Funding for this research was provided by:
Major Program of National Fund of Philosophy and Social Science of China (17ZDA079)
Received: 27 March 2020
Accepted: 3 February 2021
First Online: 17 February 2021
Ethics approval and consent to participate
: This study used the secondary data from Shaanxi Provincial Center for Disease Control and Prevention and was thus exempted by the IRB of Xi’an Jiaotong University.
: Not applicable
: The authors declare that they have no competing interests.