1. Medical Research Council Centre for Outbreak Analysis and Modelling, Imperial College London, London W2 1PG, UK.
2. Department of Epidemiology, Department of and Immunology and Infectious Diseases, and Center for Communicable Disease Dynamics, Harvard School of Public Health, Boston, MA, USA.
3. Centre of Influenza Research and State Key Laboratory of Emerging Infectious Diseases, The University of Hong Kong, Hong Kong SAR, China.
4. Department of Epidemiology, University of Michigan School of Public Health, Ann Arbor, MI USA.
5. Division of Virology, Department of Infectious Diseases, St. Jude Children's Research Hospital, Memphis, TN, USA.
6. Health Protection Agency, London NW9 5HT, UK.
7. European Centre for Disease Prevention and Control, Stockholm, Sweden.
8. School of Public Health, The University of Hong Kong, Pokfulam, Hong Kong SAR, China.