Clade Ib mpox spread poorly inside Burundi households — except to children

A prospective cohort study published on 6 October reports how often clade Ib mpox actually passed between people living in the same house in Burundi, and the answer is: not often, and mostly to children [s1].

Researchers enrolled 88 laboratory-confirmed primary mpox cases and 432 of their household contacts across two health districts, Bujumbura and Kayanza, between 23 January and 20 March 2025 [s1]. The primary outcome was a laboratory-confirmed secondary infection among contacts during follow-up [s1]. The study was funded by the UNICEF Burundi Country Office [s1].

The numbers

Of the 88 households, 18 — 20 percent — experienced any secondary transmission, and most primary cases generated a single secondary case [s1].

The overall household secondary attack rate was 6.15 percent (95% CI 4.02–8.95) [s1]. Split by age, it was 8.77 percent (5.44–13.22) among contacts under 15 and 2.84 percent (0.92–6.50) among those 15 and older — a significant difference [s1].

The estimated basic reproduction number inside households was 0.30 (95% CI 0.17–0.46), again split by age: 0.43 (0.21–0.70) for under-15s and 0.15 (0.03–0.27) for those 15 and over [s1].

An R0 below one inside households means household transmission alone cannot sustain an epidemic. The authors' interpretation follows directly: intrahousehold transmission of clade Ib mpox in Burundi was limited and unlikely to sustain broader community spread [s1].

The sensitivity analysis is the caveat

The study ran a sensitivity analysis to assess the effect of potentially misclassifying index cases younger than 15 — that is, cases where a child recorded as the household's first case may not have been [s1]. Under that analysis the estimates rose substantially: R0 of 0.9 (0.71–1.09) and a secondary attack rate of 17 percent (13.57–21.03) [s1].

That is a wide gap, and it is stated in the paper rather than buried. Whether household transmission of clade Ib is negligible or borderline self-sustaining depends on an assumption about who infected whom in households where a child was the first case identified. The honest reading is that the point estimate is low and the upper bound of plausible misclassification pushes it close to one.

Where the transmission is coming from instead

If households are not driving the outbreak, something else is. The authors conclude that infection outside the household, with adults serving as the source of initial household introductions, might be the primary driver [s1]. Their recommended response is a dual approach: interventions for household settings combined with targeted prevention for adults at risk in settings where community transmission is more probable [s1].

That is a meaningfully different operational posture from the one household-focused contact tracing implies. It shifts effort toward wherever adults are acquiring infection outside the home, while keeping household work aimed at protecting children who are exposed once the virus arrives.

Why the age gradient matters

The three-fold higher attack rate in children under 15 is the finding with the clearest implication. Children in this cohort were more likely to be infected once mpox entered their household, and the authors note this underscores their vulnerability and the need for accurate household investigation, early detection, and strategies to protect them [s1].

A linked commentary published the same day frames the pattern the same way — household transmission of mpox in Africa as limited in adults but more prevalent in children [s2].

Limits worth stating

This is a two-district cohort in one country over a two-month window, with 88 index households. It describes clade Ib transmission in Burundian household settings during early 2025; it does not establish what clade Ib does in other household structures, other countries, or later phases of an outbreak. Household size, sleeping arrangements and care-giving patterns all plausibly affect secondary attack rates, and a cohort of this size cannot resolve those.

It also cannot say why children were more affected. Higher contact intensity with a symptomatic adult caregiver, differences in susceptibility, and differences in how mild paediatric cases are detected are all consistent with the same result.

What to watch

The useful next data would be comparable household cohorts from other clade Ib settings, so the 6.15 percent figure can be checked against a different population, and better resolution on the index-case misclassification problem that separates an R0 of 0.30 from one of 0.9. Until that exists, the practical takeaway from Burundi is narrow but real: in this setting, the house was not where the outbreak was being sustained, and children inside it were the ones most likely to be infected.

Sources

• Characterising household transmission dynamics of clade Ib mpox in Burundi: a prospective cohort study — The Lancet Infectious Diseases, 6 October 2025
• Household transmission of mpox in Africa: limited in adults but more prevalent in children — The Lancet Infectious Diseases, 6 October 2025