Population-Level Sexual Mixing By HIV Status and Pre-exposure Prophylaxis Use Among Men Who Have Sex with Men in Montreal, Canada: Implications for HIV Prevention

: There are limited data on population-level mixing patterns by HIV status or pre-exposure prophylaxis (PrEP) use. Using cross-sectional survey data (Engage, 2017-2018) of 1137 men who have sex with men ≥16 year-old in Montreal, we compared observed seroconcordance in the past-6-month sexual partnerships to what would have been observed by chance if zero individuals serosort. Of 5 recent partnerships where both individuals were HIV-negative, we compared observed concordance in PrEP use to the counterfactual if zero individuals selected partners based on PrEP use. We estimated the concordance by chance using a balancing-partnerships approach assuming proportionate-mixing. HIV-positive respondents had a higher proportion of HIV-positive partners (66.4% (95% confidence interval: 64.0%-68.6%)) than by chance (23.9%(23.1%-24.7%)). HIV-negative respondents (both on and not on PrEP) had higher proportions of HIV-negative partners (82.9%(81.1%-84.7%), and 90.7%(89.6%-91.7%), respectively) compared with by chance (76.1%(75.3%-76.9%)); but those on PrEP had a higher proportion of HIV-positive partners than those not on PrEP (17.1%(15.3%-18.9%) vs. 9.3%(8.3%-10.4%)). Those on PrEP also had a higher proportion of partners on PrEP among their HIV-negative partners (50.6%(42.5%-58.8%)) than by chance (28.5%(27.5%-29.4%)). The relationship between PrEP and sexual-mixing patterns demonstrated by less population-level serosorting among those on PrEP and PrEP-matching warrants consideration during PrEP roll-out.