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A Waning Outbreak of the mpox Virus Among Men Who Have Sex with Men

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In a recent study posted to the medRxiv* server, researchers developed a mathematical model to describe the mpox (Monkeypox) transmission among men who have sex with men (MSM). They used this model to understand how MSM behavioral changes shaped the mpox outbreak in the Netherlands in June/July 2022.

In addition, they disentangled behavioral changes from the impact of vaccination to understand what could trigger a new disease outbreak. Likewise, they quantified the contribution of infection-induced immunity in reducing cases among MSM. Notably, the model accounted for mpox transmission only via sexual or intimate contact with steady and casual sex partners.

Study: The fading of the mpox outbreak among men who have sex with men: a mathematical modelling study. Image Credit: Dotted Yeti / Shutterstock


By the time the mpox vaccination program started in the Netherlands, i.e., in July 2022, the mpox outbreak, with most cases among MSM, had already begun to decline. So, the researchers investigated what all factors contributed to this decline, e.g., a decrease in the number of susceptible MSM or their behavioral changes, such as fewer casual sex partners and abstaining from sexual or intimate contact even with a steady partner.

About the study

In the present study, researchers developed a compartmental model to describe mpox transmission among MSM that fitted the daily numbers of mpox notifications in the Netherlands retrieved from their national surveillance system. Further, they used the fitting process to assess the magnitude and timing of the changes (all scenarios) that occurred only in July 2022 and combined for June and July 2022.

The study model classified MSM into four groups based on the number of partners and sexual activity levels, ranging from very low to very high. It worked on the assumption that infectious MSM abstains from sexual activity for a few days, which lowers their transmissibility compared to those who are not. Also, only a few mpox infected individuals need hospitalization, while the rest recover and become immune.

The team assessed the number of confirmed mpox cases per the date of symptom onset. They evaluated two behavioral changes in MSM - a decrease in the duration of the infectiousness while not abstaining from sex and making contact with fewer casual partners. Furthermore, they used a Bayesian approach to calibrate the study model to the number of daily mpox cases.

First, they computed the daily number of mpox cases by symptom onset using the model with every combination of parameter values. Next, they calculated the Poisson likelihood till 5 and 17 July 2022 for the scenarios with changes only in July 2022 and June plus July 2022, respectively. In totality, the team examined hypothetical scenarios for the mpox outbreak ending in 2022.

Study findings

The present study model remarkably explored counterfactual situations and showed that the 2022 mpox outbreak in the Netherlands was large but self-limited and could have faded by October 2022 even without interventions, such as MSM behavioral changes or vaccination. Notably, mpox had infected 64% of MSM with very high sexual activity in the model by the time of vaccine rollout. In addition, by that time, infection-induced immunity was adequate to prevent new outbreaks in the short term.

By incorporating heterogeneity in sexual activity, the study model captured the assortative mixing of highly sexually active individuals. Such sexual networks enhance the likelihood of the rapid spread of infection. However, the study model showed that even a smaller MSM group with a high level of sexual activity could cause a large disease outbreak in a short time. On the other hand, a reduction in the number of casual partners would have reduced the mpox epidemic scale. These findings demonstrate the importance of timely communication with at-risk populations without stigmatization or fear-chasing. In fact, spreading awareness even helped individuals with asymptomatic and pre-symptomatic mpox infections.

The study model showed that had there been no behavioral changes, the decline in mpox cases in a larger MSM group would have started about a week later, suggesting that the observed decrease in mpox cases was feasible only with behavioral changes. It is also noteworthy that the owners and organizers of MSM sex venues helped raise awareness per scientific evidence. Such collaborated efforts could help contain similar outbreaks by other pathogens (not sexually transmitted) among MSMs, a subpopulation having contact levels much higher than normal.

Furthermore, the study highlighted the positive impact of the mpox vaccination program. It reduced the risk of disease resurgence, enhanced immunity at the population level, and amplified the effect of awareness and behavioral changes. Thus, the researchers noted that the peak in modeled mpox cases matched observations without behavioral changes. In July 2022, with behavioral changes, the study model estimated a 13 to 24% decrease in the number of casual partners and a 50% decrease in infectiousness till sexual abstinence.


The study demonstrated that the depletion of susceptibles among MSM might have limited further growth of the mpox epidemic. However, behavioral changes in July 2022 accelerated the decrease in mpox cases among MSM in the Netherlands after the peak. Later, infection-induced immunity and vaccination of the most sexually active diminished the risk of resurgence of mpox. Thankfully, the resurgence risk of mpox in the short term appears to be very low, even with sexual behavior returning to pre-endemic levels or the import of mpox infections. In addition, the study model could be used to investigate the impact of vaccination in the long term for some subgroups of MSM and extended to account for transitioning across sexually active MSM groups.

Since prevention is better than cure, first and foremost, it is crucial to maintain high immunity in the MSM group with the most sexual activity though it is not a closed fixed group. Over time, unvaccinated MSM with low sexual activity at present might become highly active and supplement the pool of more susceptible MSM with a high level of sexual activity, thus causing a new mpox outbreak. Secondly, spread public health messages in the venues that the target group visit. Thirdly, as sexual mixing according to age is assortative, modeling should include the changing distribution of immunity in MSM less than 45 years or older.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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Schedule22 Jun 2024