Randomized Controlled Trial

. 2022 Dec;175(12):1629-1638.

doi: 10.7326/M22-1966. Epub 2022 Nov 29.

Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers : A Randomized Trial

Mark Loeb¹, Amy Bartholomew², Madiha Hashmi³, Wadea Tarhuni⁴, Mohamed Hassany⁵, Ilan Youngster⁶, Ranjani Somayaji⁷, Oscar Larios⁷, Joseph Kim⁷, Bayan Missaghi⁷, Joseph V Vayalumkal⁸, Dominik Mertz⁹, Zain Chagla¹⁰, Maureen Cividino¹¹, Karim Ali¹², Sarah Mansour¹³, Lana A Castellucci¹⁴, Charles Frenette¹⁵, Leighanne Parkes¹⁶, Mark Downing¹⁷, Matthew Muller¹⁸, Verne Glavin¹⁹, Jennifer Newton², Ravi Hookoom⁹, Jerome A Leis²⁰, James Kinross²¹, Stephanie Smith²², Sayem Borhan²³, Pardeep Singh², Eleanor Pullenayegum²⁴, John Conly⁷

Affiliations

¹ Department of Pathology and Molecular Medicine and Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada (M.L.).
² Department of Pathology and Molecular Medicine, McMaster University, Hamilton, Ontario, Canada (A.B., J.N., P.S.).
³ Ziauddin University, Karachi, Pakistan (M.Hashmi).
⁴ University of Saskatchewan, Saskatoon, Saskatchewan, and Canadian Cardiac Research Centre, Windsor, Ontario, Canada (W.T.).
⁵ National Hepatology and Tropical Medicine Research Institute, Cairo, Egypt (M.Hassany).
⁶ Shamir Medical Center, Tzrifin, and Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel (I.Y.).
⁷ Department of Medicine, University of Calgary and Alberta Health Services, Calgary, Alberta, Canada (R.S., O.L., J.Kim, B.M., J.C.).
⁸ Department of Pediatrics, University of Calgary and Alberta Health Services, Calgary, Alberta, Canada (J.V.V.).
⁹ Department of Medicine, McMaster University and Hamilton Health Sciences, Hamilton, Ontario, Canada (D.M., R.H.).
¹⁰ Department of Medicine, McMaster University, and St. Joseph's Healthcare, Hamilton, Ontario, Canada (Z.C.).
¹¹ St. Joseph's Healthcare, Hamilton, Ontario, Canada (M.C.).
¹² Niagara Health System, Niagara, Ontario, Canada (K.A.).
¹³ Montfort Hospital, Ottawa, Ontario, Canada (S.M.).
¹⁴ Department of Medicine, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, Ontario, Canada (L.A.C.).
¹⁵ McGill University Health Centre, Montreal, Quebec, Canada (C.F.).
¹⁶ Jewish General Hospital, Montreal, Quebec, Canada (L.P.).
¹⁷ Unity Health-St. Joseph's, Toronto, Ontario, Canada (M.D.).
¹⁸ Unity Health-St. Michael's, Toronto, Ontario, Canada (M.M.).
¹⁹ Brantford Community Health System, Brantford, Ontario, Canada (V.G.).
²⁰ Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada (J.A.L.).
²¹ Imperial College, London, England (J.Kinross).
²² University of Alberta Hospital, Edmonton, Alberta, Canada (S.S.).
²³ Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada (S.B.).
²⁴ University of Toronto, Toronto, Ontario, Canada (E.P.).

PMID: 36442064
PMCID: PMC9707441
DOI: 10.7326/M22-1966

Randomized Controlled Trial

Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers : A Randomized Trial

Mark Loeb et al. Ann Intern Med. 2022 Dec.

. 2022 Dec;175(12):1629-1638.

doi: 10.7326/M22-1966. Epub 2022 Nov 29.

