1 School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China. Electronic address: gmleung@hku.hk.
2 Division of Infectious Disease, Key Laboratory of Surveillance and Early Warning on Infectious Disease, China Centers for Disease Control and Prevention, Beijing, China.
3 School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, Hong Kong, China.