Clinical Topics & News

Study comparing surgical and N95 masks sparks concern



A randomized trial indicating that surgical masks are not inferior to N95 masks in protecting health care workers against COVID-19 has sparked international criticism.

The study’s senior author is John Conly, MD, an infectious disease specialist and professor at the University of Calgary (Alta.), and Alberta Health Services. The findings are not consistent with those of many other studies on this topic.

Commenting about Dr. Conly’s study, Eric Topol, MD, editor-in-chief of Medscape, wrote: “It’s woefully underpowered but ruled out a doubling of hazard for use of medical masks.”

The study, which was partially funded by the World Health Organization, was published online in Annals of Internal Medicine.

This is not the first time that Dr. Conly, who also advises the WHO, has been the subject of controversy. He previously denied that COVID-19 is airborne – a position that is contradicted by strong evidence. In 2021, Dr. Conly made headlines with his controversial claim that N95 respirators can cause harms, including oxygen depletion and carbon dioxide retention.

A detailed examination by the Center for Infectious Disease Research and Policy (CIDRAP) at the University of Minnesota, Minneapolis, pointed out numerous scientific flaws in the study, including inconsistent use of both types of masks. The study also examined health care workers in four very different countries (Canada, Israel, Egypt, and Pakistan) during different periods of the pandemic, which may have affected the results. Furthermore, the study did not account for vaccination status and lacked a control group. CIDRAP receives funding from 3M, which makes N95 respirators.

In a commentary published alongside the study, Roger Chou, MD, professor of medicine at Oregon Health & Science University, Portland, said that the results were “not definitive,” with “a generous noninferiority threshold” that is actually “consistent with up to a relative 70% increased risk ... which may be unacceptable to many health workers.”

Lead study author Mark Loeb, MD, professor of infectious diseases at McMaster University, Hamilton, Ont., defended the findings. “The confidence intervals around this, that is, what the possible results could be if the trial was repeated many times, range from −2.5% to 4.9%,” he told this news organization. “This means that the risk of a COVID-19 infection in those using the medical masks could have ranged from anywhere from 2.5% reduction in risk to a 4.9% increase in risk. Readers and policy makers can decide for themselves about this.”

“There is no point continuing to run underpowered, poorly designed studies that are designed to confirm existing biases,” Raina MacIntyre, PhD, professor of global biosecurity and head of the Biosecurity Program at the Kirby Institute, Sydney, said in an interview. “The new study in Annals of Internal Medicine is entirely consistent with our finding that to prevent infection, you need an N95, and it needs to be worn throughout the whole shift. A surgical mask and intermittent use of N95 are equally ineffective. This should not surprise anyone, given a surgical mask is not designed as respiratory protection but is designed to prevent splash or spray of liquid on the face. Only a respirator is designed as respiratory protection through both the seal around the face and the filter of the face piece to prevent inhalation of virus laden aerosols, but you need to wear it continually in a high-risk environment like a hospital.”

“It makes zero sense to do a randomized trial on something you can measure directly,” said Kimberly Prather, PhD, an atmospheric chemist, professor, and director of the NSF Center for Aerosol Impacts on Chemistry of the Environment at the University of California, San Diego. “In fact, many studies have shown aerosols leaking out of surgical masks. Surgical masks are designed to block large spray droplets. Aerosols (0.5-3 mcm), which have been shown to contain infectious SARS-CoV-2 virus, travel with the air flow, and escape.”

“This study ... will be used to justify policies of supplying health care workers, and perhaps patients and visitors, too, with inadequate protection,” Trish Greenhalgh, MD, professor of primary care health sciences at the University of Oxford (England), told this news organization.

“These authors have been pushing back against treating COVID as airborne for 3 years,” David Fisman, MD, an epidemiologist and infectious disease specialist at the University of Toronto, said in an interview. “So, you’ll see these folks brandishing this very flawed trial to justify continuing the infection control practices that have been so disastrous throughout the pandemic.”

The study was funded by the World Health Organization, the Canadian Institutes of Health Research, and the Juravinski Research Institute. Dr. Conly reported receiving grants from the Canadian Institutes for Health Research, Pfizer, and the WHO. Dr. Chou disclosed being a methodologist for WHO guidelines on infection prevention and control measures for COVID-19. Dr. Loeb disclosed payment for expert testimony on personal protective equipment from the government of Manitoba and the Peel District School Board. Dr. MacIntyre has led a large body of research on masks and respirators in health workers, including four randomized clinical trials. She is the author of a book, “Dark Winter: An insider’s guide to pandemics and biosecurity” (Syndey: NewSouth Publishing, 2022), which covers the history and politics of the controversies around N95 and masks. Dr. Prather reported no disclosures. Dr. Greenhalgh is a member of Independent SAGE and an unpaid adviser to the philanthropic fund Balvi. Dr. Fisman has served as a paid legal expert for the Ontario Nurses’ Association in their challenge to Directive 5, which restricted access to N95 masks in health care. He also served as a paid legal expert for the Elementary Teachers’ Federation of Ontario in its efforts to make schools safer in Ontario.

A version of this article first appeared on

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