1. Health Services and Research Group, Kenya Medical Research Institute/Wellcome Trust Research Programme, PO Box 43640, Nairobi 00100, Kenya
2. Nuffield Department of Medicine & Department of Paediatrics, University of Oxford, Henry Wellcome Building for Molecular Physiology, Old Road Campus, Headington, Oxford OX3 7BN, UK
3. School of Public Health and Family Medicine, Faculty of Health Sciences, University of Cape Town, Observatory, 7925 Cape Town, South Africa
4. Department of Global Health and Development, London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK