Incubation period for novel mpox variant slightly longer than for previous variants
Patients infected with mpox virus clade Ib, first detected in September 2023, had a median incubation period from exposure to rash of 13.6 days with high viral load, suggesting that currently recommended monitoring may be too short, according to the authors of a new analysis.
The novel mpox variant clade Ib may have a longer incubation period than previous variants, a recent study found.
Researchers used clinical surveillance data from the Democratic Republic of the Congo, where clade Ib was first detected in September 2023, to estimate incubation periods of the mpox variant and determine whether they varied by route of exposure and other factors. The main measures were demographic characteristics, exposure history, symptom onset, and transmission route. Results were published Dec. 9 by Annals of Internal Medicine.
The data examined in the study were collected between June and October 2024. Among 37 cases with high viral load that were confirmed by polymerase chain reaction testing, the median incubation period from exposure to rash was 13.6 days (95% credible interval [CrI], 9.6 to 19.0 days). Five percent of cases were expected to develop a rash within 3.1 days (95% CrI, 1.3 to 5.5 days), and 95% were expected to develop a rash within 32.3 days (95% CrI, 22.4 to 45.8 days). The median incubation period was shorter among patients with sexual versus nonsexual transmission (10.3 days [95% CrI, 3.1 to 20.3 days] vs. 13.5 days [95% CrI, 9.5 to 19.1 days]), although the study authors noted that the 95% CrIs overlapped.
Detailed exposure histories and a lower bound for exposure periods were missing from the surveillance data, among other limitations, the authors noted. They concluded that clade Ib may have a longer incubation period than earlier mpox variants and that incubation may vary by transmission route.
“Our estimates suggest that the current 3-week monitoring period recommended by the [World Health Organization] may be insufficient for a nonnegligible fraction of clade Ib nonsexually transmitted infections, potentially leading to missed cases and ongoing transmission, although this finding should be confirmed by future [mpox virus] clade Ib studies,” they wrote. They called for stronger surveillance, extended monitoring, and targeted interventions based on dominant transmission routes to help improve containment efforts in countries affected by clade Ib, which includes the U.S., according to the latest report from the CDC.