1 Department of Health Services Research and Policy, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK. Electronic address: may.vanschalkwyk@lshtm.ac.uk.
2 Department of Public Health, Environments and Society, London School of Hygiene & Tropical Medicine, London WC1E 7HT, UK.
3 Global Health Policy Unit, School of Social and Political Science, University of Edinburgh, Edinburgh, UK.
4 MRC Epidemiology Unit, University of Cambridge, Cambridge, UK.