1. Graduate School, Peking Union Medical College, Beijing, China
2. Department of Epidemiology, Capital Institute of Paediatrics, Beijing, China
3. Chinese Centre for Disease Control and Prevention, Beijing, China
4. Tulane University Health Sciences Center, School of Public Health and Tropical Medicine, New Orleans, Louisiana, USA
5. Department of Maternal and Child Health Care, School of Public Health, Shandong University, Jinan, China
6. Laboratory Department, Beijing Jishuitan Hospital, Beijing, China
7. Department of Epidemiology and Statistics, Weifang Medical College, Weifang, China