1 1Centre for the Mathematical Modelling of Infectious Diseases, Department of Infectious Disease Epidemiology, Faculty of Epidemiology and Population Health, LSHTM, London, UK.
2 2Department of Population Health Sciences, Bristol Medical School, University of Bristol, Bristol, UK.
3 3The Milner Centre for Evolution, Department of Biology and Biochemistry, University of Bath, Bath, UK.
4 4Department of Veterinary Medicine, Cambridge, UK.
5 5Institute of Infection and Immunity, St George's, University of London, London, UK.