Funding for this research was provided by:
Pilot project on chronic diseases and immune prevention policy of China Preventive Medicine Association (20170101)
Received: 9 July 2019
Accepted: 17 December 2019
First Online: 27 December 2019
Ethics approval and consent to participate
: The informed consent obtained from study participants was written. Ethical approval was given by the medical ethics committee of Ethics Committee of Beijing Chaoyang District Center for Disease Control and Prevention with the following reference number CYCDPCIRB-YJ-20181015-1.
: Not applicable.
: The authors declare that they have no competing interests.