1. Chinese Center for Disease Control and Prevention, Beijing, People's Republic of China;
2. State Key Laboratory for Infectious Disease Prevention and Control (ICDC), Beijing, China;
3. Beijing Ditan Hospital, Beijing, People's Republic of China;
4. Sichuan Center for Disease Control and Prevention, Sichuan, People's Republic of China;
5. Jiangsu Center for Disease Control and Prevention, Nanjing, People's Republic of China;
6. The Chinese University of Hong Kong, Hong Kong Special Administrative Region, People's Republic of China;
7. Centre for Health Protection, Hong Kong Special Administrative Region, People's Republic of China