Monkeypox: Early estimation of basic reproduction number R 0 in Europe.

This world outbreak of Monkeypox (MPX) infections outside Africa emerged on May 2022 in Europe and spread worldwide with unique characteristics: inter-human contagion and infection in men without specific previous immunization, prevalently men-who-have-sex-with-men (MSM). Phylogenetic analysis confirmed a unique clade, the West African clade, subclade IIb. On August 30, WHO stated 48 895 laboratory confirmed cases from 101 different countries, of which 28 050 were in Europe. It has therefore become important to define new epidemiological indices. Starting from our new surveillance system EpiMPX open data, we defined an early R 0 measure, using European ECDC confirmed cases from the epidemic start to the end of August 2022; our early R 0 pooled median is 2.44, with high variability between countries. We observed the higher R 0 in Portugal and Germany, followed by Italy, Spain, and France. Anyway, these high estimates refer to the MSM group rather than to the general population. Early estimation of R 0 can be used to support the epidemiological understanding of transmission dynamics and contain MPX from spreading in naive populations and core groups with risk factors. MPX is in an evolving situation with much to learn and to do to combat the current epidemic outbreak.