1. Centre of Influenza Research and School of Public Health, LKS Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China
2. Department of Pathology, The University of Hong Kong, Queen Mary Hospital, Pokfulam, Hong Kong SAR, China
3. Departments of Epidemiology and Microbiology and Immunology, Gillings School of Global Public Health, and School of Medicine, The University of North Carolina, Chapel Hill, North Carolina, USA
4. State Key Laboratory of Emerging Infectious Diseases, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong SAR, China