1. Department of Epidemiology, School of Public Health, Boston University, Boston, Massachusetts;
2. Department of Paediatrics and Child Health, Red Cross War Memorial Children’s Hospital,
3. SA–Medical Research Council Unit on Child and Adolescent Health, and
4. Division of Medical Microbiology, University of Cape Town, Cape Town, South Africa; and
5. Marshall Centre for Infection and Immunity, School of Biomedical Sciences, University of Western Australia, Perth, Western Australia, Australia