1 Centre for the Mathematical Modelling of Infectious Diseases, and Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK. Electronic address: alexis.robert@lshtm.ac.uk.
2 Centre for the Mathematical Modelling of Infectious Diseases, and Department of Infectious Disease Epidemiology, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK.