Human Monkeypox Virus outbreak among Men who have Sex with Men in Amsterdam and Rotterdam, the Netherlands: no evidence for undetected transmission prior to May 2022 in a retrospective study

Since May 2022 over 20.000 human monkeypox virus (hMPXV) cases have been reported from 29 EU/EEA countries, predominantly among men who have sex with men. With over 1200 cases, and a crude notification rate of 70.7 per million population, the Netherlands was in the top 5 European countries most affected. The first national case was reported from May 10th, yet potential prior transmission remains unknown. Insight into prolonged undetected transmission can help to understand the current outbreak dynamics and aid future public health interventions. We therefor performed a retrospective study to elucidate whether undetected transmission of hMPXV occurred prior to the first reported cases in Amsterdam and Rotterdam. In 401 anorectal- and ulcer samples from visitors of the Centers for Sexual Health of Amsterdam and Rotterdam dating back to February 14th, we identified 2 new cases, the earliest from the first week of May (week 18), 2022. This coincides with the first cases reported in the United Kingdom, Spain and Portugal. We found no evidence of widespread hMPXV transmission in Dutch sexual networks of MSM prior to May 2022. Likely, the hMPXV outbreak expanded across Europe within a short period in the spring of 2022 in an international highly intertwined network of sexually active MSM. Ethical statementThe Amsterdam University Medical Centre Ethical committee approved the study and deemed a full review not necessary according to the Medical Research Involving Human Subjects Act (reference letter: W22_257 # 22.313, dd July 14, 2022). Samples included from Rotterdam were required to be collected more than 3 weeks prior to the analysis (i.e. the duration of quarantine in case of a positive result) as they were deemed to be of no consequence for the index or his/her contacts. All samples and collected data were fully de-identified before sample analysis assuring anonymity.