The purpose of this study is to evaluate an investigational smallpox vaccine, called IMVAMUNE®, with respect to safety and immune (body's defense system) response. Participants will include healthy adults, age 18 or older born after 1971, who have not had smallpox vaccine before. Participants were originally assigned to 1 of 5 groups. In July 2007, a hold was placed on the Dryvax® groups and the study was modified. Participants, will be assigned by chance to one of 3 groups to be vaccinated twice with IMVAMUNE® vaccine or placebo (inactive substance) in Groups A and B, or to receive a single vaccination with IMVAMUNE® or placebo in Group F. Participants will complete a memory aid (diary) for 15 days following vaccination. Blood samples will be collected. Participants may participate for up to 425 days.
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Geometric Mean Titer (GMT) of Bavarian Nordic's (BN) Plaque Reduction Neutralizing Antibody Titer (PRNT) Assay in Groups A and B
Timeframe: Day 14 after the second vaccination
GMT of Saint Louis University's (SLU) PRNT Assay in Group A and Group B
Timeframe: Day 14 after the second vaccination
Frequency of Serious Adverse Events (SAEs)
Timeframe: Day 0 after first vaccination to study completion through Day 365 after last vaccination
Frequency of Non-Serious AEs
Timeframe: Day 0 after first vaccination through Day 28 after last vaccination
Frequency of Local Solicited Reactogenicity AEs for Groups A, B, and D and Placebo A, B, and D
Timeframe: Day 0 through Day 15 after first vaccination; Day 0 through Day 15 after second vaccination
Frequency of Local Solicited Reactogenicity AEs for Groups C and F and Placebo F
Timeframe: Day 0 through Day 15 after first vaccination
Frequency of Local Solicited Reactogenicity AEs for Group E and Placebo E
Timeframe: Day 0 through Day 15 after first vaccination
Frequency of Systemic Solicited Reactogenicity AEs for Groups A, B, and D and Placebo A, B, and D
Timeframe: Day 0 through Day 15 after first vaccination; Day 0 through Day 15 after second vaccination
Frequency of Systemic Solicited Reactogenicity AEs for Groups C, E, and F and Placebo E and F
Timeframe: Day 0 through Day 15 after first vaccination