Characteristics of the sexual networks of gay, bisexual, and other men who have sex with men in Montréal, Toronto, and Vancouver: implications for the transmission and control of mpox in Canada.

BACKGROUND:The 2022-2023 global mpox outbreak disproportionately affected gay, bisexual, and other men who have sex with men (GBM). In Canada, almost all cases occurred among GBM and >70% of them were from the country's three largest cities: Montréal, Toronto, and Vancouver. We examined how the distributions of sexual partners 1) varied by city and over time (2017-2023) and 2) were associated with mpox transmission. METHODS:The Engage Cohort Study (2017-2023) recruited GBM via respondent-driven sampling in Montréal, Toronto, and Vancouver (n=2,449). We compared reported numbers of sexual partners in the past 6 months across cities and three time periods: pre-COVID-19 pandemic (2017-2019), pandemic (2020-2021), and post-restrictions (2021-2023). We modeled the distribution of sexual partners using Bayesian negative binomial regressions and post-stratification, adjusting for sampling design and attrition. We estimated mpox's basic reproduction number (ℛ0) using a risk-stratified compartmental model. RESULTS:The pre-COVID-19 pandemic distributions of sexual partner numbers were similar across cities: participants' mean number of partners over the last 6 months was 10.4 (95%CrI: 9.4-11.5) in Montréal, 13.1 (11.3-15.1) in Toronto, and 10.7 (9.5-12.1) in Vancouver. Partner numbers decreased during the pandemic in all cities. Post-restrictions, sexual activity increased but remained below pre-pandemic levels. Based on reported cases and post-restrictions distributions of sexual partners, the estimated ℛ0 for mpox varied from 2.4-2.7 between cities. The estimated mpox per-partnership transmission probability was 84% (uncertainty ranging from 51-98%). Cumulative incidences (0.7-0.9%) were similar across cities. CONCLUSION:GBM sexual activity after restrictions were lifted remained below pre-pandemic levels. Comparable sexual partner distributions may explain similarities in mpox ℛ0 and cumulative incidence across cities. With potential for further recovery in sexual activity, mpox vaccination and surveillance strategies should be maintained.