. 2020 Oct 28;371:m3873. doi: 10.1136/bmj.m3873
© Author(s) (or their employer(s)) 2019. Re-use permitted under CC
BY. No commercial re-use. See rights and permissions. Published by
BMJ.
This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/4.0/.
Fig 1.
Flow chart of included cases and controls