Funding for this research was provided by:
National Key R&D Program of China (2017YFC0907300)
Science & Technology Project of Science and Technology Department of Sichuan Province (No. 2018JY0647)
Received: 23 July 2019
Accepted: 9 December 2019
First Online: 23 December 2019
Ethics approval and consent to participate
: Data involved in this study are statutory data as required by the central government, and the need for ethics approval is deemed unnecessary according to National Health Statistics Survey System of China. This study focused on aggregate data rather than individual, so consent to participate is unnecessary.
: Not applicable.
: The authors declare that they have no competing interests.