1 Hunan Provincial Center for Disease Control and Prevention, Changsha, China.
2 National Institute for Viral Disease Control and Prevention, Key Laboratory for Medical Virology, National Health and Family Planning Commission, China CDC, Beijing, China.
3 National Institute for Viral Disease Control and Prevention, Key Laboratory for Medical Virology, National Health and Family Planning Commission, China CDC, Beijing, China. Electronic address: yshu@cnic.org.cn.
4 Hunan Provincial Center for Disease Control and Prevention, Changsha, China. Electronic address: gldlj@hotmail.com.