Concurrent Vaccinations and U.S. Military Hospitalizations

Purpose To investigate whether concurrent vaccinations (≥2 vaccinations on consecutive days) are associated with hospitalization risk among U.S. military personnel. Methods We analyzed Defense Medical Surveillance System data from January 1998 through December 2003 for 117,876 active component U.S. military personnel. We performed a time-to-event analysis of a historical cohort using a Cox proportional hazards model comparing hospitalizations during a 120-day postvaccination exposure interval to hospitalizations within a 120-day pre-exposure interval. We excluded personnel who were deployed during these intervals and those having hospitalizations 60 days prior to the concurrent vaccination exposure. Hazards ratios (HRs) with 95% confidence intervals were calculated, adjusting for demographic, occupational, health, and calendar variables. Results We analyzed 19,743 persons having concurrent vaccinations. Receiving two or more vaccinations concurrently was not statistically associated with the adjusted risk of hospitalization (HR = 0.90 [0.75, 1.09]). Furthermore, no statistically significant associations were detected for 3 concurrent vaccinations (HR = 0.86 [0.58, 1.28]), 4 concurrent vaccinations (HR = 1.08, [0.66, 1.74]), or five or more concurrent vaccinations (HR = 0.86 [0.49, 1.51]). Conclusions No evidence was found that the concurrent receipt of multiple vaccinations is related to hospitalization risk among this sample of U.S. military personnel.