The effectiveness of the MVA vaccine against mpox in men who have sex with men: a retrospective cohort analysis, Seattle, Washington

Abstract Background Data on Modified Vaccinia Ankara (MVA)vaccine effectiveness against mpox in real world settings are limited. Methods We performed a retrospective cohort analysis using Cox proportional hazards regression to estimate the association between vaccination and laboratory-confirmed mpox incidence. Study subjects included all MSM seen in a sexual health clinic in Seattle, WA between 1/1/2020 and 12/31/2022. Subjects’ receipt of vaccine and diagnosis with mpox were ascertained from public health vaccine registry and surveillance data. Analyses were adjusted for demographic factors, HIV status, and sexual risk behaviors. Results The incidence of mpox per 100 person-years was 8.83 among patients with 0 doses, 3.32 among patients with 1 dose, and 0.78 among patients with 2 two doses of MVA vaccine. Mpox diagnosis was significantly associated with age category 30-39 and 40-51, HIV positivity, syphilis diagnosis in the prior year, >10 sex partners in the last year, and having a clinic visit in the last year. In the multivariate model adjusting for these factors, vaccine effectiveness was 81% for 1 dose and 83% for 2 doses. Conclusion These data support the effectiveness of the MVA vaccine - including a single dose of the vaccine – in preventing mpox disease and highlight the appropriateness of risk factor-based prioritization of immunization early in the epidemic. The durability of MVA vaccine-induced immunity is unknown, and at-risk persons should receive two doses of MVA.