1. The Aurum Institute, Johannesburg, 29 Queens Road, Parktown Johannesburg, Gauteng, 2193 South Africa
2. Department of Global Health and Development, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK
3. Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK
4. Department of Clinical Research, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK