This Surveillance Snapshot provides an overview of the 2024-2025 mid-season analysis of influenza vaccine effectiveness against medically-attended ambulatory influenza infections among active component U.S. service members.

A case test-negative study design was implemented among the population of active component service members from all services who were tested for influenza between December 1, 2024 and February 8, 2025—the period of peak influenza activity for the season. Data from the Defense Medical Surveillance System and standardized laboratory data provided by the Defense Centers for Public Health–Portsmouth were used for this analysis.¹ Cases were defined as individuals with a positive influenza result from a rapid antigen, reverse transcription polymerase chain reaction, or culture influenza assay. Test-negative controls were individuals with a negative influenza result from a RT-PCR or culture influenza assay. Crude odds ratios were calculated, and multivariate logistic regression was used to calculate adjusted ORs (adjusted for sex, age category, any influenza vaccination in the previous five years, and month of diagnosis) and 95% confidence intervals. Estimates of VE were defined as (1 – OR) x 100.

There were 4,677 cases (4,413 A any subtype, 389 A[H3N2], 163 A[H1N1]pdm09, 274 B any subtype) and 12,787 TNC. Ten cases were both influenza A- and influenza B-positive. VE varied by influenza type (Table). Adjusted VE estimates for all influenza types and subtypes reached statistical significance. The results indicate moderate mid-season protection against influenza A(H1N1)pdm09 (adjusted VE 41%; 95% CI, 14-60) and A(H3N2) (adjusted VE 50%; 95% CI, 36-60) and low mid-season protection against influenza A (any subtype) (adjusted VE 14%; 95% CI, 5-22) and influenza B (adjusted VE 33%; 95% CI, 9-50).

The results of this analysis show low to moderate mid-season protection of the 2024-2025 seasonal influenza vaccines against medically-attended influenza A and B infections that resulted in an ambulatory care visit among ACSMs. As these estimates were obtained during the middle of the influenza season, VE estimates and confidence intervals may change when data for the full season are available and the sample sizes increase.

Authors’ Affiliation

Epidemiology and Analysis Branch, Armed Forces Health Surveillance Division, Public Health Directorate, Defense Health Agency, Silver Spring, MD: Dr. Cost, Ms. Hu

References 

1. Rubertone MV, Brundage JF. The Defense Medical Surveillance System and the Department of Defense serum repository: glimpses of the future of public health surveillance. Am J Public Health. 2002;92(12):1900-1904. doi:10.2105/ajph.92.12.1900