1. Health Economics and Epidemiology Research Office, Department of Internal Medicine, School of Clinical Medicine, Faculty of Health Sciences, University of the Witwatersrand , 39 Empire Road, , Johannesburg 2193, South Africa
2. School of Public Health, Faculty of Health Sciences, University of the Witwatersrand , Johannesburg, South Africa
3. Department of Health Policy, London School of Economics and Political Science , London, United Kingdom