1. Real-time Syndromic Surveillance Team, Field Service, National Infection Service, Public Health England, Birmingham B3 2PW, UK
2. Nuffield Department of Primary Care Health Sciences, University of Oxford, Oxford OX2 6GG, UK
3. Royal College of General Practitioners Research and Surveillance Centre, London NW1 2FB, UK
4. Immunisation and Countermeasures, National Infection Service, Public Health England, London NW9 5EQ, UK