Weifeng Shi1,2, George F Gao3,4. 1. School of Public Health, Shandong First Medical University and Shandong Academy of Medical Sciences, Ji'nan 250000, China. shiwf@ioz.ac.cn gaof@im.ac.cn. 2. Key Laboratory of Etiology and Epidemiology of Emerging Infectious Diseases in Universities of Shandong, Shandong First Medical University and Shandong Academy of Medical Sciences, Taian 271000, China. 3. National Institute for Viral Disease Control and Prevention, Chinese Center for Disease Control and Prevention, Beijing 102206, China. shiwf@ioz.ac.cn gaof@im.ac.cn. 4. CAS Key Laboratory of Pathogenic Microbiology and Immunology, Institute of Microbiology, Chinese Academy of Sciences (CAS), Beijing 100101, China.