Heterogeneity in Mycobacterium tuberculosis immunoreactivity in young children in Blantyre, Malawi: a community-based survey

Background: As tuberculosis (TB) incidence declines, transmission increasingly concentrates into vulnerable populations. There is an urgent need for affordable surveillance strategies to monitor trends, identify high-risk groups and target interventions. Mycobacterium tuberculosis (Mtb) immunoreactivity surveys indirectly detect transmission and therefore undiagnosed infectious disease. Methods: We conducted a cross-sectional community-based interferon-gamma release assay (IGRA) survey in children aged 1-4 years in Blantyre, Malawi. Community-representative participants were recruited using novel convenience sampling in health facilities alongside random household sampling, and tested for Mtb immunoreactivity using QFT-Plus IGRA. We constructed hierarchical Bayesian logistic regression models for IGRA positivity, with neighbourhood-level random effects. Findings: Of 1,545 participants, 102 (6.6%) had a positive IGRA: an annual risk of Mtb infection (ARTI) of 2.7% (95% CrI 2.2-3.2%). Immunoreactivity was higher in the poorest third of households (8.7% vs 4.9%; adjusted odds ratio: 1.88, 95% CrI 1.08-3.01) compared to the richest, but was not associated with HIV exposure, malnutrition or reported household TB exposure. There was substantial between-neighbourhood heterogeneity (ARTI range 1.1-4.1%). There was no association between neighbourhood-level TB case notifications and ARTI. Interpretation: An innovative convenience sampling approach identified a high burden and substantial spatial variation of recent TB transmission, which did not correspond to case notification rates. This strategy could support identification of high-risk populations, monitoring of trends and targeted public health interventions.