last year, by contrast, H3N2 became globally dominant, specifically a strain featuring several unique mutations that left some people vulnerable.
Data from August 2025 led the CDC to first identify a new influenza A (H3N2) virus subclade, which has been labeled subclade J.2.4.1 or subclade K.
“Because it certainly, on paper, looks like a virus that could be a little bit more easy to spread and maybe infect more people because it’s better at evading that immunity that you have.”
Dr. Andrew Pekosz, a professor at Johns Hopkins Bloomberg School of Public Health in Baltimore, via HuffPost
Additional factors may also be playing a role in the accelerated timeline.
Officials Issue Vaccination Guidance
With virus circulation already elevated, public health departments and clinical specialists are advising residents not to wait until late fall to get immunized (urged by the San Bernardino County Department of Public Health). Dr. Sharon Wang, San Bernardino County health officer, noted that simple steps like getting vaccinated and testing when symptoms appear keep communities healthy and protect vulnerable people at higher risk of severe infection. The official respiratory virus season spans from November 1 through April 30 (noted by San Bernardino County officials), but doctors argue that the current trajectory necessitates earlier action (reported by NPR).
Despite the early rise in outpatient visits, health authorities emphasize that an early start does not automatically signal an overall more severe season (noted by the Los Angeles Times). Hospital admissions for influenza remain at low levels across California and other affected regions (reported by the Los Angeles Times). Even so, clinicians recommend securing an updated flu shot promptly to mitigate the risk of severe complications and hospitalization (advised by city and county health officials). Readers with questions regarding vaccine coverage, specific symptom recognition, or personal risk factors are encouraged to consult qualified healthcare professionals or local health departments to determine the appropriate clinical steps for their health needs.
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