The incubation periods of Monkeypox virus clade Ib

BackgroundMonkeypox virus (MPXV) clade Ib, first detected in the Democratic Republic of the Congo (DRC) in September 2023, spread internationally within months, prompting a WHO emergency declaration. Data on its incubation period, which both shapes outbreak dynamics and informs epidemic response strategies remain limited. ObjectiveTo estimate the incubation periods of mpox clade Ib examining evidence for differences by route of exposure and demographic factors. DesignBayesian analysis of clinical surveillance data collected between June and October 2024. SettingSouth Kivu, Democratic Republic of Congo, the epicenter of the current mpox clade Ib global outbreak. ParticipantsClinically attended persons with confirmed mpox clade Ib infection. MeasurementsDemographics, exposure history, symptom onset, and transmission route. ResultsAmong 37 PCR-confirmed cases with high viral load (Cycle Threshold [Ct] values less than 34), the median incubation period from exposure to rash was 13.6 days (95% CrI: 10.7-20.2). Five percent of cases are expected to develop a rash within 3.8 days (95% CrI; 1.7-6.6) and 95% within 33.4 days (95% CrI: 24.1-46.4). The incubation period appeared to differ by putative transmission route: sexual transmission had a shorter median (10.3 days, 95% CrI 3.1-20.3) than non-sexual transmission (13.5 days, 95% CrI: 9.5-19.1), though the confidence intervals overlapped. LimitationSurveillance data lacked detailed exposure histories and a lower bound for exposure periods, but models accounted for these uncertainties, yielding robust median estimates. ConclusionEvidence from this study suggests that clade Ib may have a longer incubation period than other MPXV clades, and this may vary by transmission route. The shorter incubation for sexual transmission mirrors patterns seen in the predominantly sexually transmitted clade IIb outbreak, highlighting the potential role of exposure route in disease progression. These findings have im-plications for global recommendation on post-exposure monitoring periods and prophylaxis. Primary Funding SourceThis work was supported by the Gates Foundation (INV-079976) and funds from the Geneva Centre for Emerging Viruses. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.