Seasonal influenza hospitalization incidence rates among U.S. active component service members, 2010-2024

Despite a longstanding U.S. Department of Defense (DOD) requirement for seasonal influenza vaccination of active component service members (ACSMs), quantifying the impact of the DOD immunization program is challenging. To measure the burden of severe influenza among this highly immunized ACSM population, this study evaluated seasonal and cumulative seasonal influenza hospitalization rates among ACSMs from 2010 through 2024, stratifying by sex, age group, race and ethnicity, service branch, recruit site, and location (U.S. vs. non-U.S.). In contrast to Centers for Disease Control and Prevention (CDC) U.S. population data, the highest ACSM cumulative seasonal influenza hospitalization rate was in the age group under 25 years (9.3 per 100,000 person-years [p-yrs]) and recruits (70.1 per 100,000 p-yrs). Non-U.S.-based ACSMs had lower influenza hospitalization rates (4.8 per 100,000 p-yrs) compared to ACSMs in the U.S. (8.0 per 100,000 p-yrs). Within the DOD, cumulative seasonal influenza hospitalization rates were highest in the youngest age group, particularly among recruits. This may influence DOD influenza vaccine distribution priority considerations in the future. Compared to U.S. national data, in which adult seasonal influenza hospitalization rates increase with age, the highest cumulative hospitalization rate among ACSMs occurred in the youngest age group, those younger than age 25 years, especially in recruit settings. Lower cumulative rates of seasonal influenza hospitalization in older age groups of ACSMs help quantify the impacts of the longstanding DOD vaccination requirement for influenza. The higher burden of hospitalization among recruits offers DOD vaccine distribution priority considerations in the future.