RT Journal Article SR Electronic T1 What is the most appropriate respiratory protection against COVID-19? JF BMJ Evidence-Based Medicine JO BMJ EBM FD BMJ Publishing Group Ltd SP e5 OP e5 DO 10.1136/bmjebm-2020-111441 VO 26 IS 6 A1 Nicholas E Burr A1 Christopher J Black YR 2021 UL http://ebm.bmj.com/content/26/6/e5.abstract AB