Influenza vaccine effectiveness against influenza A-associated hospitalization and severe in-hospital outcomes among adults in the United States, 2024-2025.

Background

The U.S. 2024-2025 influenza season was characterized by sustained elevated activity from November 2024-April 2025, with circulation of both influenza A(H1N1)pdm09 and A(H3N2), the latter of which included some antigenically drifted viruses.

Methods

From October 1, 2024, to April 30, 2025, a multistate respiratory virus surveillance network enrolled adults hospitalized with acute respiratory illness in 26 U.S. medical centers. Influenza vaccine effectiveness (VE) against influenza-associated hospitalization and severe in-hospital outcomes was estimated using a test-negative study. The odds of influenza vaccination among influenza-positive case patients and influenza-negative control patients were compared using multivariable logistic regression; VE was calculated as (1 - adjusted odds ratio for vaccination) × 100, expressed as a percent.

Results

The 2024-2025 seasonal influenza vaccine was effective against influenza-associated hospitalization (VE: 40% [95% confidence interval (CI): 32%-47%]), consistent across age group and influenza type/A subtype. Influenza vaccination also reduced the overall risk of all severe in-hospital outcomes evaluated, including standard oxygen therapy (VE: 41% [95% CI: 31%-50%]), non-invasive advanced respiratory support (VE: 38% [95% CI: 19%-52%]), invasive organ support (VE: 58% [95% CI: 44%-69%]), ICU admission (VE: 58% [95% CI: 47%-67%]), and death (VE: 52% [95% CI: 18%-71%]) with effectiveness varying by influenza A subtype and age.

Conclusions

Influenza vaccination reduced the risk of influenza-related hospitalization and severe in-hospital outcomes in adults during the severe 2024-2025 influenza season compared with those not vaccinated.