Influenza vaccine effectiveness against outpatient acute respiratory illness with laboratory-confirmed influenza, United States, 2024-25 season.

BACKGROUND

Influenza A(H1N1)pdm09 and A(H3N2) viruses predominated during the 2024-25 U.S. influenza season. We estimated influenza vaccine effectiveness (VE) in the United States against mild-to-moderate outpatient influenza illness by influenza type and subtype in the 2024-25 season.

METHODS

We enrolled outpatients aged ≥8 months with acute respiratory illness symptoms including cough from outpatient health clinics, urgent care clinics, and emergency departments in 7 states. Upper respiratory specimens were tested for influenza type/subtype by reverse-transcriptase polymerase chain reaction (RT-PCR). Influenza VE was estimated with a test-negative design comparing odds of testing positive for influenza among vaccinated versus unvaccinated participants controlling for age, study site, underlying health status, and month of illness onset. We also estimated VE of current season vaccination among adults stratified by prior season vaccination status.

RESULTS

Among 6,793 enrolled patients, 2,016 (30%) tested positive for influenza including 961 A(H3N2), 770 A(H1N1)pdm09, and 183 B/Victoria. Overall vaccine effectiveness against any influenza illness was 33% (95% Confidence Interval [CI]: 24 to 41): 27% (95% CI: 14 to 39) against influenza A(H3N2), 37% (95% CI: 24 to 48) against A(H1N1)pdm09, and 40% (95% CI: 12 to 59) against B/Victoria. We did not detect statistically significant interaction between current and prior season vaccination.

CONCLUSIONS

Influenza vaccination during the 2024-25 season protected against circulating influenza viruses, reducing the risk of outpatient medically attended influenza overall by approximately one-third among people who were vaccinated.