Cloth Masks May Prevent Transmission of COVID-19
Authors: Catherine M. Clase, Edouard L. Fu, Meg Jardine, Johannes F.E. Mann, Juan J. Carrero
Journal: Annals of Internal Medicine (2021)

LettersApril 2021Cloth Masks May Prevent Transmission of COVID-19FREECatherine M. Clase, MB BChir, MSc, Edouard L. Fu, BSc, Meg Jardine, MBBS, PhD, Johannes F.E. Mann, MD, PhD, Juan J. Carrero, Pharm, PhDCatherine M. Clase, MB BChir, MScMcMaster University and St. Joseph's Hospital, Hamilton, Ontario, CanadaSearch for more papers by this author, Edouard L. Fu, BScLeiden University Medical Center, Leiden, the Netherlands Search for more papers by this author, Meg Jardine, MBBS, PhDMcMaster University and St. Joseph's Hospital, Hamilton, Ontario, CanadaSearch for more papers by this author, Johannes F.E. Mann, MD, PhDUniversity of Erlangen-Nürnberg and KfH Kidney Center, Munich-Schwabing, Germany Search for more papers by this author, Juan J. Carrero, Pharm, PhDKarolinska Institutet, Stockholm, SwedenSearch for more papers by this authorAuthor, Article, and Disclosure Informationhttps://doi.org/10.7326/L21-0091 SectionsAboutVisual AbstractPDF ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinkedInRedditEmail IN RESPONSE: We found a body of evidence on a first, mechanistic question that showed that some cloth masks substantially reduce the passage of particles. The second question was whether this makes a difference in clinical outcomes. We characterized this answer as “unknown,” citing Brainard and colleagues' evidence synthesis.Drs. Harada and Harada Sassa describe Brainard and colleagues' review in more detail than we were able to because of space limitations. Brainard and colleagues classified the evidence that they summarized as low certainty and very low certainty. The evidence is also highly indirect, containing no data on COVID-19 transmission. Furthermore, Brainard and colleagues do not mention whether any of their included studies in Figure 1 of their review were conducted during a pandemic. Like Greenhalgh and associates (1), who summarized many other meta-analyses and publications that discussed this second question, we regarded this as absence of evidence.Consistent, convincing evidence showed that most (but not all) cloth masks reduce droplet and aerosol transmission. Evidence that public mask wearing protects either the wearer or others was lacking. Applying the ideas of uncertainty and risk to this question, we thought that during a pandemic that at that point had claimed more than 300 000 lives, community wearing of cloth masks was likely to be a useful adjunct to the public health interventions of physical distancing and hand hygiene.Good evidence that cloth and cloth masks have useful properties, no evidence on whether wearing a cloth mask in the community makes a difference in transmission, and the presence of an unprecedented pandemic led to our decision to recommend using cloth masks first and conduct necessary further research second.Like Drs. FitzGerald and Anderson, we, too, had converted the log numbers in Bae and associates' brief research report to natural numbers and calculated filtration efficiency using standard methods. When we did so, the results and our conclusions differed from those of Bae and associates and supported use of cloth masks. At approximately the same time, others noted that many of the numbers used in Bae and associates' calculations might be lower than the limit of detection of the assay. At the time of publication, we chose to be silent rather than disseminate uncertain information. Bae and associates' brief research report has since been retracted (2).Ueki and coworkers (3) have subsequently used mannequins with and without masks to study aerosolized SARS-CoV-2. They found consistent reductions in transmission when cloth masks, medical masks, and respirators were worn by the index mannequin and protection of the wearer when medical masks and respirators were worn by the receiving mannequin. The filtration efficiency of the cloth masks used in the experiment is not known.Our evidence summary of the filtration efficiency of cloth and cloth masks has since been peer-reviewed and published (4). We created a parallel plain-language website (clothmasks.ca) (5). The between-country comparisons of Leffler and coworkers have also been peer-reviewed and published, finding that per capita coronavirus mortality increased on average each week by 16% in countries that supported masks compared with 62% in those that did not (6).In summary, subsequent evidence suggests that community use of cloth masks is effective in reducing transmission of SARS-CoV-2. However, we believe that the important philosophical point is the recognition that decision making under uncertainty is not counter to evidence-based medicine but at its heart. Had masks not been proved effective, authorities who recommended them in April 2020 still would have been right to have done so.References1. Greenhalgh T, Schmid MB, Czypionka T, et al. Face masks for the public during the COVID-19 crisis. BMJ. 2020;369:m1435. [PMID: 32273267] doi: 10.1136/bmj.m1435 CrossrefMedlineGoogle Scholar2. Bae S, Kim MC, Kim JY, et al. Effectiveness of surgical and cotton masks in blocking SARS-CoV-2: a controlled comparison in 4 patients [Letter]. Ann Intern Med. 2020;173:W22-3. [PMID: 32251511]. doi: 10.7326/M20-1342 LinkGoogle Scholar3. Ueki H, Furusawa Y, Iwatsuki-Horimoto K, et al. Effectiveness of face masks in preventing airborne transmission of SARS-CoV-2. mSphere. 2020;5. [PMID: 33087517] doi: 10.1128/mSphere.00637-20 CrossrefMedlineGoogle Scholar4. Clase CM, Fu EL, Ashur A, et al. Forgotten technology in the COVID-19 pandemic: filtration properties of cloth and cloth masks—a narrative review. Mayo Clin Proc. 2020;95:2204-24. [PMID: 33012350] doi: 10.1016/j.mayocp.2020.07.020 CrossrefMedlineGoogle Scholar5. Clothmasks.ca. Evidence-informed cloth masks. 2020. Accessed at www.clothmasks.ca on 11 August 2020. Google Scholar6. Leffler CT, Ing E, Lykins JD, et al. Association of country-wide coronavirus mortality with demographics, testing, lockdowns, and public wearing of masks. Am J Trop Med Hyg. 2020;103:2400-11. [PMID: 33124541] doi: 10.4269/ajtmh.20-1015 CrossrefMedlineGoogle Scholar Comments 0 Comments Sign In to Submit A Comment Author, Article, and Disclosure InformationAuthors: Catherine M. Clase, MB BChir, MSc; Edouard L. Fu, BSc; Meg Jardine, MBBS, PhD; Johannes F.E. Mann, MD, PhD; Juan J. Carrero, Pharm, PhDAffiliations: McMaster University and St. Joseph's Hospital, Hamilton, Ontario, CanadaLeiden University Medical Center, Leiden, the Netherlands University of Erlangen-Nürnberg and KfH Kidney Center, Munich-Schwabing, Germany Karolinska Institutet, Stockholm, SwedenDisclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M20-2567. PreviousarticleNextarticle Advertisement FiguresReferencesRelatedDetailsSee AlsoCloth Masks May Prevent Transmission of COVID-19: An Evidence-Based, Risk-Based Approach Catherine M. Clase , Edouard L. Fu , Meera Joseph , Rupert C.L. Beale , Myrna B. Dolovich , Meg Jardine , Johannes F.E. Mann , Roberto Pecoits-Filho , Wolfgang C. Winkelmayer , and Juan J. Carrero Cloth Masks May Prevent Transmission of COVID-19 Kouji H. Harada , Mariko Harada Sassa Cloth Masks May Prevent Transmission of COVID-19 Maxwell G. Anderson Cloth Masks May Prevent Transmission of COVID-19 George FitzGerald Metrics April 2021Volume 174, Issue 4 Page: 580 ePublished: 20 April 2021 Issue Published: April 2021 Copyright & PermissionsCopyright © 2021 by American College of Physicians. All Rights Reserved.PDF downloadLoading ...