Expanded Characterization of Immune Response to Merck Adenovirus 5 Gag/Pol/Nef Vaccine Given to HIV Uninfected Adults

This study will look for relationships among the immune responses induced by MRKAd5 HIV-1 gag/pol/nef vaccine. The study will also determine if the T cells that respond to different vaccine epitopes have correspondingly different functional profiles. The study will evaluate the safety and tolerability of the vaccine regimen as well. This study will last 60 weeks. All enrolled participants will receive vaccinations at Weeks 0, 4, and 26. There will be between 8 and 20 study visits including the screening visit, depending on the site location. A physical exam, interview, and blood collection will occur at most or all visits. All participants will undergo leukapheresis approximately 4 weeks after their last vaccination and at Week 52. Medical history, an HIV test, a pregnancy test, and HIV and risk reduction counseling will occur at selected visits. Additional blood collection is now occurring in this study to collect more information about the relationship between the immune response and efficacy to the vaccine.