Objectives: To compare rates of myopericarditis, severe and serious dermatological or neurological events, and other adverse events in deploying US military personnel who received or did not receive ACAM2000® (Smallpox [Vaccinia] Vaccine, Live) vaccine and to evaluate potential risk factors for development of myopericarditis.

Methods: Prospective observational cohort study enrolling up to 15,000 ACAM2000 recipients (Cohort 1) and up to 5000 persons otherwise eligible for ACAM2000 vaccination but not vaccinated due to recency of vaccination or characteristics of their contacts (Cohort 2). Data and specimens were collected initially and 10 (6-17) days later. Those with clinical or laboratory evidence of possible myopericarditis were referred for further evaluation and adjudication by a blinded independent review committee. The adjusted odds ratio for myopericarditis was determined by a logistic regression model controlling for age, race, gender, and exercise regimen.

Results: 14,667 subjects provided initial data and specimens (Cohort 1, 10,825; Cohort 2, 3842); 12,110 (Cohort 1, 8945; Cohort 2, 3165) completed Visit 2 per-protocol. A total of 125 (Cohort 1, 111; Cohort 2, 14) were referred for myopericarditis adjudication, yielding 54 (Cohort 1, 44, Cohort 2, 10) subclinical myopericarditis, 5 suspected myocarditis, 1 confirmed myocarditis, and 1 suspected pericarditis. Unadjusted myopericarditis rates were: Cohort 1, 5.7/1000 (95% CI, 4.3-7.5); Cohort 2, 3.2/1000 (95% CI, 1.7-5.8). Unadjusted and adjusted odds ratios for myopericarditis were 1.8 (95% CI: 0.9-3.6) and 1.3 (95% CI: 0.6-2.6), respectively. One hundred seventeen subjects (1.1%) in Cohort 1 and 13 (0.3%) in Cohort 2 experienced at least 1 serious adverse event. No instances of serious and severe neurological or dermatological adverse events were reported.

Conclusions: In this carefully screened, generally young and healthy service-member population, ACAM2000 vaccination was associated with modest non-significant increases in the risk of myopericarditis (adjusted OR, 1.3; unadjusted OR, 1.8); all but seven cases were subclinical.

Clinical trials registration: ClinicalTrials.gov NCT00928577.