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<0.001). HIV-negative or status-unknown respondents had higher proportions of HIV-negative partners (87.0% and 87.5%, respectively, vs chance 75.4%, p<0.001). HIV-negative respondents on PrEP had a higher proportion of HIV-positive partners than those not on PrEP (20.6% vs 8.4%; p<0.001). HIV-negative respondents on PrEP had a higher proportion of HIV-negative partners on PrEP (55.8% vs 34.7%); those not on PrEP had a higher proportion of HIV-negative partners not on PrEP (78.6% vs 65.3%), than chance (p<0.001).
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.
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