Junfang Xu1, Yu Shi2, Feng Cheng3, Wannian Liang4. 1. Institute of Social Medicine, School of Public Health, Zhejiang University School of Medicine, Hangzhou, China. 2. Vanke School of Public Health, Tsinghua University, Beijing 100084, China. 3. Vanke School of Public Health, Tsinghua University, Beijing 100084, China; Institute for Healthy China, Tsinghua University, Beijing 100084, China. 4. Vanke School of Public Health, Tsinghua University, Beijing 100084, China; Institute for Healthy China, Tsinghua University, Beijing 100084, China. Electronic address: liangwn@tsinghua.edu.cn.