Antigenic Competition in HIV Preventive Vaccines

HIV vaccines are designed to create an immune response to certain parts of the HIV virus called peptides. Researchers believe that eliciting a response to a peptide called Gag is particularly important. Most HIV vaccines in current clinical trials combine multiple peptides, but including these other peptides may cause antigenic competition. Antigenic competition occurs when the body's immune system reaction to one part of a vaccine weakens or inhibits the response to another part of the vaccine. Specifically, this study is concerned that having too many other peptides in a vaccine might weaken the specific immune response to Gag. This study will test whether a vaccine which only includes the peptides Gag and Pol elicits a stronger immune response to Gag and Pol than a vaccine that also includes the peptides Env A, B, and C. Participation in this study will last 6 months. The number of study visits will vary by study site. Participants will be randomly assigned to receive injections of one of two vaccines in their upper arm. One group of participants will receive rAd5 gag/pol, which contains only the Gag and Pol peptides, while the other group of participants will receive rAd5 gag/pol Env A/B/C, which contains the Gag, Pol, and Env A, B, and C peptides. For 3 days after injection, participants will need to record their temperature and any side effects, and they will be contacted by study staff 7 days after the injection for follow-up monitoring. During study visits, participants will complete the following assessments: an HIV test; a physical exam; collection of blood samples; a pregnancy test; and an interview about health, medications, HIV risk behaviors, and experiences with the study. Participants will be contacted by study staff once a year for 5 years after the vaccination for follow-up health and safety monitoring.