The risk of future mpox outbreaks among men who have sex with men: a modelling study based on cross-sectional seroprevalence data

Structured abstract Background In the wake of the 2022-2023 mpox outbreak, crucial knowledge gaps exist regarding orthopoxvirus-specific immunity in risk groups and its impact on future outbreaks. Aim This study combined cross-sectional seroprevalence studies in two cities in the Netherlands with mathematical modelling to evaluate the risk of future mpox outbreaks among men who have sex with men (MSM). Methods Serum samples were obtained from 1,065 MSM visiting the Centres for Sexual Health (CSH) in Rotterdam or Amsterdam after the introduction of vaccination and the peak of the Dutch mpox outbreak. For MSM visiting the CSH in Rotterdam, sera were linked to epidemiological and vaccination data. An in-house developed ELISA was used to detect vaccinia virus (VACV)- specific IgG. These observations were combined with literature data on infection dynamics and vaccine effectiveness to inform a stochastic transmission model to estimate the risk on future mpox outbreaks. Results The seroprevalence of VACV-specific antibodies was 45.4% and 47.1% in Rotterdam and Amsterdam, respectively. Transmission modelling showed that the impact of risk group vaccination on the original outbreak was likely small; however, the number of mpox cases in a future outbreak would be markedly reduced because of vaccination. Simultaneously, the current level of immunity alone may not prevent future outbreaks. Maintaining a short time-to-diagnosis will be a key component of any strategy to prevent new outbreaks. Discussion Our findings indicate a reduced likelihood of future mpox outbreaks among MSM in the Netherlands under the current conditions, but emphasise the importance of maintaining population immunity, diagnostic capacities, and disease awareness.