Extended incubation period identified in novel mpox strain

1. This surveillance study suggests that the incubation period of mpox virus (MPXV) clade Ib may be slightly longer than that of other subclades, based on data from the Democratic Republic of the Congo (DRC).

2. Incubation periods were shorter for cases likely acquired through sexual transmission compared with non-sexual transmission.

Evidence Rating Level: 3 (Average)

Study Rundown: South Kivu, a province in eastern Democratic Republic of the Congo (DRC), has experienced an outbreak of mpox linked to a novel mpox virus (MPXV) subclade, clade Ib, since late 2023. Transmission appears primarily human-to-human, shifting from sexual contact to community spread, disproportionately affecting young children. While incubation periods for clade Ib are often inferred from clade II data, differences in natural history between clade Ib and other MPXV variants remain unclear. This study estimated the incubation period of clade Ib using surveillance data from the Mpox Treatment Center in the Uvira Health Zone, with attention to exposure route and patient demographics. Only a subset of cases had complete contact information; among these, non-sexual physical and respiratory contacts were most common, while sexual contact was rare. In high-confidence confirmed cases, verified by low cycle threshold quantitative polymerase chain reaction (qPCR), median time from exposure to rash or fever onset was approximately two weeks. Rash incubation was shorter for sexual transmission, in males, early in the outbreak, and in children under fifteen. Limitations include the absence of clade typing to confirm clade Ib, incomplete exposure data, and assumptions regarding human-to-human transmission. Overall, these findings suggest clade Ib may have a slightly longer incubation period than other MPXV variants, varying by transmission route, age, sex, and epidemic phase, with implications for surveillance, outbreak response, and public health planning.

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Relevant Reading: Incubation Period and Serial Interval of Mpox in 2022 Global Outbreak Compared with Historical Estimates

In-Depth [cross-sectional study]: This surveillance study aimed to estimate the incubation period of mpox clade Ib cases in South Kivu. Data were collected from the Mpox Treatment Center in the Uvira Health Zone through structured interviews with suspected cases—defined as individuals presenting with a skin rash or papulovesicular or pustular eruptions—between June and October 2024. Suspected case contacts were defined as individuals with a rash, lesions, or similar symptoms who had contact with a patient or confirmed mpox case within three weeks prior to symptom onset or diagnosis. Although all suspected cases were intended to undergo quantitative polymerase chain reaction (qPCR) testing, supply shortages limited this. Cases were classified by PCR cycle threshold (Ct), with Ct <34 designated as “high-confidence confirmed cases,” Ct <39 as “confirmed cases,” and untested or unconfirmed cases as “suspected cases.” During the study period, 973 suspected cases were recorded, with 35.6% reporting contact with a suspected mpox case; 70% of these provided temporal exposure information. Males comprised 48% of cases, 30% were children under five, and 2% were 45 years or older; 44% were hospitalized. Most reported non-sexual physical contact (95%) and respiratory contact (84%), while sexual contact was uncommon (8%). Among 123 cases with complete contact and laboratory data, 72% were PCR-positive, and 40% of these were high-confidence confirmed cases. Among confirmed cases, the median time from most recent contact to rash onset was 10 days (IQR, 3–14), 9 days to fever onset (IQR, 3–14), and 9 days to any symptom (IQR, 2–13). High-confidence confirmed cases had a median incubation from exposure to rash onset of 13.6 days (95% CrI, 9.6–19.0), with fever and symptom onset slightly shorter. Incubation was shorter for sexual versus non-sexual transmission and in the early epidemic phase. Sensitivity analyses including confirmed and suspected cases suggested longer incubation, particularly for non-sexual exposures. Overall, these findings indicate that MPXV clade Ib may have a slightly longer incubation period than other variants, influenced by transmission route and epidemic phase, with implications for surveillance and outbreak response.

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