Effectiveness of modified vaccinia Ankara-Bavarian Nordic vaccine against mpox infection: emulation of a target trial

OBJECTIVE To estimate the real world effectiveness of modified vaccinia Ankara-Bavarian Nordic (MVA-BN) vaccine against mpox infection. DESIGN Emulation of a target trial. SETTING Linked databases in Ontario, Canada. PARTICIPANTS 9803 men aged >= 18 years with a history of being tested for syphilis and a laboratory confirmed bacterial sexually transmitted infection (STI) in the previous year, or who filled a prescription for HIV pre-exposure prophylaxis in the previous year. On each day between 12 June 2022 and 27 October 2022, those who had been vaccinated 15 days previously were matched 1:1 with unvaccinated men by age, geographical region, past HIV diagnosis, number of bacterial STI diagnoses in the previous three years, and receipt of any nonMVA-BN vaccine in the previous year. MAIN OUTCOME MEASURE The main outcome measure was vaccine effectiveness ((1-hazard ratio)x100) of one dose of subcutaneously administered MVA-BN against laboratory confirmed mpox infection. A Cox proportional hazards model was used to estimate hazard ratios to compare the rate of laboratory confirmed mpox between the two groups. RESULTS 3204 men who received the vaccine were matched to 3204 unvaccinated controls. A total of 71 mpox infections were diagnosed, with 0.09 per 1000 person days (95% confidence interval (CI) 0.05 to 0.13) in the vaccinated group and 0.20 per 1000 person days (0.15 to 0.27) in the unvaccinated group over the study period of 153 days. Estimated vaccine effectiveness of one dose of MVA-BN against mpox infection was 58% (95% CI 31% to 75%). CONCLUSION The findings of this study, conducted in the context of a targeted vaccination programme and evolving outbreak of mpox, suggest that one dose of MVA-BN is moderately effective in preventing mpox infection.