Real-world effectiveness of MBA-VN vaccine against mpox, a test-negative case-control study.

Background: Mpox is a viral illness for which the MVA-BN vaccine was authorized during the 2022 global outbreak. Although initial studies reported high immunogenicity, real-world evidence on vaccine effectiveness (VE) remains scarce. This study aimed to estimate the real-world VE of at least one dose of the MVA-BN vaccine against symptomatic mpox in adults tested at a tertiary hospital in Barcelona, Spain.

Methods: We conducted a test-negative case-control study between July 2022 and December 2023. Adults tested for mpox by PCR were included. Cases were PCR-positive; controls were PCR-negative. Vaccine effectiveness of at least one dose was calculated as (1 - adjusted odds ratio) × 100 %, using multivariable logistic regression.

Results: Among 301 participants, 126 were cases and 175 controls. Crude VE of ≥1 dose ≥14 days before testing was 47 % (95 % CI: -17 % to 77 %). After adjusting for sex, age, testing center, sexually transmitted infection (STI) testing in the last year and risk criteria for mpox VE increased to 74 % (95 % CI: 38 % to 90 %) but decreased to 44 % (95 % CI: -52 % to 79 %) when additionally adjusting for epidemiological weeks. Similar patterns were observed in a subgroup of men with at least one risk criteria for mpox.

Conclusions: Our study provides context-specific evidence on the effectiveness of a single MVA-BN vaccine dose against symptomatic mpox in a real-world setting. Vaccine effectiveness estimates varied depending on adjustment strategy, highlighting the importance of accounting for epidemiological weeks to avoid overestimation.