1. Department of Infection Biology, Faculty of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel St, London WC1E 7HT, UK
2. Center for Global Health, Perelman School of Medicine, University of Pennsylvania, Philadelphia 19104, PA, USA
3. Department of Pathology, University of Cambridge, Tennis Court Rd, Cambridge CB2 1QP, UK