Mid-Season Influenza Vaccine Effectiveness Against Laboratory-Confirmed Ambulatory Influenza: U.S. Active Component Service Members

Image of 48088663. This report provides an overview of the 2023-2024 mid-season analysis of influenza vaccine effectiveness against medically-attended ambulatory influenza infections.

This Surveillance Snapshot provides an overview of the 2023-2024 mid-season analysis of influenza VE against medically-attended ambulatory influenza infections among active component service members. Data from the Defense Medical Surveillance System, the NHRC’s respiratory surveillance program, and standardized laboratory data provided by the Defense Centers for Public Health–Portsmouth were used for this analysis.1,2 A case test-negative study design was implemented among the population of ACSMs from all services who were tested for influenza between December 1, 2023 and February 23, 2024—the period of peak influenza activity for the season. 

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, prior vaccination [any influenza vaccine in previous 5 years], and month of diagnosis) and 95% confidence intervals. VE estimates were defined as (1 - OR) x 100. 

Click on the table to access a 508-compliant PDF version

There were 3,540 cases—2,794 A (any subtype), 246 A(H3N2), 119 A(H1N1)pdm09, 751 B (any type)—and 16,411 TNCs. TNCs were more likely vaccinated (85.4%) than cases (82.5%). VE varied by influenza type (Table). Statistically significant VE was found against any influenza case with an adjusted VE of 21% (95% CI: 13, 29) and influenza A (any subtype) with an adjusted VE of 26% (95% CI: 18, 34). The VE point estimate against influenza A(H1N1)pdm09 and A(H3N2) showed effectiveness, but did not reach statistical significance (23% [95% CI: -23, 51] and 28% [95% CI: -1, 49], respectively). This mid-season assessment did not find the vaccine to be effective against influenza B ambulatory infections (-5% [95% CI: -30, 15]). 

The results of this analysis show low protection of the 2023-2024 seasonal influenza vaccines against medically-attended influenza A infections that resulted in ambulatory care visits among ACSMs. As these estimates were obtained during the middle of the influenza season, VE estimates and CIs may change when data from the full season are available and sample sizes increase. 

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 
  2. https://www.med.navy.mil/Naval-Medical-Research-Command/R-D-Commands/Naval-Health-Research-Center/Core-Research/Operational-Infectious Diseases/respiratory-surveillance-2023-2024

You also may be interested in...

Topic
Feb 3, 2026

Medical Surveillance Monthly Report

The Medical Surveillance Monthly Report, a peer-reviewed journal launched in 1995, is the Armed Forces Health Surveillance Division's flagship publication. The MSMR provides monthly evidence-based estimates of the incidence, distribution, impact, and trends of health-related conditions among service members.

Report
Dec 1, 2025

MSMR Vol. 32 No. 12 - December 2025

.PDF | 2.90 MB

The December 2025 issue of MSMR features four full reports: the annual update on cold weather injuries among the active and reserve components; a report on trends in the prevalence of obesity among U.S. active component service members and civilians; the annual update on diagnoses of mental health disorders among active component service members; a ...

Article
Dec 1, 2025

Update: Cold weather injuries among the active and reserve components of the U.S. Armed Forces, July 2020–June 2025

Since 2004, MSMR has published annual updates on the incidence of cold weather injuries affecting U.S. Armed Forces members for the five most recent cold seasons. This 2025 report discusses the occurrence of frostbite, immersion hand and foot injuries, hypothermia, as well as “other specified and unspecified effects of reduced temperature.”

Report
Nov 1, 2025

MSMR Vol. 32 No. 11 - November 2025

.PDF | 3.56 MB

The November 2025 MSMR presents four full reports on sexually transmitted infections (STIs), led by the annual update on STIs among active component service members, 2016–2024; followed by a report on follow up testing for chlamydia or gonorrhea among male Air Force basic trainees, 2017–2023; accompanied by a report on sexual networks of service ...

Report
Oct 1, 2025

MSMR Vol. 32 No. 10 - October 2025

.PDF | 2.90 MB

The October 2025 MSMR presents five full reports: measles, mumps, rubella and varicella within the Military Health System; seasonal influenza hospitalization among service members; BMI, physical fitness, and COVID-19 hospitalization in the Army; adverse pregnancy outcomes in female service members after COVID-19 infection or vaccination; long COVID ...

Refine your search