Zhicheng Wang1, Ge Yang2, Yan Guo3. 1. Vanke School of Public Health, Tsinghua University, Beijing, China; Research Center for Public Health, School of Medicine, Tsinghua University, Beijing, China. Electronic address: wanzc61@gmail.com. 2. Department of Neonatology, Guangzhou Women and Children's Medical Center, Guangzhou, Guangdong, China. 3. Department of Health Policy and Management, School of Public Health, Peking University, Beijing, China. Electronic address: guoyan@bjmu.edu.cn.