On how role versatility boosts an STI.

The prevalence of the HIV-1 infection has decayed in the last decades in western heterosexual populations. However, among men who have sex with men (MSM) the prevalence is still high, despite intensive campaigns and treatment programs that keep infected men as undetectable (Beyrer et al. 2012). Promiscuity and condom fatigue (Adam et al. 2005), which are not unique to the MSM community, are making unprotected anal intercourse (UAI) more common and sexually transmitted infections (STIs) presumably harder to track. Yet, MSM communities are peculiar in the sense that men can adopt fixed (insertive or receptive) or versatile (both practices) roles. Some old theoretical work (Wiley & Herschkorn 1989, Van Druten et al. 1992, Trichopoulos et al. 1998) predicted that the transmission of HIV-1 would be enhanced in MSM populations engaged more in role versatility than in role segregation, in which fixed roles are predominantly adopted. These predictions were based on the assumption that the probability of acquisition from unprotected insertive anal (UIA) sex was neglectable. However, as later shown (Vittinghoff et al. 1999, Goodreau et al. 2005), this assumption is inappropriate and HIV-1 may still be acquired via UIA sex. Here I show through a stochastic model that the increase of the HIV-1 prevalence among MSM due to role versatility holds under a stronger assumption of bidirectional virus transmission.