Mathematical modelling of the influence of serosorting on the population-level HIV transmission impact of pre-exposure prophylaxis




Wang, Linwei
Moqueet, Nasheed
Simkin, Anna
Knight, Jesse
Ma, Huiting
Lachowsky, Nathan J.
Armstrong, Heather L.
Tan, Darrell H.S.
Burchell, Ann N.
Hart, Trevor A.

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Objectives: HIV pre-exposure prophylaxis (PrEP) may change serosorting patterns. We examined the influence of serosorting on the population-level HIV transmission impact of PrEP, and how impact could change if PrEP users stopped serosorting. Design: We developed a compartmental HIV transmission model parameterized with bio-behavioural and HIV surveillance data among MSM in Canada. Methods: We separately fit the model with serosorting and without serosorting [counterfactual; sero-proportionate mixing (random partner-selection proportional to availability by HIV status)], and reproduced stable HIV epidemics with HIV-prevalence 10.3–24.8%, undiagnosed fraction 4.9–15.8% and treatment coverage 82.5–88.4%. We simulated PrEP-intervention reaching stable pre-specified coverage by year-one and compared absolute difference in relative HIV-incidence reduction 10 years postintervention (PrEP-impact) between models with serosorting vs. sero-proportionate mixing; and counterfactual scenarios when PrEP users immediately stopped vs. continued serosorting. We examined sensitivity of results to PrEP-effectiveness (44–99%; reflecting varying dosing or adherence levels) and coverage (10–50%). Results: Models with serosorting predicted a larger PrEP-impact than models with sero-proportionate mixing under all PrEP-effectiveness and coverage assumptions [median (interquartile range): 8.1% (5.5–11.6%)]. PrEP users’ stopping serosorting reduced PrEP-impact compared with when PrEP users continued serosorting: reductions in PrEP-impact were minimal [2.1% (1.4–3.4%)] under high PrEP-effectiveness (86– 99%); however, could be considerable [10.9% (8.2–14.1%)] under low PrEP effectiveness (44%) and high coverage (30–50%). Conclusion: Models assuming sero-proportionate mixing may underestimate population- level HIV-incidence reductions due to PrEP. PrEP-mediated changes in serosorting could lead to programmatically important reductions in PrEP-impact under low PrEP effectiveness. Our findings suggest the need to monitor sexual mixing patterns to inform PrEP implementation and evaluation.


Some of the model parameters in the current modelling article drew on estimates published in Wang et al. 2019 ( We acknowledge the Engage study and its funders (Canadian Institutes of Health Research (CIHR) Team Grant [TE2-138299]; CIHR Canadian HIV Trials Network [CTN 300]; Canadian Foundation for AIDS Research [Engage]; Canadian Blood Services [MSM2017LP-OD]; Ontario HIV Treatment Network (OHTN) [1051]; Ryerson University [no related grant number]; and Public Health Agency of Canada [4500370314]), which supported the independently published results in Wang et al. 2019 ( We would like to thank Kristy Yiu for supporting submission and project coordination, and Steven Tingley for helpful discussions surrounding model structure.


HIV, MSM, pre-exposure prophylaxis, serosorting, sexual mixing patterns


Wang, L., Moqueet, N., Simkin, A., Knight, J., Ma, H., Lachowsky, N. J., Armstrong, H. L., Tan, D. H. S., Burchell, A. N., Hart, T. A., Moore, D. M., Adam. B. D., Macfadden, D. R., Baral, S., & Mishra, S. (2021). “Mathematical modelling of the influence of serosorting on the population-level HIV transmission impact of pre-exposure prophylaxis.” AIDS, 35(7), 1113-1125. DOI: