Study area. A. The infant cohort is drawn from two catchment populations in southern Vietnam, shaded grey: 1) District 8 in Ho Chi Minh City (HCMC), and 2) Cao Lanh District (including Cao Lanh Town) in Dong Thap Province, showing the location of the collaborating site Dong Thap Hospital (star). B. Spatial distribution of the residence of cohort participants in Cao Lanh Town and District, Dong Thap (left) and District 8, HCMC (right). Lines show administrative wards (n = 333 in DT and n = 16 in HCMC) and symbols show location of collaborating hospitals: Dong Thap Hospital (star), Hospital for Tropical Diseases (pentagon), Hung Vuong Hospital (triangle) and District 8 Hospital (diamond).