1. MRC Centre for Outbreak Analysis and Modelling, School of Public Health, Imperial College London, Norfolk Place, London W2 1PG, UK.
2. Department of Epidemiology, Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe Street, Baltimore, MD 21205, USA.
3. Department of Biology and Emerging Pathogens Institute, University of Florida, Post Office Box 100009, Gainesville, FL 32610, USA.