1. Department of AIDS/STD Control and Prevention, Guangzhou Center for Disease Control and Prevention, No. 1, Qide Road, Jiahe, Baiyun District, Guangzhou, Guangdong Province 510440, China
2. University of North Carolina Project-China, No. 2, Lujing Road, Yuexiu District, Guangzhou, Guangdong Province 510095, China