1. Department of Population Health, London School of Hygiene and Tropical Medicine, London, United Kingdom
2. Department of Epidemiology and Biostatistics, Kilimanjaro Christian Medical University College, Moshi, Tanzania
3. MRC/WITS Rural Public Health and Health Transitions Research Unit (Agincourt), School of Public Health, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa