What led to the sharp decline in mpox diagnoses in Berlin after the outbreak peaked in July 2022?

Background The 2022 outbreak of mpox clade IIb among men having sex with men (MSM) in Europe was marked by a sharp and rapid rise in reported cases from May until July/August in many countries, followed by an equally swift decline by the end of the year. There is ongoing debate about the relative contributions of infection saturation within a core group, changes in sexual behaviour, and vaccination to the rapid decrease. Methods We used data from two sources to assess the potential roles of the three factors mentioned in the decline of mpox cases among MSM in Berlin. First, we analysed data from a mpox seroprevalence study conducted between April and June 2023. Second, we incorporated routine data from three Berlin community-based voluntary counselling and testing (CBVCT) centres to provide additional context. Factors associated with vaccination status and changes in sexual behaviour were identified through multivariable regression analysis based on self-reported data from an online questionnaire completed by study participants. Serological results have been published previously. We describe the distribution of acquired immunity and reported behavioural changes within core groups. Results Questionnaire data from 728 study participants were analysed, with serological data available for all. While 60% had received vaccination, cases declined soon after the campaign began, casting doubts on its impact. Reported behavioural changes were moderate and short-lived. Antibodies indicating prior mpox infection were found in 39–47% of a potential core group, most of whom had not been clinically diagnosed but were identified retrospectively. Conclusions Infection saturation within a core group played a key role in the decline of mpox transmission. Temporary behavioural changes contributed by slowing transmission until vaccination took effect. To prevent the resurgence of a susceptible core group and future outbreaks, ongoing vaccination efforts in core groups will be essential.