[1] Shaanxi Univ Chinese Med, Sch Publ Hlth, Xianyang 712046, Shaanxi, Peoples R China
[2] Xi An Jiao Tong Univ, Natl Hlth & Family Planning Commiss, Key Lab Trace Elements & Endem Dis, Inst Endem Dis,Sch Publ Hlth,Hlth Sci Ctr, Xian 710061, Shaanxi, Peoples R China
[3] Xian Med Univ, Dept Publ Hlth, Xian 710021, Shaanxi, Peoples R China
[4] Ctr Dis Control & Prevent, Immunizat Dept, Xianyang 712046, Shaanxi, Peoples R China