Reconciling Estimates of Per-Act Infectivity of HIV: Couples vs Commercial Relationship

Hongmei Yang, Population Services International (PSI)

Estimates from past studies on the per-act infectivity of HIV during unprotected sex vary by study type and region. Although differences between frequent (regular) and casual sexual contacts, and cofactors like STD infection are recognized as contributing to the variation, no study has tested the comparability of estimates after adjusting for these factors. To address this, we tested two scenarios using probability models on data from a Thai study among military conscripts who frequented FSWs (Nelson 1993). The overall unadjusted per-act infectivity was 0.0406 (scenario A) and 0.0761 (scenario B). Adjusted infectivity was 3.7 times less than unadjusted values. Adjusted infectivity decreased with increased number of sexual contacts, following power functions. After adjusting for STD infection and stage of HIV infection, and considering differences in number of sexual contacts, estimates were comparable to those found in studies among discordant couples. These results have important implications on HIV epidemic modeling.