1. Department of Global Health and Development, London School of Hygiene and Tropical Medicine, 15–17 Tavistock Place, London WC1H 9SH, UK,
2. Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, UK and
3. School of Medicine, King’s College London, Guy’s Campus, Great Maze Pond, London SE1 9RT, UK