• Average of log10 HIV-1 RNA viral load [ Time Frame: At Weeks 58 and 63 ] [ Designated as safety issue: No ]
  
• Frequency of Grade 3 or 4 systemic adverse events, the occurrence of a severe or life-threatening injection site adverse event, or death [ Time Frame: Throughout study ] [ Designated as safety issue: Yes ]
  

• Distribution of plasma HIV RNA viral load [ Time Frame: At Weeks 63 and 87 ] [ Designated as safety issue: No ]
  
• Proportion of patients with controlled viremia and their respective distribution of plasma HIV RNA viral load [ Time Frame: At Weeks 63 and 87 ] [ Designated as safety issue: No ]
  
• Mean viral burden, defined as time-averaged area under the log10 HIV-1 RNA viral load curve [ Time Frame: At Weeks 63 and 87 ] [ Designated as safety issue: No ]
  
• HIV DNA levels [ Time Frame: At Weeks 30, 38, 63, and 87 ] [ Designated as safety issue: No ]
  
• HIV-1 DNA levels [ Time Frame: At Weeks 30, 38, 46, 50, 63, and 87 ] [ Designated as safety issue: No ]
  
• Magnitude and absolute change in CD4 and CD8 counts [ Time Frame: At Weeks 63 and 87 ] [ Designated as safety issue: No ]
  
• Percent and absolute change in the number of HIV-1- specific CD8 cells, as measured by interferon (IFN)-gamma Elispot assay and intracellular cytokine staining [ Time Frame: Through Week 30 ] [ Designated as safety issue: No ]
  
• Percent and absolute change in the cytokine secretion patterns and the proliferative capacity of HIV-1- specific CD4 and CD8 cells, as measured by multiparameter flow cytometry [ Time Frame: Through Week 30 ] [ Designated as safety issue: No ]
  
• Breadth and magnitude of HIV-1- specific CD4 and CD8 cell responses [ Time Frame: Throughout study ] [ Designated as safety issue: No ]
  
• Time to reach the specified virologic or immunologic criteria for reinitiating antiretroviral therapy after treatment interruption [ Time Frame: Throughout study ] [ Designated as safety issue: No ]
  
• Frequency of Grade 3 or 4 systemic adverse events or the occurrence of a severe or life-threatening injection site adverse event, or HIV-related events, AIDS-defining infections, and death from time of vaccination [ Time Frame: Throughout study ] [ Designated as safety issue: Yes ]
  
• Cell-associated infectivity in latently infected cells [ Time Frame: At Week 63 ] [ Designated as safety issue: No ]
  
• Cell-associated infectivity at Week 63 and immunologic responses [ Time Frame: At Weeks 63 and 87 ] [ Designated as safety issue: No ]
  

While the advent of highly active antiretroviral therapy (HAART) has contributed to the increasing control of HIV infection and viral replication, ultimate control of HIV infection will require the development of effective HIV-specific immunity in HIV infected individuals. Therapeutic vaccination within the earliest weeks following acute or recent HIV infection may increase the immune system's response to HIV. This study will determine whether MRKAd5 HIV-1 gag/pol/nef vaccine followed by treatment interruption can maintain viral suppression in patients with acute or recent HIV infection.

The interventional part of the study will last 102 weeks. Participants will be randomly assigned to receive either MRKAd5 HIV-1 gag/pol/nef or placebo vaccine at baseline and Weeks 4 and 26. Participants must remain on HAART from study entry until Week 38. Participants whose HIV viral load rebounds two times or more above 500 copies/ml by Weeks 39 to 41 will not enter Step 2 of the study. Participants whose viral load drops to 500 copies/ml or less by Weeks 39 to 41 will enter Step 2, where they will discontinue HAART for 24 weeks. Participants in Step 2 will have plasma HIV viral loads measured every 2 weeks for the first 4 weeks and weekly for the next 3 weeks. Study participants will continue in Step 2 until they experience virologic and immunologic failure or they need to restart HAART for another reason; they will then enter Step 3, where they will reinitiate HAART. Step 3 participants will continue on HAART until Week 102. A long-term safety follow-up period will occur from Weeks 103 to 240.

Timing of the study visits will be determined by which steps a participant enters. A physical exam and blood and urine collection will occur at most study visits throughout the study until Week 102. Follow-up phone calls to study participants will occur every 6 months from Week 102 until Week 240 to collect long-term safety data, including clinical status, CD4 count, and medication history. During the long-term safety follow-up, participants will also have study visits every 6 months. Visits will include medical and medication history.