1. a Department of Epidemiology and Population Health , London School of Hygiene and Tropical Medicine , London , UK
2. b School of Public Health , University of the Witwatersrand , South Africa
3. c Department of Public Health and Policy , London School of Hygiene and Tropical Medicine , London , UK
4. d Department of Infectious and Tropical Diseases , London School of Hygiene and Tropical Medicine , London , UK