1. Field Epidemiology Training Programme, Public Health England (East Midlands Office), Nottingham, England.
2. University of Nottingham Health Protection and Influenza Research Group, Clinical Sciences Building, Nottingham City Hospital, Hucknall Road, Nottingham, NG5 1PB, England.
3. Field Epidemiology Service, Public Health England (North East Office), Newcastle upon Tyne, England.