Monitoring Influenza in a Military Setting in Germany: Its Relevance in the 2023/2024 Season and Comparison to the Civil Population.

Abstract

Introduction: Acute airway infections (AAI) can jeopardize force readiness. Influenza, as one of the main viral agents that cause AAI, is highly contagious and may lead to severe illness and even death. Consequently, AAI and influenza surveillance in the German Armed Forces (Bundeswehr) is crucial for timely monitoring its seasonal impact. With this retrospective analysis, we assessed the burden of disease in comparison with civilian surveillance data in order to evaluate potential risk and preventive factors for influenza infection.

Materials and methods: From October to March of each year, military outpatient sentinel medical treatment facilities (MTF) report weekly numbers of AAI cases. Additionally, voluntary virological laboratory tests are conducted via RT-qPCR and aggregated influenza vaccination rates of all military personnel are monitored. With this retrospective analysis, we report AAI proportions and incidences as well as positivity rates (PR) of 7 viral agents for the 2023/2024 season. Influenza PR were analyzed for potentially confounding risk factors. Finally, we conducted a stratified analytic comparison with data from the national influenza surveillance system by the Robert Koch Institute (RKI).

Results: During the 2023/2024 season, our surveillance observed 18,999 soldiers served by 14 MTF throughout Germany. A total of 13,431 cases of AAI represented 27% (weekly range: 18.4%-44.8%) of all acute medical encounters. The 7-day-incidences of AAI varied from 420/100,000 to 5,686/100,000. A total of 3,636 virological laboratory tests were conducted. SARS-CoV-2 (17.3%), human rhinovirus (15.5%), and influenza A + B (4.4%) were the most relevant viruses detected. The statistical analysis of the military dataset showed a significantly elevated odds ratio (OR) for influenza and fever. Odds ratio for influenza and sex were slightly elevated for men in both groups but without clinical relevance [military OR 1.25 (95% CI 0.78-2.02); civilian OR 1.26 (95% CI 1.03-1.51)]. Age seemed to be a risk factor, however, as evident in rising influenza PR with higher age and a significant mean value difference (MVD) between positive and negative patients in the military but not in the civilian dataset. Age-stratified comparative analyses between the military and the civilian dataset showed a significantly elevated influenza OR (P < .001) for civilian patients of all age groups except for those >60 years. Differences between age groups were greatest in subgroup 1 (17-29 years) and least in subgroup 5 (60-67 years). Overall, the age-stratified Mantel-Haenszel influenza OR estimate for an influenza infection in civilian versus military patients was 4.06 (95% CI 3.32-4.96).

Conclusions: Our analysis adds new aspects to the knowledge on influenza infections in soldiers and reveals relevant differences in terms of burden of disease between the observed German military personnel and the civilian population. With regard to the significantly lower influenza OR for soldiers, we assume that the considerably higher vaccination rate could have the greatest impact of all factors discussed.