Quantifying sexual mixing by HIV status and pre-exposure prophylaxis (PrEP) use among men who have sex with men

Background Existing measures of preferential partner selection do not account for attribute-concordance by chance. We quantified network-level sexual mixing by HIV status and PrEP use using a balancing partnership approach. Methods Data were from Engage, a cross-sectional survey of MSM ≥16 year-old in three Canadian cities (2017–2018). MSM with ≥1 anal/oral sex partners in the past six months (P6M) reported their own and partners’ HIV status and PrEP use. After stratifying by respondents’ HIV status (positive/negative/unknown) and P6M PrEP use (yes/no), we compared observed seroconcordance to that expected by chance among P6M-partnerships with known-status. Within HIV negative-concordant recent partnerships, we compared observed concordance in PrEP use at last sex to chance. Concordance by chance is calculated under proportionate-mixing assumption, which means the distribution of partnerships by partners’ attributes equals that by respondents’ attributes as a result of partnership balancing. We used chi-squared tests for all comparisons. Results Of the 22,102 P6M-partnerships reported by 1881 respondents (17.0%, 74.5% and 8.5% HIV-positive, negative and unknown, respectively), 60.2% comprised partners’ of known-status. 64.3% of HIV-positive respondents’ partnerships were HIV-positive (vs chance 24.6%, p Conclusion Network-level serosorting and PrEP matching were evident after accounting for distribution of partnerships by chance. PrEP-mediated changes to mixing, such as less serosorting among MSM on PrEP, may indirectly influence the population-level HIV prevention impact of PrEP and should be included in the monitoring and evaluation of PrEP roll-out. Disclosure No significant relationships.