[1] Peking Univ, Hosp 3, Beijing, Peoples R China
[2] Peking Univ, Sch Publ Hlth, Inst Child & Adolescent Hlth, Xueyuan Rd 38, Beijing, Peoples R China
[3] Stanford Univ, Dept Med, Stanford Prevent Res Ctr, Stanford, CA 94305 USA
[4] Beijing Ctr Dicease Prevent & Control, Beijing, Peoples R China