1. Centre for Mathematical Modelling of Infectious Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK
2. Centre for Infection, Division of Cellular and Molecular Medicine, St George’s, University of London, Cranmer Terrace, London SW17 0RE, UK
3. CoMPLEX (Centre for Mathematics and Physics in the Life Sciences and Experimental Biology), University College London, Gower Street, London WC1E 6BT, UK