Community-based assessment of mpox seroprevalence and undocumented exposures in Manitoba and Alberta, Canada

During the 2022 global mpox outbreak, gay, bisexual and other men who have sex with men (gbMSM) were disproportionately affected,s however, the true number of cases is likely higher than reported due in part to the stigma associated with mpox. This stigma may have played an even more influential role on health-seeking behaviour in politically conservative regions, such as the Canadian Prairies. The shifting epidemiology may have also led to misdiagnoses during initial months of the outbreaks. Seroprevalence studies can help assess these gaps in surveillance and population-level immunity. To address this, we conducted a cross-sectional seroprevalence study among 275 individuals who identified as potentially eligible to receive the mpox vaccine in the Canadian provinces of Manitoba and Alberta from June 2024 to December 2025 based on provincial vaccination criteria. This eligibility included, but was not exclusively limited to, gbMSM, gender diverse individuals, and sex workers. Dried blood spots were analyzed using the Meso Scale Discovery Orthopoxvirus assay targeting five orthologous mpox and vaccinia virus antigens. In total, 14.5% of individuals self-reported receiving the mpox vaccine and of those without reported Orthopoxvirus vaccination, 5.9% were seroreactive to multiple mpox antigens. These results suggest the potential for undetected mpox circulation within Manitoba and Alberta. While primarily men who have sex with men were eligible for the mpox vaccine, an expansive range of gender and sexual orientation identities were classified as seroreactive, suggesting a misalignment between those who were seroreactive and vaccine eligibility criteria. This emphasizes how broader mpox vaccine eligibility, not restricted by specific gender and sexuality requirements, may better encompass those at risk of infection.