1. Xiang Ya School of Public Health, Central South University, Changsha, Hunan 410078, People's Republic of China
2. Institute for HIV/AIDS, The First Hospital of Changsha, Changsha, Hunan 410011, People's Republic of China
3. Department of AIDS/STD Control and Prevention, Hunan Provincial Center for Disease Control and Prevention, Changsha, Hunan 410005, People's Republic of China