X Wu1, H Wang2, Y Yang3, Y Zheng3, L Sha3. 1. Jiangsu Provincial Center for Disease Control and Prevention, Nanjing, China; The 4th China Medical Team of Assistance to Sierra Leone. Electronic address: sxwbox@163.com. 2. Jiangsu Provincial Commission of Health and Family Planning, Nanjing, China; The 4th China Medical Team of Assistance to Sierra Leone. 3. The Second Hospital of Nanjing, Affiliated to South East University, Nanjing, China; The 4th China Medical Team of Assistance to Sierra Leone.