1. London School of Hygiene and Tropical Medicine; Centre for the Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
2. Public Health England; Field Epidemiology Services London, 151 Buckingham Palace Road, London SW1W 9SZ, UK.
3. Public Health England; Field Epidemiology Service Liverpool, 5th Floor, Rail House, Lord Nelson Street, Liverpool L1 1JF, UK.
4. Public Health England; NHS England Lancashire & Greater Manchester, 4th Floor, 3 Piccadilly Place, Manchester M1 3BN, UK.