The Step Study found that men who had sex with men (MSM) who received an adenovirus type 5 (Ad5) vector-based vaccine and were uncircumcised or had prior Ad5 immunity had a higher HIV incidence than MSM who received placebo. We investigated whether differences in HIV exposure, measured by reported sexual risk behaviors, may explain the increased risk.
Among 1,764 MSM in the trial, 724 were uncircumcised, 994 had prior Ad5 immunity and 560 were both uncircumcised and had prior Ad5 immunity. Analyses compared sexual risk behaviors and perceived treatment assignment among vaccine and placebo recipients, determined risk factors for HIV acquisition and examined the role of insertive anal intercourse in HIV risk among uncircumcised men.
Few sexual risk behaviors were significantly higher in vaccine vs. placebo recipients at baseline or during follow-up. Among uncircumcised men, vaccine recipients at baseline were more likely to report unprotected insertive anal intercourse with HIV negative partners (25.0% vs. 18.1%; p=0.03). Among uncircumcised men who had prior Ad5 immunity, vaccine recipients were more likely to report unprotected insertive anal intercourse with partners of unknown HIV status (46.0% vs. 37.5%; p=0.05). Vaccine recipients remained at higher risk of HIV infection compared to placebo recipients (HR =2.8; 95% CI:1.7, 6.8) controlling for potential confounders.
These analyses do not support a behavioral explanation for the increased HIV infection rates observed among uncircumcised men in the Step Study. Identifying biologic mechanisms to explain the increased risk is a priority.This study is registered with ClinicalTrials.gov, number NCT00095576.