How change in behaviour influences the impact of mpox vaccination among men who have sex with men

Abstract Background The 2022 outbreak of mpox subclade IIb in the Netherlands affected primarily men who have sex with men (MSM). The recent outbreak of subclade Ib in African countries and its introduction in Europe and America have raised concerns about future mpox outbreaks. We investigated the potential impact of mpox introductions in the Netherlands, accounting for vaccination, behavioural changes, and introductions of subclades with different characteristics. Methods We developed a model for mpox transmission among MSM. We accounted for subgroups of MSM with different risk levels for mpox and allowed individuals to change risk level due to change in their behaviour. We examined several scenarios with mpox introductions. Results From the model, we calculated that 54% of high-risk MSM had immunity in December 2023. This percentage declined to 39% a year later, as unvaccinated MSM switched from low to high risk level due to changes in their behaviour, thus reducing the percentage immune in the high-risk group. In hypothetical model scenarios without vaccination in 2024-2025 and with mpox introductions in May 2025, small outbreaks were possible. The numbers of cases were lowered by 34% or 48% in a scenario with vaccination carried out in August-October 2024 or February-April 2025, respectively. Assuming that MSM took precautions in response to the outbreak resulted in 17-51% less mpox cases. Introduction of a subclade with 10% higher transmission probability than subclade IIb resulted in 77% more cases. Conclusions Due to changes in behaviour, the fraction immune among high-risk MSM will decline, leading to greater possibilities for future mpox outbreaks. The number of cases can be reduced with preventive vaccination, but also with precautionary measures when an outbreak starts. Recurring vaccination campaigns should prioritise individuals most at risk, ensuring those entering high-risk groups are reached. Campaigns promoting positive changes in behaviour remain crucial. Key messages • Due to changes in behaviour, not-immune MSM enter the high-risk group, thus reducing the fraction immune within the group and increasing the possibility of future mpox outbreaks. • Preventive mpox vaccination and behavioural adaptations can impede mpox spread and hence should be promoted with social marketing, informative campaigns, and innovative public health interventions.