1. Chaoyang Center for Disease Control and Prevention, Beijing, P. R. China.
2. Institute for Global Health, Vanderbilt University School of Medicine, Nashville Tennessee.
3. Department of Pediatrics, Vanderbilt University School of Medicine, Nashville Tennessee.
4. State Key Laboratory for Infectious Disease Prevention and Control, and National Center for AIDS/STD Control and Prevention (NCAIDS), Chinese Center for Disease Control and Prevention (China CDC), Beijing, P. R. China.