The Fading of the Mpox Outbreak Among Men Who Have Sex With Men: A Mathematical Modelling Study

Abstract

Background In the Netherlands, the number of mpox cases started declining before mpox vaccination was initiated. Most cases were men who have sex with men (MSM). We investigated whether the decline in mpox could be attributed to infection-induced immunity or behavioural adaptations. Methods We developed a transmission model and accounted for possible behavioural adaptations: less casual partners and shorter time until MSM with mpox refrain from sexual contacts. Results Without behavioural adaptations, the peak in modelled cases matched observations, but the decline was less steep than observed. With behavioural adaptations in the model, we found a decline of 16-18% in numbers of casual partners in June and 13-22% in July 2022. Model results showed a halving of the time before refraining from sex. When mpox vaccination started, 57% of MSM with very high sexual activity in the model had been infected. Model scenarios revealed that the outbreak could have waned by November 2022 even without vaccination. Conclusions The limited duration of the mpox outbreak in the Netherlands can be ascribed primarily to infection-induced immunity among MSM with high sexual activity levels. The decline was accelerated by behavioural adaptations. Immunity among those most sexually active is essential to impede mpox resurgence.

Factors potentially contributing to the decline of the mpox outbreak in the Netherlands, 2022 and 2023

“…A likely contributor to the declining course of the outbreak may be infection-acquired immunity in a substantial proportion of those most at risk. This is also in line with results from several modelling studies that conclude that immunity due to infection in the most sexually active GBMSM may have been an important driver of the decline [ 32 - 34 ]. However, these studies also acknowledge that behavioural changes might have had a substantial impact on the course of the epidemic.…”

“…However, these studies also acknowledge that behavioural changes might have had a substantial impact on the course of the epidemic. Modelling results using data on Dutch mpox cases before the start of the PPV programme indicate that increased immunity due to infections may have limited further growth of the mpox epidemic but could not exclusively explain the fast decline after the peak [ 34 ]. The model suggests that behavioural adaptations among GBMSM (including reduction in the number of sexual partners) accelerated the decrease observed before the start of PPV.…”

“…Fast implementation of future vaccination programmes is essential to be effective in preventing new cases [ 37 ]. Immunity due to infection and vaccination among the most sexually active GBMSM reduces the risk of resurgence of mpox [ 32 , 34 ] and will protect against (severe) disease. The mpox vaccination programme in the Netherlands continued in 2023 and took newly identified risk factors, such as visiting sex venues, into account [ 30 , 38 ].…”

A survey-based assessment of rates and covariates of mpox diagnosis and vaccination provides evidence to refine eligibility criteria for mpox vaccination among gay, bisexual and other men who have sex with men in the Netherlands

“…In the second half of 2022, the number of mpox cases declined in the Netherlands ( 16 ), as well as in Europe ( 18 ) and globally ( 8 ). Future trends in mpox infections remain uncertain, although new outbreaks are likely to be of smaller magnitude than the 2022 outbreak ( 19 , 20 ). In this context, mpox remains a public health concern, and achieving and sustaining elimination are priorities ( 21 ).…”

Impact of interventions on mpox transmission during the 2022 outbreak in Canada: a mathematical modeling study of three different cities

“…Various factors that govern the underlying transmission dynamics could have contributed to reductions in transmission, including saturation of groups at high risk of infection [10][11][12][13], changes in sexual behaviours following community outreach [14][15][16], contact tracing and isolation of traced contacts by local public health units [17,18], and use of vaccination [19,20].…”