Background Less is known about HIV prevalence and incidence among men who have sex with men (MSM) in Saint Petersburg, Russia than in other large European cities, but it is clear that in that city the public health impact of HIV on drug injectors and their sex partners is remarkably high. We estimated the public health impact

Methods Respondent-driven sampling (RDS) yielded behavioural and HIV testing data and specimens for HIV antibody testing from 201 MSM who did not report injecting drugs and 691 drug injectors who did not report male-male sex. We estimated the potential for sexual HIV transmission from MSM and injectors unaware of their HIV infection, comparing unsafe encounters.

Results One quarter of people who inject drugs and 5% of MSM were HIV infected and unaware of their infection. Based on our prior estimate that there were 83,100 drug injectors living in Saint Petersburg; about 20,000 injectors are unaware of their HIV infection. If 1% of the general population are MSM, we estimate that there are about 3,000 MSM with undiagnosed HIV infection in Saint Petersburg. The 169 injectors with undiagnosed infection reported unprotected sex with 37 partners who themselves injected drugs, and that the proportion of injectors who were uninfected was 56%. Injectors with undiagnosed infection reported unprotected sex with 26 partners who did not inject drugs; the uninfected proportion of non-injecting partners of drug injectors was 83%. Therefore, undiagnosed, HIV+ injectors exposed an estimated mean 0.25 uninfected partners in six months time. Those injectors expose 5,821 (95% CI 3,092–11,095) uninfected sex partners during a six-month period, whereas the 3,000 MSM with undiagnosed HIV infection in Saint Petersburg expose 4,800 (95% CI 4,320–12,000).

Conclusion New sexual infections eminating from MSM may become comparable to those from drug injectors.