1. National AIDS Reference Laboratory, National Center for AIDS & STD Control and Prevention, Chinese Center for Disease Control and Prevention, Beijing 100050, People's Republic of China
2. Department of Epidemiology, School of Public Health, University of Alabama at Birmingham, Birmingham, Alabama 35294-0022
3. Institute for Global Health and Department of Pediatrics, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232-0242