Funding for this research was provided by:
National Science Foundation of China (71774104)
Chinese Medical Board (CMB-16-257)
Cheeloo Youth Scholar Grant (IFYT1810)
Shandong University (2012DX006)
Received: 16 April 2019
Accepted: 20 September 2019
First Online: 15 October 2019
Ethics approval and consent to participate
: The study protocol for this study involving human participants was approved by the Ethical Committee of Shandong University School of Public Health and was in accordance with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Before starting the survey, trained interviewers explained the details of the study to all participants, and written informed consent was obtained from each of the participants.
: Not applicable.
: The authors declare that they have no competing interests.