1. Faculty of Medicine, University of Southampton, Southampton General Hospital, Southampton, UK
2. Academic Unit of Primary Care and Population Sciences and Department of Social Statistics and Demography, University of Southampton, Southampton, UK
3. School of Nursing & Public Health, Africa Health Research Institute, University of KwaZulu-Natal, KwaZulu-Natal, South Africa
4. Research Department of Epidemiology & Public Health, University College London, London, UK