Authors

Affiliations

¹ Department of Pathology and Molecular Medicine and Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada (M.L.).
² Department of Pathology and Molecular Medicine, McMaster University, Hamilton, Ontario, Canada (A.B., J.N., P.S.).
³ Ziauddin University, Karachi, Pakistan (M.Hashmi).
⁴ University of Saskatchewan, Saskatoon, Saskatchewan, and Canadian Cardiac Research Centre, Windsor, Ontario, Canada (W.T.).
⁵ National Hepatology and Tropical Medicine Research Institute, Cairo, Egypt (M.Hassany).
⁶ Shamir Medical Center, Tzrifin, and Sackler School of Medicine, Tel Aviv University, Tel Aviv, Israel (I.Y.).
⁷ Department of Medicine, University of Calgary and Alberta Health Services, Calgary, Alberta, Canada (R.S., O.L., J.Kim, B.M., J.C.).
⁸ Department of Pediatrics, University of Calgary and Alberta Health Services, Calgary, Alberta, Canada (J.V.V.).
⁹ Department of Medicine, McMaster University and Hamilton Health Sciences, Hamilton, Ontario, Canada (D.M., R.H.).
¹⁰ Department of Medicine, McMaster University, and St. Joseph's Healthcare, Hamilton, Ontario, Canada (Z.C.).
¹¹ St. Joseph's Healthcare, Hamilton, Ontario, Canada (M.C.).
¹² Niagara Health System, Niagara, Ontario, Canada (K.A.).
¹³ Montfort Hospital, Ottawa, Ontario, Canada (S.M.).
¹⁴ Department of Medicine, Ottawa Hospital Research Institute, University of Ottawa, Ottawa, Ontario, Canada (L.A.C.).
¹⁵ McGill University Health Centre, Montreal, Quebec, Canada (C.F.).
¹⁶ Jewish General Hospital, Montreal, Quebec, Canada (L.P.).
¹⁷ Unity Health-St. Joseph's, Toronto, Ontario, Canada (M.D.).
¹⁸ Unity Health-St. Michael's, Toronto, Ontario, Canada (M.M.).
¹⁹ Brantford Community Health System, Brantford, Ontario, Canada (V.G.).
²⁰ Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada (J.A.L.).
²¹ Imperial College, London, England (J.Kinross).
²² University of Alberta Hospital, Edmonton, Alberta, Canada (S.S.).
²³ Department of Health Research Methods, Evidence, and Impact, McMaster University, Hamilton, Ontario, Canada (S.B.).
²⁴ University of Toronto, Toronto, Ontario, Canada (E.P.).

PMID: 36442064
PMCID: PMC9707441
DOI: 10.7326/M22-1966

Erratum in

Correction: Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
[No authors listed] [No authors listed] Ann Intern Med. 2023 Jun;176(6):884. doi: 10.7326/L23-0158. Epub 2023 May 16. Ann Intern Med. 2023. PMID: 37186923 No abstract available.

Expression of concern in

Expression of Concern: Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Conly J. Conly J. Ann Intern Med. 2025 Sep;178(9):1367-1368. doi: 10.7326/ANNALS-25-03203. Epub 2025 Aug 5. Ann Intern Med. 2025. PMID: 40759021 No abstract available.

Abstract

Background: It is uncertain if medical masks offer similar protection against COVID-19 compared with N95 respirators.

Objective: To determine whether medical masks are noninferior to N95 respirators to prevent COVID-19 in health care workers providing routine care.

Design: Multicenter, randomized, noninferiority trial. (ClinicalTrials.gov: NCT04296643).

Setting: 29 health care facilities in Canada, Israel, Pakistan, and Egypt from 4 May 2020 to 29 March 2022.

Participants: 1009 health care workers who provided direct care to patients with suspected or confirmed COVID-19.

Intervention: Use of medical masks versus fit-tested N95 respirators for 10 weeks, plus universal masking, which was the policy implemented at each site.

Measurements: The primary outcome was confirmed COVID-19 on reverse transcriptase polymerase chain reaction (RT-PCR) test.

Results: In the intention-to-treat analysis, RT-PCR-confirmed COVID-19 occurred in 52 of 497 (10.46%) participants in the medical mask group versus 47 of 507 (9.27%) in the N95 respirator group (hazard ratio [HR], 1.14 [95% CI, 0.77 to 1.69]). An unplanned subgroup analysis by country found that in the medical mask group versus the N95 respirator group RT-PCR-confirmed COVID-19 occurred in 8 of 131 (6.11%) versus 3 of 135 (2.22%) in Canada (HR, 2.83 [CI, 0.75 to 10.72]), 6 of 17 (35.29%) versus 4 of 17 (23.53%) in Israel (HR, 1.54 [CI, 0.43 to 5.49]), 3 of 92 (3.26%) versus 2 of 94 (2.13%) in Pakistan (HR, 1.50 [CI, 0.25 to 8.98]), and 35 of 257 (13.62%) versus 38 of 261 (14.56%) in Egypt (HR, 0.95 [CI, 0.60 to 1.50]). There were 47 (10.8%) adverse events related to the intervention reported in the medical mask group and 59 (13.6%) in the N95 respirator group.

Limitation: Potential acquisition of SARS-CoV-2 through household and community exposure, heterogeneity between countries, uncertainty in the estimates of effect, differences in self-reported adherence, differences in baseline antibodies, and between-country differences in circulating variants and vaccination.

Conclusion: Among health care workers who provided routine care to patients with COVID-19, the overall estimates rule out a doubling in hazard of RT-PCR-confirmed COVID-19 for medical masks when compared with HRs of RT-PCR-confirmed COVID-19 for N95 respirators. The subgroup results varied by country, and the overall estimates may not be applicable to individual countries because of treatment effect heterogeneity.

Primary funding source: Canadian Institutes of Health Research, World Health Organization, and Juravinski Research Institute.

