<qa>  What is the primary objective of the phase III study of MVA vaccine?This is a phase III, randomized, placebo-controlled, double-blinded, study of MVA vaccine. The hypothesis is that MVA will be safe in vaccinia-immune adults when administered by intramuscular injection, and that this vaccine will result in an immunologic response comparable to that observed after Dryvax Registered Trademark vaccination in previously vaccinia-immunized persons. The primary objectives include: evaluating the safety of MVA administered by intramuscular IM injection at 108 PFU, and determining whether MVA provides clinical evidence of protection against a 12-week Dryvax challenge when compared to placebo groups. The secondary objectives are to identify the schedules of MVA administration that provide clinical evidence of protection against Dryvax Registered Trademark challenge and compare the Immunogenicity of Dryvax Registered Trademark and MVA as measured by vaccinia-specific neutralizing antibody, ELISA and intracellular cytokine assays, as well as variola-specific neutralizing antibody assays that will be conducted at the CDC on a subset of samples. 