1. Population Health Research Institute, St George University of London, London, SW17 0RE, UK
2. School of Primary Care Research, National Institute for Health Research, University of Manchester, Manchester, M13 9PL, UK
3. South African Medical Research; and Faculty of Medicine and Health Sciences, Stellenbosch University, Stellenbosch,7505, South Africa
4. Department of Social and Behavioral Sciences, Harvard T.H. Chan School of Public Health (HSPH), Harvard University, Boston, Massachusetts 02215, USA
5. Department of Sociology, University of Oxford and Nuffield College, Oxford, OX1 1NF, UK