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

<p><strong>Background: </strong>The 2022–2023 global mpox outbreak disproportionately affected gay, bisexual, and other men who have sex with men (GBM). We investigated differences in GBM's sexual partner distributions across Canada's 3 largest cities and over time, and how they shaped transmission.</p> <p><strong>Methods: </strong>The Engage Cohort Study (2017–2023) recruited GBM via respondent-driven sampling in Montréal, Toronto, and Vancouver (n = 2449). We compared reported sexual partner distributions across cities and periods: before COVID-19 (2017–2019), pandemic (2020–2021), and after lifting of restrictions (2021–2023). We used Bayesian regression and poststratification to model partner distributions. We estimated mpox's basic reproduction number (<em>R0</em>) using a risk-stratified compartmental model.</p> <p><strong>Results: </strong>Pre–COVID-19 pandemic distributions were comparable: fitted average partners (past 6 months) were 10.4 (95% credible interval: 9.4–11.5) in Montréal, 13.1 (11.3–15.1) in Toronto, and 10.7 (9.5–12.1) in Vancouver. Sexual activity decreased during the pandemic and increased after lifting of restrictions, but remained below prepandemic levels. Based on reported cases, we estimated <em>R0</em> of 2.4 to 2.7 and similar cumulative incidences (0.7%–0.9%) across cities.</p> <p><strong>Conclusions: </strong>Similar sexual partner distributions may explain comparable <em>R0</em> and cumulative incidence across cities. With potential for further recovery in sexual activity, mpox vaccination and surveillance strategies should be maintained.</p>