1. a Shandong Center for Disease Control and Prevention , Jinan , Shandong Province , P. R. China
2. b Qingdao Center for Disease Control and Prevention , Qingdao , Shandong Province , P. R. China
3. c Department of Epidemiology and Biostatistics, School of Public Health , Shandong University , Jinan , Shandong Province , P. R. China
4. d Health Counseling Center , Qingdao , Shandong Province , P. R. China
5. e Institute for Global Health , Vanderbilt University , Nashville , TN , USA
6. f Department of Biostatistics , Vanderbilt University , Nashville , TN , USA
7. g Department of Pediatrics , Vanderbilt University , Nashville , TN , USA