Abstract
INTRODUCTION: The 2022 outbreak of mpox subclade IIb in the Netherlands affected primarily men who have sex with men (MSM). Despite the sharp decline in cases, concerns remain about future mpox outbreaks. This study investigated the effect of mpox introductions, accounting for vaccination, behavioural changes and introductions of subclades with different characteristics. METHODS: This study developed a compartmental model for mpox among MSM. Three levels of sexual activity were distinguished: low, medium and high. The group with high activity accounted for 5% of the population with the highest numbers of sexual partners, and the group with low activity included 60% with the lowest numbers of partners. In the model, individuals were allowed to change sexual activity level. Several scenarios with mpox introductions were examined. RESULTS: In January 2024, 54% of MSM with high sexual activity level had immunity; this declined to 39% a year later due to changes in sexual activity. The introduction of five cases in May 2025 resulted in 759 cases in the first 4 months without vaccination after 2023; however, the number of cases was 34% or 48% lower with 3000 vaccinations in August-October 2024 or February-April 2025, respectively. With earlier initiation or greater magnitude of behavioural adaptations, the number of mpox cases was 17%-51% smaller. The introduction of a subclade with 10% higher transmission probability than subclade IIb resulted in 77% more cases. CONCLUSIONS: Owing to changes in sexual activity, the fraction immune in the group with high sexual activity will decline, leading to greater possibilities for future mpox outbreaks. The number of cases can be reduced with preventive vaccination and behavioural adaptations. Recurring vaccination campaigns should prioritise individuals with high sexual activity levels, ensuring those entering high-activity groups are reached. Campaigns promoting timely behavioural changes remain crucial.