1 Global Health and Development Group and Medical Research Council Centre for Global Infectious Disease Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London SW7 2AZ, UK. Electronic address: a.gheorghe@imperial.ac.uk.
2 Global Health and Development Group and Medical Research Council Centre for Global Infectious Disease Analysis, Department of Infectious Disease Epidemiology, Imperial College London, London SW7 2AZ, UK.
3 Department of Economics and Related Studies, University of York, York, UK.