1. MRC Centre for Outbreak Analysis and Modelling; Department of Infectious Disease Epidemiology; Imperial College London; London; UK
2. European Centre for Disease Prevention and Control; Stockholm; Sweden
3. Division of Virology; National Institute for Biological Standards and Control; Health Protection Agency; Potters Bar; Hertfordshire; UK
4. Formerly Division of Virology; National Institute for Biological Standards and Control; Health Protection Agency; Potters Bar; Hertfordshire; UK