Characterising household transmission dynamics of clade Ib mpox in Burundi: a prospective cohort study

Knowledge of intrahousehold transmission dynamics of clade Ib mpox, especially in recently epidemic African contexts, is scarce. Our study aimed to quantify household transmission patterns of clade Ib mpox in Burundi, with a focus on children.

We conducted a prospective cohort study in two health districts, Bujumbura and Kayanza, in Burundi from Jan 23 to March 20, 2025, enrolling 88 laboratory-confirmed primary mpox cases and 432 of their household contacts. We estimated household secondary attack rates (SARs), serial intervals, and basic reproduction number (R₀), including a sensitivity analysis to assess the effect of potential misclassification of mpox index cases younger than 15 years. The primary outcome was occurrence of a secondary mpox infection within the household, defined as any laboratory-confirmed mpox case identified among contacts during the follow-up period.

Of the 88 households, 18 (20%) experienced secondary transmission, with most primary mpox cases generating a single secondary case. The overall SAR across all households was 6·15% (95% CI 4·02-8·95) and was significantly higher among those younger than 15 years (8·77% [5·44 -13·22]) than among those aged 15 years or older (2·84% [0·92-6·50]). The overall R₀ was 0·30 (95% CI 0·17-0·46), and was significantly higher for those younger than 15 years (0·43 [0·21-0·70]) than those aged 15 years or older (0·15 [0·03-0·27]). The sensitivity analysis showed significantly higher estimates (R₀ 0·9 [0·71-1·09]; SAR 17% [13·57-21·03]).

Intrahousehold transmission of clade 1b mpox in Burundi was limited, and unlikely to sustain a broader community spread. The involvement of children in transmission chains within the household underscores their vulnerability, emphasising the need for accurate household investigation, early detection, and strategies to protect them. Our findings suggest that infection outside the household, with adults serving as a source for initial household introductions, might be a primary driver of the outbreak. The mpox outbreak response should adopt a dual approach combining interventions for household settings and targeted prevention strategies for adults at risk where community transmission is more probable.

UNICEF Burundi Country Office.