Effectiveness of one dose of MVA-BN vaccine against mpox infection in males in Ontario, Canada: A target trial emulation

BackgroundLimited evidence is available on the real-world effectiveness of modified vaccinia Ankara-Bavarian Nordic vaccine (MVA-BN) against mpox infection. MethodsWe emulated a target trial using linked databases in Ontario, Canada to estimate the effectiveness of one dose of MVA-BN. Our study included males aged [≥]18 years who: (1) had a history of syphilis testing and a laboratory-confirmed bacterial sexually transmitted infection (STI) in the prior year; or (2) filled a prescription for HIV pre-exposure prophylaxis in the prior year. On each day between June 12, 2022 and October 27, 2022, males who had been vaccinated 15 days prior were matched 1:1 with unvaccinated males by age, geographic region, prior HIV diagnosis, number of bacterial STI diagnoses in the previous three years, and receipt of any non- MVA-BN vaccine in the previous year. We used a Cox proportional hazards model to estimate the hazard ratio comparing the incidence of mpox between the two groups, and calculated vaccine effectiveness as (1-HR)x100. ResultsEach group included 3,204 males. A total of 71 mpox infections were diagnosed over the study period, with 0.09 (95% confidence interval [CI], 0.05-0.13) per 1000 person-days for the vaccinated group and 0.20 (95%CI, 0.15-0.27) per 1000 person-days for the unvaccinated group. Estimated vaccine effectiveness of one dose of MVA-BN against mpox infection was 59% (95%CI, 31-76%). ConclusionsThis study, conducted in the context of a targeted vaccination program and evolving outbreak, suggests that one dose of MVA-BN is moderately effective in preventing mpox infection.