1. School of Public Health, Zhejiang Chinese Medical University, Hangzhou, China
2. Influenza Division, Centers for Disease Control and Prevention, Atlanta, GA, USA
3. Department of Infectious Disease control and Prevention, Guangzhou Center for Disease Control and Prevention, Guangzhou, China
4. School of Public Health, Southern Medical University, Guangzhou, China
5. Huashan Hospital, Fudan University, Shanghai, China