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P11.05 Proportion of hiv and sexually transmitted infection (sti) cases among men who have sex with men (msm) attributable to serosorting
  1. CM Khosropour1,
  2. LA Barbee1,2,
  3. JC Dombrowski1,2,
  4. RP Kerani1,2,
  5. DA Katz1,2,
  6. F Swanson3,
  7. MR Golden1,2
  1. 1University of Washington
  2. 2Public Health – Seattle & King County
  3. 3Gay City Health Project


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.

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