1 Department of Public Health, Environments and Society, London School of Hygiene & Tropical Medicine, University of London, London WC1E 7HT, UK; Centre for Statistical Methodology, London School of Hygiene & Tropical Medicine, University of London, London WC1E 7HT, UK. Electronic address: antonio.gasparrini@lshtm.ac.uk.
2 Department of Public Health, Environments and Society, London School of Hygiene & Tropical Medicine, University of London, London WC1E 7HT, UK.