aHealth Economics and Epidemiology Research Office, Department of Internal Medicine, School of Clinical Medicine, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South AfricabDepartment of Epidemiology, Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USAcWitkoppen Health and Welfare Centre, Johannesburg, South AfricadSchool of Medicine, University of North Carolina at Chapel Hill, Chapel Hill, North CarolinaeCenter for Global Health and DevelopmentfDepartment of Epidemiology, Boston University School of Public Health, Boston University, Boston, Massachusetts, USA.