Funing Bian1, Wenge Li1, Xianping Li2, Jinxing Lu3. 1. State Key Laboratory for Communicable Diseases Prevention and Control, Chinese Center for Disease Control and Prevention, Beijing 102206, China. 2. State Key Laboratory for Communicable Diseases Prevention and Control, Chinese Center for Disease Control and Prevention, Beijing 102206, China. Email: lixianping@icdc.cn. 3. State Key Laboratory for Communicable Diseases Prevention and Control, Chinese Center for Disease Control and Prevention, Beijing 102206, China. Email: lujinxing@icdc.cn.