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, >70% of cases thus far have been among GBM in Montréal, Toronto, and Vancouver. We examined how the distributions of sexual partners 1) varied by city and over time related to the COVID-19 pandemic 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 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 partner numbers using Bayesian negative binomial regressions and post-stratification, adjusting for sampling design and attrition. We estimated the basic reproduction number ( R 0 ), secondary attack rate (SAR), and cumulative incidence proportion of mpox using the fitted distributions and case timeseries. Results The pre-COVID-19 pandemic distribution of sexual partner numbers was similar across cities: participants’ mean number of partners was 10.3 (95%CrI: 9.3-11.3) in Montréal, 12.8 (11.1-14.7) in Toronto, and 10.6 (9.41-11.9) in Vancouver. Partner numbers decreased during the pandemic in all cities. Post-restrictions, sexual activity increased but remained well below pre-pandemic levels. Based on reported cases and post-restrictions distributions, the estimated R 0 (2.4-2.6) and cumulative incidences (0.6-0.9%) were similar across cities. The estimated average SAR across cities was 79%. Conclusion GBM sexual activity after restrictions were lifted remained below pre-pandemic levels. Comparable sexual partner distributions across cities may explain similarities in mpox R 0 and cumulative incidence across cities. Public health authorities should consider the risk of mpox resurgence for future vaccination and surveillance strategies as sexual activity is expected to recover.