1. Helen Ward, Christina Atchison, Matthew Whitaker, Bethan Davies, Deborah Ashby, Marc Chadeau-Hyam, Steven Riley, and Paul Elliott are with the School of Public Health, Imperial College London, UK. Ara Darzi is with the Institute of Global Health Innovation, Imperial College London. Christl A. Donnelly is with the Department of Statistics, University of Oxford, Oxford, UK. Wendy Barclay and Graham S. Cooke are with the Department of Infectious Disease, Imperial College London.