3 Fengtai District Center for Disease Control and Prevention of Beijing, 3 Xi An St., Feng-Tai District, Beijing, 100071, China. lijie7111@sina.com. 4 National Institute for Viral Diseases Control and Prevention, China CDC.100 Ying-Xin St., Xuan-Wu District, Beijing, 100052, China. zhaojund@126.com. PMID: 27424027.