1. Guangzhou Municipal Center for Disease Control and Prevention, Guangdong, People's Republic of China
2. National Institute for Communicable Disease Control and Prevention, Beijing, People's Republic of ChinaState Key Laboratory of Infectious Diseases Prevention and Control, Bejing, People's Republic of China
3. University of Hong Kong, Hong Kong Special Administrative Region, People's Republic of China
4. National Institute for Communicable Disease Control and Prevention, Beijing, People's Republic of China