Assessing interventions among men who have sex with men during the clade-IIb Mpox outbreak in Japan in 2023

Objectives Mpox is an infectious viral disease caused by monkeypox virus, which can be divided into two distinct clades (I and II), each with subclades (a and b). Mpox clade IIb has spread globally since 2022. Transmission occurs via sexual contact, mostly among men who have sex with men (MSM). Japan experienced the first surge of cases in early 2023, with 252 reported cases including one fatality by March 2025. In consultation with the government, non-profit organisations launched risk-awareness campaigns using social media, dating applications and in-person events. This study aimed to estimate the effective reproduction number (Rt) during the outbreak to evaluate the efficacy of interventions in reducing secondary transmission. Methods Weekly PCR-verified Mpox case data from January 2023 to May 2024 were analysed. Information on the timing and nature of interventions was retrospectively collected. A renewal equation model was applied to estimate weekly Rt, accounting for the delay from infection to illness onset. A simplified step function model for Rt was selected to approximately describe the outbreak. Results Weekly Rt values revealed substantial variation, frequently falling below 1 during the outbreak. Model selection identified the best three-step model that changed step in accordance with two key interventions: (1) announcement of the Mpox outbreak on non-profit organisation websites and (2) dissemination of a risk-alerting advertisement on social media, dating applications and in-person events among MSM. Rt decreased from 1.24 (95% CI 0.39 to 2.09) to 0.92 (95% CI 0.60 to 1.24) in the second week of intervention. Conclusions Rt changed with interventions, and model selection identified the best simplified function to describe the epidemic. Early risk awareness campaigns and advertisements for high risk groups using online platforms appeared to alter the secondary transmission patterns in a timely manner.