US military service members vaccinated against smallpox in 2003 and 2004 experience a slightly higher risk of hospitalization postvaccination

This study explores adverse events severe enough to warrant hospitalization that may have been associated with receiving the smallpox vaccine in conjunction with military service. Cox proportional hazards modeling was used to identify the risk of hospitalization among US active-duty military personnel during a 1-year period following receipt of the smallpox vaccine. The reference group consisted of active-duty military personnel who also received the smallpox vaccine after the conclusion of their health care observation period, allowing for comparison to a temporally and demographically similar population. The risk of hospitalization was slightly elevated among the postvaccine group for any-cause hospitalization and for hospitalization in several broad diagnostic categories. Hospitalizations for asthma, autoimmune diseases, and myopericarditis, were more likely in the postvaccine group. The increased risk of hospitalization for varied outcomes does not necessarily imply a cause–effect relationship, but it does offer areas for more focused study, using longitudinal data to explore the long-term impact of smallpox vaccination on the health of young adults.