1. School of Public Health, Department of Infectious Disease Epidemiology, Imperial College London, Medical School Building, St Mary’s Campus, Norfolk Place, W2 1PG, London, UK
2. Department of Economics, University of Bath, Claverton Down, Bath BA2 7JP, Bath, UK
3. Institute for Health Policy and Organisation, University of Manchester, Booth Street West, M15 6PB, Manchester, UK