Biological and demographic causes of high HIV and sexually transmitted disease prevalence in men who have sex with men

Objectives: HIV disproportionately affects men who have sex with men (MSM). MSM and heterosexual networks are distinguished by biologically determined sexual role segregation among heterosexual individuals but not MSM, and anal/vaginal transmissibility differences. To identify how much these biological and demographic differences could explain persistent disparities in HIV/sexually transmitted disease prevalence in the United States, even were MSM and heterosexual individuals to report identical numbers of unprotected sexual partnerships per year. Methods: A compartmental model parameterized using two population-based surveys. Role composition was varied between MSM and heterosexual subjects (insertive-only and receptive-only versus versatile individuals) and infectivity values. Results: The absence of sexual role segregation in MSM and the differential anal/vaginal transmission probabilities led to considerable disparities in equilibrium prevalence. The US heterosexual population would only experience an epidemic comparable to MSM if the mean partner number of heterosexual individuals was increased several fold over that observed in population-based studies of either group. In order for MSM to eliminate the HIV epidemic, they would need to develop rates of unprotected sex lower than those currently exhibited by heterosexual individuals in the United States. In this model, for US heterosexual individuals to have a self-sustaining epidemic, they would need to adopt levels of unprotected sex higher than those currently exhibited by US MSM. Conclusions: The persistence of disparities in HIV between heterosexual individuals and MSM in the United States cannot be explained solely by differences in risky sexual behavior between these two populations.