Risk behaviors for HIV in sexual partnerships of San Francisco injection drug users.

While injection drug users (IDU) in the USA are known to form sexual partnerships with IDU as well as non-IDU, scientific research is lacking regarding risk behaviors for HIV transmission within these partnerships. Such information could aid HIV-prevention efforts among IDU and could also explain the relatively low prevalence of HIV among non-IDU heterosexuals in US cities such as San Francisco. Using data from a cross-sectional sample of San Francisco IDU we estimated (1) the prevalence of IDU-IDU and IDU-non-IDU sexual partnerships, (2) the frequency of serodiscordant unprotected intercourse in IDU-IDU and IDU-non-IDU sexual partnerships, and (3) the frequency of concurrence of sexual risk and injection-related risk within IDU-IDU sexual partnerships. An estimated 68% of sexually active San Francisco IDU is in IDU-IDU partnerships. Our analysis suggests that compared to IDU-non-IDU partnerships, IDU-IDU partnerships include a greater rate of episodes of serodiscordant unprotected intercourse (incidence rate ratio: 10.2; 95% confidence interval: 2.1-50.7). In fact, our data suggest that 92% of serodiscordant sexual episodes involving IDU are attributable to IDU-IDU pairings. Unprotected intercourse and needle sharing occur concurrently in an estimated 29% of IDU-IDU partnerships. Our data suggest that HIV-transmission risk is higher within IDU-IDU partnerships than it is within IDU-non-IDU partnerships. This disparity could explain the relatively low prevalence of HIV among non-IDU heterosexuals in San Francisco. We recommend that HIV-prevention efforts among IDU continue to address sexual risk behaviors for HIV transmission in addition to injection-related risk behaviors, with emphasis on IDU-IDU partnerships.