Combining mpox vaccination and behavioural changes to control possible future mpox resurgence among men who have sex with men: a mathematical modelling study

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, there are concerns about future mpox outbreaks. We investigated the impact of mpox introductions, accounting for vaccination, behavioural changes, and introductions of subclades with different characteristics.

We developed a compartmental model for mpox among MSM. We distinguished three levels of sexual activity: low, medium, and high. The group with high activity was the 5% of the population with the highest numbers of sexual partners; the group with low activity was the 60% with the lowest numbers of partners. In the model, individuals were allowed to change sexual activity level. We examined several scenarios with mpox introductions.

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. Introduction of 5 cases in May 2025 resulted in 759 cases in the first four months without vaccination after 2023, but the numbers of cases were 34% or 48% lower with 3,000 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. Introduction of a subclade with 10% higher transmission probability than subclade IIb resulted in 77% more cases.

Due 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 level, ensuring those entering high-activity groups are reached. Campaigns promoting timely behavioural changes remain crucial. What is already known on this topic The numbers of mpox cases among men who have sex with men (MSM) in the Netherlands were low in 2023-2024, but there have been many introductions of new cases infected outside the Netherlands. Therefore, there are concerns about future mpox outbreaks, but it is uncertain whether new mpox vaccination campaigns are needed. What this study adds Due to behavioural changes, not-immune MSM enter the group with high sexual activity, 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. However, outbreak sizes will increase as the time between vaccination roll-out and new introductions is extended. How this study might affect research, practice or policy Social marketing and interventions promoting timely behavioural changes and vaccination are essential. Recurring vaccination campaigns are necessary for mpox prevention, prioritising individuals with high sexual activity level and ensuring those entering high-activity groups are reached. The findings also emphasize that research is needed to understand determinants of behaviour and monitor behavioural changes over time. Competing Interest Statement The authors have declared no competing interest. Funding Statement This study did not receive any funding Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study used ONLY simulated data or openly available human data: Data with the numbers of daily mpox cases in the Netherlands are freely available on the website of the National Institute of Public Health and Environment of the Netherlands: https://www.rivm.nl/en/mpox/current-information-about-mpox. The weekly numbers of mpox vaccinations are available in the report: Sexually transmitted infections in the Netherlands in 2023 (page 152) of Kayaert L, et al. (RIVM report 2024-0038; Center for Infectious Diseases Control, National Institute of Public Health and the Environment (RIVM); Bilthoven, 2024. Available online: https://www.rivm.nl/bibliotheek/rapporten/2024-0038.pdf). Data from the PrEP pilot are third-party data from the Dutch national registration of Sexual Health Centre consultations (SOAP); they can be obtained via request to soap{at}rivm.nl. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes Figure 3 and Figures in the Supplement were updated.