 Despite the fact that the World Health Organization WHO officially declared smallpox to be eradicated, a new threat exists due to the potential use of variola virus as an agent for biological warfare andor bio-terrorism. As a consequence, there is an urgent need for a safe and efficacious vaccine to protect the public against smallpox. To date, the majority of clinical studies with Modified Vaccinia Ankara MVA have studied a prime-boost vaccination regimen, with a dose of up to 1×108 tissue culture infectious dose 50 TCID50 of MVA administered on Days 0 and 28. While this vaccination regimen induces a robust immune response that is protective in a variety of animal models and is appropriate for a pre-smallpox release scenario, in the event of a confirmed release of smallpox, a more rapid vaccination regimen that provides a protective immune response would be desirable. Ideally, at least short-term protection could be obtained with a single dose of vaccine. While a single dose of MVA at 1×108 TCID50, does induce an immune response in the majority of recipients, it is possible that a higher dose of MVA could provide a more rapid andor stronger immune response relative to a single, standard 1×108 TCID50 dose of MVA. The goal of this study is to examine the kinetics and magnitude of the immune response of a single high dose of MVA 5×108 TCID50 relative to both a single and primeboost regimen using the standard doses 1×108 TCID50 of MVA. This study will complement a current, ongoing study, DMID Protocol 06-0012, which is examining the immune response to compressed primeboost dosing regimens of MVA administered at 1×108 TCID50. Upon the completion of these studies, clinical data will be generated which will inform policy makers about different options for post-event utilization of available smallpox vaccines. The study is designed as a randomized, non-placebo controlled, double blinded study containing two arms: Group A N45 will receive a single high dose of IMVAMUNE 5x108 TCID50, consisting of two 0.5 mL injections vaccine on Day 0 and a single saline placebo dose single 0.5 mL injection on Day 28 to match the two dose regimen of Group B. Group B N45 will receive a standard two dose regimen of IMVAMUNE 1x108 TCID50 vaccine on Day 0 consisting of 0.5 mL injection of vaccine and 0.5 mL injection of saline placebo and Day 28 single 0.5 mL injection of vaccine. Safety will be measured by assessment of adverse events for 28 days following the last vaccination 56 days following the initial vaccination for those subjects that fail to receive the second dose and for serious adverse events at six months post the final vaccination, and reactogenicity to the vaccines for 15 days following each vaccination. Immunogenicity testing will include antibody testing enzyme linked immunosorbent assay ELISA and plaque reduction neutralizing antibody titers PRNT and cellular immune responses INF-gamma enzyme linked immunospot ELISPOT following each vaccination and at six months post the final vaccination. In addition, ELISA responses, using MVA VR-1508 as the target antigen and PRNT using vaccinia WR Western Reserve as the target antigen will be explored. Participants will include 90 healthy, vaccinia-naïve adults, aged 18 and older born after 1971. Study duration will be approximately 13 months 7 monthssubject. 