1. National Center for Women and Children's Health, China Center for Disease Control and Prevention, Beijing 100013, People's Republic of China
2. National AIDS Reference Laboratory, National Center for AIDS/STD Control and Prevention, China Center for Disease Control and Prevention, Beijing 100050, People's Republic of China
3. Institute for Global Health and Departments of Medicine, Pathology, and Pediatrics, Vanderbilt University School of Medicine, Nashville, Tennessee 37232