Biological and Demographic Causes of High HIV and Sexually Transmitted Disease Prevalence in Men Who Have Sex With Men

Men who have sex with men (MSM) are disproportionately affected by HIV. MSM and heterosexual networks are distinguished by biologically determined sexual role segregation among heterosexual individuals but not among MSM, and by anal/vaginal transmissibility differences.

The authors of the current study set out to identify how much these biological and demographic differences could explain persistent disparities in HIV/STD prevalence in the United States -- even if MSM and heterosexuals report identical numbers of unprotected sexual partnerships per year. They created a compartmental model parameterized using two population-based surveys. Role composition was varied between MSM and heterosexual subjects (insertive-only and receptive only vs. versatile individuals) and infectivity values.

Considerable disparities in equilibrium prevalence resulted from the absence of sexual role segregation in MSM and differential anal/vaginal transmission probabilities. "The U.S. heterosexual population would only experience an epidemic comparable to MSM if the mean partner number of heterosexual individuals was increased by several fold over that observed in population-based studies of either group," the authors wrote. "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 U.S. heterosexual individuals to have a self-sustaining epidemic, they would need to adopt levels of unprotected sex higher than those currently exhibited by U.S. MSM."

The authors concluded, "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 those two populations."

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