Flu vaccine effectiveness (VE) changes every season, depending on how well the vaccine matches circulating viruses and on who is being vaccinated. California can estimate it mid-season, because state law requires labs to report flu test results — positive since October 1, 2019, negative since June 15, 2023 — and, since January 1, 2023, requires providers to record flu shots in the state immunization information system.
A worrying season
From October 1, 2025 to January 31, 2026, flu caused an estimated 22–38 million illnesses, 280,000–590,000 hospitalizations and 12,000–60,000 deaths across the United States; California's season was of moderate severity, like the nation's. There was extra concern this year: an antigenically drifted influenza A(H3N2) subclade K virus was spreading — 92% of genetically characterized U.S. viruses, and 108 of 116 sequenced A(H3N2) viruses (93%) in California.
How the estimate works
The California Department of Public Health (CDPH) compared people aged 6 months and older who tested positive for flu with those who tested negative, and checked how many in each group had a 2025–26 flu shot at least 14 days before testing. If vaccinated people are underrepresented among positives, the vaccine is working. Estimates were adjusted for age, race and ethnicity, week and county.
| Test results | 952,765 |
| Positive | 86,369 (9%) — 96% influenza A, 4% influenza B |
| Vaccinated | 22% of positives, 27% of negatives |
Of influenza A results with a subtype, 76% were A(H3N2) and 24% A(H1N1)pdm09.
The results
| Group | VE against a positive flu test |
|---|---|
| Everyone | 33% |
| Against influenza A | 32% |
| Against influenza B | 47% |
| Under 18 | 39% |
| 18–49 | 34% |
| 50–64 | 31% |
| 65 and older | 22% |
By vaccine type
| Group | Vaccine | VE |
|---|---|---|
| Ages 2–17 | nasal spray (live attenuated, LAIV) | 55% |
| Ages 2–17 | standard-dose shot | 39% |
| 65 and older | recombinant | 39% |
| 65 and older | adjuvanted or high-dose | 22% |
| 65 and older | standard-dose | 16% |
93% of vaccinated adults 65 and older received one of the preferentially recommended vaccines — high-dose, adjuvanted or recombinant.
What it means
- The vaccine protected every age group, most strongly the young, and better against influenza B.
- It still worked against the drifted H3N2 strain; Canada estimated 40% against A(H3N2).
- California's figures match earlier seasons and were shared with local health departments in January 2026 to shape vaccination and treatment messages.
- Everyone 6 months and older should get a flu vaccine each year — and it's not too late while flu is circulating. Early antiviral treatment lowers the risk of severe illness and hospitalization.
Limits: the estimates are early and California-only; test and vaccination data may be incomplete; young children needing two doses may be misclassified as fully vaccinated; there were no data on hospitalization or testing setting; and prior vaccination, health conditions and care-seeking weren't accounted for.
Sources
Based on Sophie Zhu, Joshua Quint, Tomás M. León and others, "Interim Estimates of 2025–26 Seasonal Influenza Vaccine Effectiveness — California, October 2025–January 2026," MMWR, volume 75 (as corrected), Centers for Disease Control and Prevention; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from www.cdc.gov
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