New research from the eastern Democratic Republic of the Congo indicates that mpox virus clade Ib may have a longer incubation period than previously thought, particularly when spread through nonsexual community transmission. The study, published in the Annals of Internal Medicine, used Bayesian analysis on clinical surveillance data collected between June and October 2024 at the Mpox Treatment Center in South Kivu.
The mpox clade Ib first appeared in September 2023 in South Kivu and initially spread rapidly through sexual contact. Over time, however, nonsexual community transmission became more common, especially among young children within households. Animal-to-human spillover events remain rare.
Researchers found that among high-confidence cases—those with strong PCR confirmation—the median incubation period from exposure to rash onset was 13.6 days, with a credible interval ranging from 9.6 to 19 days. Notably, about five percent of cases developed symptoms within just over three days after exposure, while 95% showed symptoms by day 32.3. This suggests that some infections may develop outside the standard 21-day monitoring window currently recommended by the World Health Organization (WHO).
The study also examined differences based on transmission route. Sexual transmission led to a shorter median incubation period of 10.3 days compared to 13.5 days for nonsexual routes; however, there was significant overlap between these groups.
“These findings suggest that clade Ib may have a longer incubation period than other MPXV clades, and this may vary by transmission route,” the researchers stated. They added: “These findings have implications for global recommendations on postexposure monitoring periods and prophylaxis.”
Currently, WHO guidelines advise monitoring exposed individuals for 21 days and recommend vaccination within 14 days after exposure based on previous knowledge from other mpox clades and smallpox vaccination data.
The authors suggest that public health interventions should be adapted to local patterns of transmission. In regions where nonsexual household or community spread is dominant—such as eastern Democratic Republic of the Congo—surveillance efforts might need to be strengthened and postexposure monitoring periods extended beyond current standards.
