To demonstrate the efficacy of MVA-BN® by assessing non-inferiority of MVA-BN® compared to ACAM2000® in terms of vaccinia-specific Plaque Reduction Neutralization Test (PRNT) antibody response at the Peak Visits (Day 42 for Group 1 and Day 28 for Group 2) and by showing that vaccination with MVA-BN® prior to administration of ACAM2000® results in an attenuation of take.