A Study of Safety, Tolerability and Immunogenicity of a 1-Dose Regimen of MRKAd5 HIV-1 Gag Vaccine Versus the ALVAC_HIV Vaccine (V520-019)

Inclusion Criteria: * Subject was previously enrolled in Merck HIV Vaccine study V520 Protocol 007 or 012, and received the complete 3-dose regimen during participation * Subject is in good general health * Subject is not infected with HIV * Subject agrees not to donate blood during the first 52 weeks of the study * Subject agrees not to donate sperm during the first 12 weeks of the study * Subject who is of reproductive potential agrees to use an acceptable method of birth control through Week 12 of the study Exclusion Criteria: * Subject has been administered immune globulin or blood product 3 months prior to receiving study vaccination * Subject has been vaccinated with a live virus vaccine in the past 30 days * Subject has been vaccinated with an inactivated vaccine in the past 14 days * Subject has an active medical disease * Subject is taking daily required prescription drugs * Female subject is pregnant, breastfeeding or expecting to conceive * Subject is positive for HIV * Subject has used injection drug within the past year * Subject has a sexual partner that is infected with HIV * Subject has a sexual partner that is an active injection drug user * Subject has been treated for or diagnosed with a new sexually transmitted disease in the past 6 months * Subject engaged in unprotected intercourse with more than 2 persons in the previous 6 months * Subject engaged in protected intercourse with more than 4 persons in the previous 6 months * Subject has previously participated in an HIV vaccine clinical trial (other than Merck V520 Protocols 007 and 012) * Subject weighs less than 110 lbs. * Subject has a recent history of alcohol abuse * Subject intends to donate blood in the first 52 weeks of the trial