1 Guangzhou Eighth People's Hospital, Guangzhou Medical University, Guangzhou, China.
2 World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, University of Hong Kong, Hong Kong, SAR, China. ehylau@hku.hk.
3 World Health Organization Collaborating Centre for Infectious Disease Epidemiology and Control, School of Public Health, University of Hong Kong, Hong Kong, SAR, China.