A Worldwide, Phase I, Dose-Escalating Study of a 3-Dose Regimen of the MRKAd5 (Clade B) Vaccine in Healthy Adults (V520-018)

This three stage study will evaluate the safety and tolerability of MRKAd5 HIV-1 Gag Vaccine. In Stage I subjects will be randomized to receive the vaccine at 1x10\^9 viral particles/dose (vp/d) or placebo. In Stage II subjects will be randomized to receive the vaccine at 1x10\^10 vp/d or placebo. In Stage III subjects will be randomized to receive the vaccine at 1x10\^9 vp/d, at 1x10\^10 vp/d, or placebo. Immunogenicity of the single dose regimen of MRKAd5 HIV-1 Gag Vaccine will also be measured.