. 2021 Nov 24;9:788370. doi: 10.3389/fpubh.2021.788370
Copyright © 2021 Liu, Xu and Ma.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Figure 1.
Literature search strategy, following PRISMA flow diagram (12).