Introduction The contribution of serosorting to HIV/STI disease burden among MSM is unknown.

Methods The study used data collected as part of routine care at an STD clinic 2002–2013 or a community-based HIV/STD testing centre 2004–2013 in Seattle, Washington. Data included men’s condom use for anal sex with HIV-positive, negative and unknown-status partners in the prior 12 months. We defined three mutually-exclusive behavioural categories: serosorting (condomless anal intercourse [CAI] only with HIV-concordant partners); non-concordant CAI (any CAI with HIV-discordant/unknown-status partners; [NCCAI]); and no CAI (consistent condom use or no AI). We estimated adjusted relative risks (aRR) between sexual behaviour and HIV/STI, and calculated the population attributable risk (PAR%) as the proportion of HIV/STI cases attributable to serosorting (assuming serosorters would have otherwise consistently used condoms) and the population prevented fraction (PF) as the proportion of hypothetical HIV/STI cases averted by serosorting (assuming serosorters would have otherwise had NCCAI).

Results Behavioural data were complete for 49,912 visits; 91% (n = 45,220) were among self-reported HIV-negative MSM. Overall, serosorting was reported by 35% and 38% of HIV-negative and positive MSM, respectively. Compared to men reporting no CAI, HIV-negative serosorters had a significantly higher risk of HIV (aRR = 2.0), syphilis (aRR = 2.0), urethral gonorrhoea/chlamydia (GC/CT) (aRR = 1.5) and rectal GC/CT (aRR = 1.7). The proportion of HIV/STI cases attributable to serosorting (PAR%) were: HIV (15.3%); syphilis (16.9%); urethral GC/CT (11.9%); rectal GC/CT (19.3%). Serosorters had a significantly lower risk of HIV (aRR = 0.53) and syphilis (aRR = 0.76) compared to men reporting NCCAI; the proportion of HIV and syphilis cases averted by serosorting (PF) were 14.6% and 8.3%, respectively. Among HIV-positive MSM, syphilis risk was significantly higher for serosorters compared to consistent condom users (aRR = 1.4; PAR% = 12.1%).

Conclusion These data suggest that serosorting is responsible for, but also prevents, 15% of HIV cases among MSM (PAR% = 15.3% and PF = 14.6%), depending on the behaviour it replaces.

Disclosure of interest statement No pharmaceutical grants were received in the development of this study.