The results of the Soweto Men’s Study confirm that MSM are at high risk for HIV infection, and demonstrate that HIV is unevenly distributed among MSM subpopulations. At 33.9%, HIV prevalence among self-identified gay men is greater than three times that of bisexual and straight-identified MSM, whose respective estimates of 6.4 and 10.6% are comparable to the 11.7% HIV prevalence found among South African men aged 15–49 in the 2005 national survey [19]. In their meta-analysis of HIV prevalence for MSM in developing countries, Baral et al. found MSM in Africa were 3.8 times more likely to be HIV infected than the general population [8], yet our overall prevalence estimate of 13.2% is comparable to men in the general population. However, as the samples from which Baral et al.’s pooled estimates were drawn were largely derived from convenience samples of men who self-identified as gay or another homosexual identity and were not adjusted for sampling designs, our finding that gay-identified men have a substantially higher odds of HIV infection than non-gay identified MSM is largely consistent with the findings of the meta-analysis.

In reporting this finding, we recognize that gay identity is not a behavior, although in township MSM communities gay identity is itself highly correlated with the exclusive practice of RAI, and conversely, self-identifying as bisexual or a straight MSM was highly correlated with the exclusive practice of insertive anal intercourse (IAI) with male partners [5]. Since gay-identified men are also the most visible of MSM in township communities, this finding indicates that it is possible and necessary to begin working immediately with gay-identified men on HIV prevention and treatment, focused not only on individual behaviors, but the disadvantageous structural and socio-cultural contexts that influence their behaviors. Unlike other sub-Saharan African countries, there are no legal barriers to working with gay communities in South Africa.

The Soweto Men’s Study is also, to our knowledge, one of the first studies of MSM in Africa to have applied the RDSAT-generated weight outcomes described by Heckathorn [18] to estimate HIV prevalence and to develop multivariable model of HIV risk factors, and the first with an HIV prevalence outcome to account for missing outcome data through multiple imputation. That the imputed model’s HIV prevalence estimates were between the two extreme possibilities gives us confidence that the procedure has resulted in a reasonable initial HIV prevalence estimate for this population. Given RDS’s popularity with studying MSM populations in sub-Saharan Africa, future behavioral and HIV surveillance studies using RDS may need to follow similar procedures to account for missing HIV data. Other studies have found that MSM who are socially vulnerable may be reluctant to test [12], and that many MSM have prior negative experiences with HIV VCT [20]. It is therefore understandable that MSM may decline to test even in a study setting that guaranteed access to medical care.

Several studies have noted the association between alcohol consumption and sexual risk behaviors [21–23] Our sample was quite homogenous in its drinking behavior, and thus we did not detect a statistically significant association with UAI or HIV infection; we do not conclude that alcohol consumption plays no part in HIV transmission among MSM. On the contrary, purchasing alcohol in exchange for sex with other men predicted HIV infection—and was the only form of transactional sex between men with a significant effect. This speaks to the complex role that substance use and sexual exchange play in the lives of township MSM. Better understanding how all of these factors may contribute to MSM’s HIV risk will be important to intervention efforts.

A large number of participants also reported having a regular female partner, and counted at least one woman among their last five partners. Although female partnerships and UVI were associated with lower HIV risk, the comparison group in each case is composed overwhelmingly of gay-identified men who practice RAI with other men, whose risk is much higher. It is important to note that relatively high rates of self-reported UVI (45.6%) and UIAI with men (28.0%) in this population provide ample opportunity for HIV transmission to take place, and suggests that the heterosexual and MSM epidemics are behaviorally linked. MSM who also have sex with women keep their identities and behaviors hidden and are therefore difficult to target with same-sex specific HIV prevention messaging aimed at gay men; in the absence of information this group may be inadvertently increasing the long-term risk of HIV infection to themselves as well as their male and female partners.

Circumcision was associated with lower HIV prevalence. Given that most MSM in our sample reported practicing IAI exclusively, we conclude that there may be a protective association for MSM in South Africa for men who are consistently the insertive partners in anal sex with men. Although a meta-analysis of observational studies of circumcision in MSM populations by Millet et al. could not detect a consistent protective effect, the authors did find a non-significant reduction in odds in settings with less than 50% circumcision prevalence, and concluded that more data on MSM who engage primarily in IAI would be helpful [24]. Given the potential for circumcision to decrease HIV transmission at the population level, and that many MSM who are insertive partners with men also have sex with women, this association should continue to be explored in further observational research with MSM throughout the region.

Our study has several limitations. RDS studies of MSM have been critiqued for overstating claims to unbiased population estimates because the samples actually recruited may not fully represent the underlying population, even after adjustment [25]. Although we attempted systematically to select seeds that would produce a well-networked and diverse sample, it is possible that men who are unemployed and who have lower socioeconomic status (SES) and educational achievement are not well networked with better educated and resourced MSM. Thus unemployed men of lower SES may be overrepresented in our sample. However, our results are likely representative of the most socially vulnerable MSM whose limited income, mobility, and economic opportunity may also limit their access to HIV prevention information, condoms and latex-compatible lubricant, and high-quality health services. It is notable that although black race and Soweto residence were not explicit inclusion criteria, the chains did not leave Soweto except in the case of one Coloured participant, suggesting that the legacy of apartheid continues to limit Soweto MSM’s geographic and social mobility. It is also possible that the offer of VCT may have operated both as an additional recruitment incentive [26, 27] as well as a disincentive to participating altogether, biasing the results in ways that would be difficult to account for. The interviewer-administered questionnaires may have introduced social desirability bias with respect to self-report of sensitive sexual behaviors, including drug use, receptive AI, and/or unprotected AI. In addition, the multiple imputation procedure we followed for missing HIV results is untested, and to the extent that risk behavior may have been underreported, our imputation method would bias our prevalence estimate towards the extreme possibility of non-testers being HIV-negative. We acknowledge that our adjusted results are likely a conservative estimate of HIV prevalence in the target population.

Finally, our experience with the Soweto Men’s Study shows that it is possible to engage MSM in HIV research in the absence of a functioning LGBT organization. It is always preferable to collaborate with and strengthen the capacity of LGBT organizations for MSM HIV research where possible, but the disadvantageous legal and social environment in much of sub-Saharan Africa may inhibit their ability to launch or sustain research and intervention initiatives in their respective communities. Like other MSM studies in the region, our findings show that it is critical to begin addressing the needs of MSM for HIV prevention and treatment, and that individual MSM, health care practitioners, and researchers can engage in effective collaborations to improve health outcomes in this highly vulnerable population.