Factors that influenced the course of the mpox outbreak in the Netherlands, 2022-2023

Background In 2022-2023 a global outbreak of mpox affected mostly gay, bisexual and other men having sex with men (GBMSM). Outbreak control in the Netherlands included isolation, quarantine, post-exposure prophylaxis vaccination, and primary preventive vaccination (PPV). We describe the course of the outbreak, the vaccination programme, vaccine effectiveness (VE) of full vaccination against symptomatic disease, and trends in behaviour to generate hypotheses about factors that influenced the outbreak’s decline. Methods Observational study. Data on notified cases, number of PPV invitations, and PPV doses administered were collected from public health services. PPV uptake and coverage were calculated. Monthly trends in behavioural data of GBMSM visiting Sexual Health Centres (SHC) were analysed for all consultations in 2022. VE was estimated using the screening method. Findings A total of 1,266 mpox cases were reported until August 1, 2023. The outbreak peaked early July 2022 and sharply declined afterwards. PPV started July 25, 2023; in total 29,851 doses were administered. 45.8% received at least one dose, 35.4% was fully vaccinated. Estimated VE was 68.2% (95% CI 4.3%-89.5%). In our analyses of behavioural data, we did not observe an evident decrease in high-risk behaviour. Interpretation It is unlikely that PPV was a driver of the outbreak’s decline, as incidence started to decline well before the start of the PPV programme. The possible impact of behavioural change could not be demonstrated with the available indicators. We hypothesise that infection-induced immunity in high-risk groups contributed most to the decline. Funding Dutch Ministry of Health, Welfare and Sport.