Background
Monkeypox virus (MPXV) clade Ib, first detected in Democratic Republic of the Congo (DRC) in September 2023, spread internationally within months, prompting a WHO emergency declaration. Data on how this clade differs from others, including the incubation period which both shapes outbreak dynamics and informs epidemic response strategies, remain limited.
Methods
We use clinical, laboratory and reported exposure data from suspected and qPCR-confirmed mpox cases in Uvira, DRC to estimate the incubation period for MPXV clade 1b. We developed a Bayesian model to estimate incubation periods of various symptom onset definitions (rash, fever, any symptom), demographic factors and putative transmission route (sexual vs. non-sexual).
Results
Among 92 PCR-confirmed cases, the median incubation period from exposure to rash was 18.6 days (95% CrI: 15.2-22.6). Five percent of cases are expected to develop a rash within 5.4 days (95% CrI; 3.4-7.6) and 95% within 37.6 days (95% CrI: 30.3-47.0). The incubation period differed by putative transmission route: sexual transmission had a shorter median (9.5 days, 95% CrI 3.7-16.4) than non-sexual transmission (18.9 days, 95% CrI: 15.3-23.0).
Conclusions
Incubation periods for clade Ib are longer than previous estimates for other MPXV clades, but vary strongly by sexual vs. non-sexual exposures, the former closely aligning with observations from the predominantly sexually transmitted clade IIa global outbreak. These findings highlight the significant differences by transmission route – likely reflecting differences in infectious viral dose and suggest the need to revise global post-exposure monitoring recommendations.