Linwei Wang, Nasheed Moqueet, Gilles Lambert, Daniel Grace, Ricky Rodrigues, Joseph Cox, Nathan J Lachowsky, Syed W Noor, Heather L Armstrong, Darrell H S Tan, Ann N Burchell, Huiting Ma, Herak Apelian, Jesse Knight, Marc Messier-Peet, Jody Jollimore, Stefan Baral, Trevor A Hart, David M Moore, Sharmistha Mishra. American Journal of Epidemiology. 2019. American Journal of Epidemiology.
Our study has three aims. First, we aimed to quantify serosorting at the population-level among men who have sex with men (MSM) by comparing observed seroconcordance in sexual partnerships to what would have been observed by chance if zero individuals serosort (preferentially selected partners of the same perceived HIV status). Second, we aimed to compare if the population-level serosorting among HIV-negative MSM differed by pre-exposure prophylaxis (PrEP) use. Finally, in sexual partnerships formed between two HIV-negative individuals, we aimed to quantify population-level PrEP-matching by comparing observed concordance in PrEP use to what would have been observed by chance if zero individuals preferentially selected partners based on PrEP use.
Despite compelling evidence of individual-level serosorting, there is no empirical estimate that quantifies population-level sexual mixing by HIV status nor its relationship with PrEP use. Existing studies either measured individuals’ intention to preferentially select partners, a measure shown to have low agreement with individual behaviours, or measured individuals’ sexual partnering behaviours, which does not distinguish intended behaviours from what could be unintentional (by chance).
Moreover, serosorting and related patterns of ‘who has sex with whom’ may be changing, with the scale-up of PrEP and antiretroviral treatment for HIV-positive individuals leading to viral suppression. Qualitative evidence suggests that PrEP may reduce stigma and anxiety around sex within serodiscordant partnerships and lead to less serosorting. Conversely, PrEP use may be associated with increased stigma if those on PrEP are perceived to have multiple sexual partners, and/or by equating PrEP use with condomless anal sex, thus leading to preferential partner selection by PrEP use.
We found evidence of population-level serosorting among HIV-positive MSM and HIV-negative MSM in Montreal, Canada, including those who used PrEP. However, there was less serosorting among HIV-negative MSM who used PrEP compared with those who did not. We also found evidence of population-level PrEP-matching among HIV-negative MSM (those on PrEP had a higher proportion of partners on PrEP among their HIV-negative partners than by chance; those not on PrEP had a higher proportion of partners not on PrEP among their HIV-negative partners than by chance).
Our empirical estimates of population-level serosorting could help in the study of HIV transmission dynamics, as well as the transmission of sexually transmitted bacteria and other viruses using mathematical models.
Our findings also suggest potential influence of PrEP on sexual mixing patterns as evidenced by less population-level serosorting among those on PrEP and PrEP-matching. These data reinforce the importance of monitoring changes in sexual mixing patterns among MSM to inform PrEP implementation and impact evaluation.
Population-level patterns of PrEP-matching and less serosorting while on PrEP could potentially lead to disparities in HIV prevention benefits between HIV-negative MSM who use PrEP and those who do not. MSM not on PrEP do not directly benefit from the decreased HIV acquisition risk through taking PrEP; and they may be less likely to benefit from partners’ PrEP use if they are less likely to have a partner on PrEP due to PrEP-matching. Moreover, their sexual-network reflects a greater extent of serosorting than HIV-negative MSM on PrEP. In a setting where undiagnosed HIV may be high, if serosorting is associated with more condomless sex, HIV-negative MSM not on PrEP may face an even higher HIV acquisition risk. Thus, with potential changes in sexual mixing because of PrEP, it is even more important to ensure high or increased HIV testing to reduce the fraction or person-years of undiagnosed HIV in the population.
Our research team is conducting a mathematical modelling study to evaluate the influence of PrEP-mediated serosorting (less serosorting among MSM on PrEP compared with HIV-negative MSM not on PrEP) on the population-level impact of PrEP in incidence reduction. We have incorporated the empirical estimates on population-level serosorting from the current study into our transmission model.
Additionally, the on-going longitudinal data collection of the Engage study would allow for future studies to investigate the changes in sexual mixing patterns with the roll-out of PrEP, as well as to examine potential reasons underlying PrEP-matching.
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