Seasonal influenza activity in California is increasing with influenza A(H3N2) comprising the majority of currently circulating specimens. A newly emerged virus strain, H3N2 subclade K, is circulating globally and is now present in California. This strain is antigenically drifted from the H3N2 component of the 2025–26 seasonal influenza vaccine formulation and has been associated with earlier waves of influenza outbreaks in Canada, Japan, and the UK.
Key Messages
- Influenza cases and hospital admissions are increasing in California. A newly emerged influenza A virus strain, H3N2 subclade K, is circulating globally, including in California. The majority of influenza A specimens subtyped in California are H3N2. Current influenza vaccines are still expected to offer protection against severe disease.
- Administer 2025-26 influenza vaccine to anyone 6 months of age and older who has not yet received it.
- Test symptomatic patients as quickly as possible. Use rapid molecular (nucleic acid amplification detection) tests for outpatients, and RT-PCR and other influenza molecular tests for hospitalized patients to confirm diagnosis and guide timely treatment. Influenza A positive hospitalized patients, especially those in the ICU, should have specimens sent for subtyping.
- Initiate antiviral treatment without delay and ideally at point of care. Do not delay treatment while waiting for test results if influenza is suspected in high-risk patients. Treat immediately with oseltamivir or single-dose baloxavir based on recommendations for use. Treatment with influenza antivirals decreases the risk of serious illness and hospitalization in those at higher risk for severe disease.
- Stay aware of respiratory virus activity in your community by visiting Orange County Respiratory Virus Data or CDPH Respiratory Virus Dashboard this respiratory season.
Advisory: Influenza Update: Vaccinate, Test, and Administer Influenza Antivirals
Resources: Orange County Respiratory Virus Data
