1. Clinical HIV Research Unit, Department of Internal Medicine, School of Clinical Medicine, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg 2041, South Africa
2. Health Economics and Epidemiology Research Office, Department of Internal Medicine, School of Clinical Medicine, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg 2193, South Africa
3. Center for Global Health and Development, Boston University, Boston, MA 02118, USA
4. Department of Epidemiology, Boston University School of Public Health, Boston, MA 02118, USA