PubMed Disclaimer

Conflict of interest statement

Disclosures: Disclosures can be viewed at www.acponline.org/authors/icmje/ConflictOfInterestForms.do?msNum=M22-1966.

Figures

**Visual Abstract.. Medical Masks Versus N95 Respirators for COVID-19.**
It is uncertain if medical masks offer similar protection against COVID-19 compared with N95 respirators. This randomized trial, which enrolled participants in Canada, Israel, Pakistan, and Egypt, aimed to determine whether medical masks are noninferior to N95 respirators to prevent COVID-19 in health care workers providing routine care.

**Figure 1.. Trial flow diagram.**
ITT = intention-to-treat; mRNA = messenger RNA. * Dates of follow-up: Canada (May 2020 to May 2021), Israel (November 2020 to January 2021), Pakistan (June 2021 to December 2021), and Egypt (December 2021 to March 2022).

**Figure 2.. Forest plot of the primary intention-to-treat analysis of RT-PCR–confirmed COVID-19.**
There were 86 of 8338 (1%) weekly surveys missing in the medical mask group and 65 of 8468 (0.8%) missing in the N95 respirator group. The subgroup analysis by country was added to show the heterogeneity of treatment effect. HR = hazard ratio; RT-PCR = reverse transcriptase polymerase chain reaction.

**Figure 3.. Cumulative incidence of primary analysis of RT-PCR–confirmed COVID-19.**
RT-PCR = reverse transcriptase polymerase chain reaction.

**Figure 4.. Cumulative incidence of primary analysis of RT-PCR–confirmed COVID-19 by country.**
RT-PCR = reverse transcriptase polymerase chain reaction.

See this image and copyright information in PMC

Comment in

Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Laine C, Wee CC, Chang S, Cotton D, Chopra V, Williams SV, Yang YX, Guallar E, Ross EA. Laine C, et al. Ann Intern Med. 2023 Jul;176(7):eL230074. doi: 10.7326/L23-0074. Ann Intern Med. 2023. PMID: 37459624 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Fisman DN, Macintyre R. Fisman DN, et al. Ann Intern Med. 2023 Jul;176(7):eL230073. doi: 10.7326/L23-0073. Ann Intern Med. 2023. PMID: 37459625 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Hedges K, Levitsky M, Marchand G, Morawska L, Bhérer L, Letellier MC. Hedges K, et al. Ann Intern Med. 2023 Jul;176(7):eL230076. doi: 10.7326/L23-0076. Ann Intern Med. 2023. PMID: 37459626 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Delaine J. Delaine J. Ann Intern Med. 2023 Jul;176(7):eL230075. doi: 10.7326/L23-0075. Ann Intern Med. 2023. PMID: 37459627 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Greenhalgh T, Cane DJ, Oliver M. Greenhalgh T, et al. Ann Intern Med. 2023 Jul;176(7):eL230072. doi: 10.7326/L23-0072. Ann Intern Med. 2023. PMID: 37459629 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Brosseau LM. Brosseau LM. Ann Intern Med. 2023 Jul;176(7):eL230071. doi: 10.7326/L23-0071. Ann Intern Med. 2023. PMID: 37459630 No abstract available.
Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers.
Loeb M, Mertz D, Chagla Z, Conly J. Loeb M, et al. Ann Intern Med. 2023 Jul;176(7):eL230077. doi: 10.7326/L23-0077. Ann Intern Med. 2023. PMID: 37459636 No abstract available.

References

1. World Health Organization. Rational use of personal protective equipment for coronavirus disease 2019 (COVID-19) and considerations during severe shortages: interim guidance. Accessed at www.who.int/publications/i/item/rational-use-of-personal-protective-equi... on 24 June 2022.
1. World Health Organization. WHO recommendations on mask use by health workers, in light of the Omicron variant of concern: WHO interim guidelines, 22 December 2021. Accessed at www.who.int/publications/i/item/WHO-2019-nCoV-IPC_Masks-Health_Workers-O... on 24 June 2022.
1. Janssen L, Ettinger H, Graham S, et al. The use of respirators to reduce inhalation of airborne biological agents. J Occup Environ Hyg. 2013;10:D97-103. [PMID: ] doi: 10.1080/15459624.2013.799964 - DOI - PMC - PubMed
1. National Institute for Occupational Safety and Health. Determination of particulate filter efficiency level for N95 series filters against solid particulates for non-powered, air-purifying respirators standard testing procedure. Accessed at www.cdc.gov/niosh/npptl/stps/pdfs/TEB-APR-STP-0059-508.pdf on 24 June 2022.
1. Centers for Disease Control and Prevention. Interim infection prevention and control recommendations for healthcare personnel during the coronavirus disease 2019 (COVID-19) pandemic. Accessed at www.cdc.gov/coronavirus/2019-ncov/hcp/infection-control-recommendations....; 2021 on 24 June 2022.

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Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers : A Randomized Trial

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Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers : A Randomized Trial

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