London School of Hygiene & Tropical Medicine - Department of Clinical Research, Biomedical Research and Training Institute, Biomedical Research & Training Institute - The Health Research Unit Zimbabwe, University Teaching Hospital, Lusaka, London School of Hygiene & Tropical Medicine, London School of Hygiene & Tropical Medicine - Department of Clinical Research, University Teaching Hospital, Lusaka, Biomedical Research and Training Institute, Department of Paediatrics, University of Zimbabwe, Biomedical Research & Training Institute - The Health Research Unit Zimbabwe, University of Oxford - Nuffield Department of Medicine, affiliation not provided to SSRN, London School of Hygiene & Tropical Medicine - MRC Tropical Epidemiology Group (TEG), University of East Anglia (UEA) - Norwich Medical School, University Teaching Hospital, Lusaka and University of Bristol - Musculoskeletal Research Unit