1. Harvard Center for Population and Development Studies, Harvard University , Cambridge, Massachusetts , USA
2. Department of Epidemiology, Harvard T.H. Chan School of Public Health , Boston, Massachusetts , USA
3. Sydney Brenner Institute for Molecular Science, University of the Witwatersrand , Johannesburg , South Africa
4. Department of Social and Behavioral Sciences, Harvard T.H. Chan School of Public Health , Boston, Massachusetts , USA
5. MRC/Wits Rural Public Health and Health Transitions Research Unit , Agincourt , South Africa