Letters
18 July 2023

Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care WorkersFREE

Publication: Annals of Internal Medicine
Volume 176, Number 7
TO THE EDITOR: Regarding their trial, Loeb and colleagues (1) state, “the overall estimates rule out a doubling in hazard”; however, no such conclusion is possible. The trial used an extraordinary threshold (a hazard ratio of 2, or a 100% relative increase in risk) for noninferiority and was underpowered to find smaller but still important risks. (We estimate that a 4-fold increase in sample size would have been needed to identify a 50% increase in relative hazard.) Power aside, design flaws biased the study toward the null result that was obtained.
The intervention under study was incorrect use of N95 respirators—intermittently rather than continuously. SARS-CoV-2 is an airborne pathogen (2). Infection occurs via inhalation of shared air, and infective aerosols accumulate over time in closed indoor settings. As such, only continuous use of N95 respirators protects health care workers against respiratory infection; intermittent use of medical masks and respirators is equally ineffective (3). Unplanned crossover (those randomly assigned to medical masks could reassign themselves to the N95 group on the basis of unrecorded risk assessment) and contamination due to failure to use a cluster design further biased study results toward the null (4).
Notwithstanding lack of power and multiple biases—and although only 21 infections developed among 301 participants recruited in Canada and Israel through May 2021—analysis according to the registered protocol reveals a doubling of risk for infection for medical masks (relative risk, 2.05 [95% CI, 0.85 to 4.95]; P = 0.10) in these participants. The study had come close to showing inferiority after recruiting only a fraction of its prespecified sample size. Around this time, the authors recalculated their required sample size (in July 2021) as 1010 participants and began recruiting participants in Pakistan at a site not mentioned in the trial's registered protocol. Six months later, recruitment in Pakistan was discontinued and was begun in Egypt (also not registered in the protocol). Final results were heavily influenced by the inclusion of the sites in Egypt, with more than 70% of infections originating there. As Altman and associates note, “when authors substitute other outcomes after the trial has started there must be concern that such changes were done with knowledge of the data. That casts doubt on the reliability and integrity of the results” (5).
Lastly, the performance of this trial lacked equipoise in the face of clear engineering evidence of the superiority of respirators for airborne pathogens. The fact that this trial was done in a flawed manner that could not provide valid results means that participants were endangered for no reason.

References

1.
Loeb MBartholomew AHashmi Met al. Medical masks versus N95 respirators for preventing COVID-19 among health care workers. A randomized trial. Ann Intern Med. 2022;175:1629-1638. [PMID: 36442064]  doi: 10.7326/M22-1966
2.
Centers for Disease Control and Prevention. How COVID-19 spreads. 2022. Accessed at www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/how-covid-spreads.html on 8 December 2022.
3.
MacIntyre CRWang QSeale Het al. A randomized clinical trial of three options for N95 respirators and medical masks in health workers. Am J Respir Crit Care Med. 2013;187:960-6. [PMID: 23413265]  doi: 10.1164/rccm.201207-1164OC
4.
Chen XChughtai AAMacIntyre CR. Herd protection effect of N95 respirators in healthcare workers. J Int Med Res. 2017;45:1760-1767. [PMID: 27789807]  doi: 10.1177/0300060516665491
5.
Altman DGMoher DSchulz KF. Harms of outcome switching in reports of randomised trials: CONSORT perspective. BMJ. 2017;356:j396. [PMID: 28196813]  doi: 10.1136/bmj.j396

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Information & Authors

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Published In

cover image Annals of Internal Medicine
Annals of Internal Medicine
Volume 176Number 7July 2023

History

Published online: 18 July 2023
Published in issue: July 2023

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David N. Fisman, MD, MPH https://orcid.org/0000-0001-5009-6926
University of Toronto Dalla Lana School of Public Health, Toronto, Ontario, Canada
Raina Macintyre, MBBS, MAppEpid, PhD
Kirby Institute, University of New South Wales, Kensington, New South Wales, Australia

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David N. Fisman, Raina Macintyre. Medical Masks Versus N95 Respirators for Preventing COVID-19 Among Health Care Workers. Ann Intern Med.2023;176:eL230073. [Epub 18 July 2023]. doi:10.7326/L23-0073

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