Modelling the outbreak risk of monkeypox virus clade Ib among MSM in the EU/EEA and the impact of targeted vaccination

Since the upsurge of monkeypox virus (MPXV) clade Ib in the Democratic Republic of the Congo and neighbouring countries in 2024, there have been multiple importations into Europe. So far, secondary transmission from the imported MPXV clade Ib has been limited. However, further importations are likely, and the associated risk of large MPXV clade Ib outbreaks is a public health concern in the EU/EEA. This work assesses the outbreak risk of MPXV clade Ib among men who have sex with men (MSM), since this population group was most affected by previous mpox outbreaks in Europe. Based on a stochastic, individual-based mathematical model on simulated importations and MSM sexual contact network data, our results are twofold. First, if there is an undetected MPXV clade Ib importation into an urban MSM community in the EU/EEA, then we estimate a relatively high probability of around 19% that a small outbreak above 10 cases occurs, and a moderately low probability of around 4% that a large outbreak above 100 cases occurs. Second, we estimate that a pre-emptive increase of the mpox vaccine uptake by 2.5 to 5 percentage points among MSM has a substantial impact on reducing the risk of large outbreaks – this effect is particularly large when targeting the most sexually active individuals. Given continued global circulation of MPXV clade Ib, our work suggests that small-to-moderate-sized outbreaks among MSM in the EU/EEA are likely and that increasing vaccination uptake remains an effective public health measure in reducing the risk of large outbreaks.