Transient behavior changes and depletion of susceptibles in high contact groups led to case decline during 2022 mpox outbreak in MSM in Berlin

Mpox denotes a viral zoonosis caused by monkeypox virus (MPXV), which is endemic in West and Central Africa. In spring 2022, notable outbreaks of mpox clade IIb were recorded in several high-income countries, predominantly affecting men who have sex with men (MSM). At the peak of the outbreak, over 200 new mpox cases per week were reported in Berlin, which constitutes one of the largest MSM communities in Europe. Within the same year, the outbreak virtually disappeared, and it is unclear which factors contributed to its rapid decline. To investigate the concomitant effects of sexual contact networks, transient contact reductions and the effect of infection- vs. vaccine-derived immunity on the 2022 mpox outbreak, we calibrated an agent-based model with epidemic, vaccination, contact- and behavioral data. Our results indicate that vaccination had a marginal effect on the epidemic decline. Rather, a combination of infection-induced immunity of high-contact individuals, as well as transient behavior changes reduced the number of susceptible individuals below the epidemic threshold. However, the 2022 mpox vaccination campaign, together with infection-derived immunity may have contributed to herd-immunity in the Berlin MSM community against ongoing clade I mpox outbreaks, which are responsible for the death of 1,140 individuals in Kongo alone. However, demographic changes and immune waning will deteriorate this herd immunity over time.