Participants will actively participate in this study for 12 months and will then be contacted and asked questions about their health once annually for 3 years following initial study injection. Participants will be randomly assigned to receive either the SAAVI prime-boost preventive vaccine regimen or placebo. Vaccination with the SAAVI DNA-C2 vaccine will occur at Months 0, 1, and 2; boost vaccinations with the SAAVI MVA-C vaccine will occur at Months 4 and 5. Additional study visits will occur at Weeks 2, 6, 10, 16, 18, and 20 and Days 147, 154, 273, and 364.