Characteristics of individuals who received a complete, 2-dose mpox vaccine regimen as part of the public health response to the mpox epidemic in Ontario, Canada

In May 2022, an outbreak of mpox emerged in Canada. In June 2022, the province of Ontario began offering first doses of a 2-dose regimen of Modified Vaccinia Ankara–Bavaria Nordic (MVA-BN) to those at high risk of exposure. Second doses became available in September 2022. To help increase dose 2 access and uptake, we sought to understand how individuals who received 2 doses differed from those who received only 1 dose. We conducted a cross-sectional study using population-level data among individuals who received ≥1 dose of MVA-BN between June 6, 2022 and October 31, 2023 in Ontario. We used age-adjusted Poisson regression to examine the association between demographic, social, and economic characteristics; co-morbidities; and proxies for sexual exposure (e.g., bacterial sexually transmitted infection [STI] diagnoses) and proxies for healthcare engagement (e.g., syphilis testing, past receipt of other vaccines) with MVA-BN dose 2 receipt. Among 33,012 individuals with ≥1 MVA-BN dose, 38.2% (12,620) received 2 doses. Receipt of dose 2 versus only dose 1 was associated with region (e.g., higher in Ottawa compared with Toronto [prevalence ratio, PR = 1.08, 95% confidence interval, CI 1.06-1.09]); syphilis testing (≥4 tests PR = 1.12, 95%CI 1.11-1.14) or receiving a COVID-19, influenza, or other vaccine (PR = 1.12, 95%CI 1.11-1.14) in the year before dose 1; and syphilis testing (≥4 tests PR = 1.19, 95%CI 1.18-1.20) or bacterial STI diagnoses >3 months after dose 1 (≥4 diagnoses PR = 1.07, 95% CI 1.05-1.08). Refugees were less likely to get dose 2 versus Canadian-born individuals or long-term immigrants (PR = 0.93, 95%CI 0.91-0.95).Our findings suggest lower healthcare access and/or engagement may play a role in limiting dose 2 receipt in Ontario. Public messaging around availability and eligibility of second doses, tailored strategies for eligible refugees, increased access outside healthcare venues, and adopting promotion strategies from regions with high uptake, may help increase dose 2 coverage.