1. Department of Child and Adolescent Health and Maternal Care, School of Public Health, Capital Medical University, No.10 You'anmenwai Xitoutiao, Fengtai District, Beijing 100069, China
2. Department of Epidemiology of Microbial Diseases, Yale School of Public Health, and Department of Pediatrics, Yale School of Medicine, P.O. Box 208034, New Haven, CT 06520-8034, USA