A declining outbreak of the mpox virus among men who have sex with men

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

In addition, they decoupled behavioral changes from the impact of vaccination to understand what could trigger a new disease outbreak. Similarly, they quantified the contribution of infection-induced immunity in reducing cases among MSM. In particular, the model took into account mpox transmission only through sexual or intimate contact with regular and casual sex partners.

Study: Fading the mpox outbreak among men who have sex with men: A mathematical model 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 the majority of cases among MSM, had already started to subside. So the researchers examined what all factors contributed to this decrease, for example a decrease in the number of susceptible MSM or their behavioral changes, such as fewer casual sex partners and refraining from sexual or intimate contact, even with a regular partner.

About the study

In the current study, researchers developed a compartmental model to describe mpox transmission among MSM that matched the daily numbers of mpox reports in the Netherlands retrieved from their national surveillance system. Further, they used the adjustment 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 levels of sexual activity ranging from very low to very high. It worked on the assumption that infectious MSM abstained from sexual activity for a few days, which lowers their transmissibility compared to those who are not. Also, only a few individuals infected with mpox require hospitalization, while the rest recover and become immune.

The team assessed the number of confirmed mpox cases by the date symptoms started. They evaluated two behavioral changes in MSM: a decrease in duration of contagiousness when not abstaining from sex and making contact with fewer casual partners. In addition, they used a Bayesian approach to calibrate the study model to the number of daily mpox cases.

First, they calculated the daily number of mpox cases based on symptom onset using the model with each combination of parameter values. They then calculated the Poisson probability through July 5 and July 17, 2022 for the scenarios with only changes in July 2022 and June plus July 2022, respectively. In total, the team examined hypothetical scenarios for the mpox outbreak ending in 2022.

Research findings

The current study model remarkably explored counterfactuals and showed that the 2022 mpox outbreak in the Netherlands was large but self-limited and could have subsided 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 the vaccine was rolled out. In addition, by then the infection-induced immunity was sufficient to prevent new outbreaks in the short term.

By including heterogeneity in sexual activity, the study model captured the assortative admixture of highly sexually active individuals. Such sexual networks increase the likelihood of rapid spread of infection. However, the study model showed that even a smaller MSM group with a lot of sexual activity can cause a major disease outbreak in a short period of time. On the other hand, a reduction in the number of casual partners would have reduced the size of the mpox epidemic. These findings demonstrate the importance of timely communication with at-risk populations without stigmatization or fear-mongering. Spreading awareness even helped people with asymptomatic and presymptomatic 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 only achievable with behavioral changes. It is also noteworthy that the owners and organizers of MSM sex venues have contributed to raising awareness based on scientific evidence. Such concerted efforts could help contain similar outbreaks by other pathogens (not sexually transmitted) among MSMs, a subpopulation with much more contact than usual.

In addition, the study highlighted the positive impact of the mpox vaccination program. It reduced the risk of disease flare-ups, improved population-level immunity, and enhanced the effect of consciousness and behavior changes. For example, the researchers noted that the peak in modeled mpox cases corresponded to observations with no behavioral changes. In July 2022, with changes in behavior, the study model estimated a 13 to 24% decrease in casual partners and a 50% decrease in contagiousness to sexual abstinence.


The study showed that the depletion of susceptible MSM could have limited further growth of the mpox epidemic. However, behavioral changes in July 2022 accelerated the decline of mpox cases among MSM in the Netherlands after the peak. Later, infection-induced immunity and vaccination of the most sexually active people reduced the risk of mpox resurgence. Fortunately, the short-term resurgence risk of mpox appears to be very low, even if sexual behavior returns to pre-endemic levels or the entry of mpox infections. In addition, the research model could be used to investigate the long-term impact of vaccination on some subgroups of MSM and expanded to account for transitions between sexually active MSM groups.

Since prevention is better than cure, it is primarily crucial to maintain high immunity in the MSM group with the most sexual activity, although it is not a closed fixed group. Over time, unvaccinated MSM with low levels of sexual activity could become highly active at this time and supplement the pool of more susceptible MSM with high levels of sexual activity, triggering another mpox outbreak. Second, distribute public health messages in the locations the target audience visits. Third, since age-related sexual admixture is assortative, modeling should also account for the changing distribution of immunity in MSM less than 45 years of age.

*Important announcement

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

A declining outbreak of the mpox virus among men who have sex with men

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