- Participant naïve to previous smallpox vaccination and received the ACAM2000 vaccine (Cohort 1), OR, Participant has not received ACAM2000 vaccine within the prior 3 years, AND is ineligible for current ACAM2000 vaccination either because of recency of prior vaccinia vaccination or for reasons solely attributable to conditions or characteristics of their contacts (such as a healthy soldier who is married to someone with a contraindicated condition) (Cohort 2).