Funding for this research was provided by:
National Natural Science Foundation of China (31371024)
EFSD/CDS/Lilly Programme award (EFSD/CDS/Lilly Programme award (2013))
Received: 15 October 2019
Accepted: 9 July 2020
First Online: 18 July 2020
Ethics approval and consent to participate
: This study was approved by the local ethics committee at Qingdao CDC, Qingdao, China. All participants provided written informed consent for participating in the study. We have acquired administrative permission to access the data which was used in the research.
: Not applicable.
: The authors declared that they had no competing interests.