1 Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK. Electronic address: may.vanschalkwyk@lshtm.ac.uk.
2 Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK.
3 Goldsmiths, University of London, London, UK.
4 Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK; SPECTRUM Consortium, UK.