Surgical masks are not inferior to N95 masks for preventing the spread of COVID-19 to health-care workers, says a study led by McMaster University researchers.

The results follow the tracking of 1,009 health-care workers providing COVID-19 patient care at 29 sites in Canada, Egypt, Israel and Pakistan between May 2020 and March 2022.

Study participants were randomly assigned to either a surgical mask, which was already the standard used at all of the study sites, or an N95 respirator. As with any randomized clinical trial, study participants were volunteers who could exit the study or switch to an N95 at anytime.

The findings have been published in the Annals of Internal Medicine (AIM).

“The surgical masks were not statistically less effective than N95s in preventing COVID-19 infections in health-care providers looking after patients with COVID-19,” said lead author Mark Loeb, professor of McMaster’s Department of Pathology and Molecular Medicine and a Hamilton infectious disease physician.

“The major thrust of this study is that there have been no other rigorous comparisons of surgical masks to N95 respirators during the pandemic.  As a randomized clinical trial – it offers the highest standards of evidence – relating to this question throughout the pandemic, including the Omicron variant.”

Loeb said the finding is consistent with a systematic review of four previous randomized controlled trials on masks done between 1990 and March 2020 which similarly demonstrated that the use of surgical masks compared to N95 respirators did not increase viral respiratory infection or clinical respiratory illness.

He added that there have been conflicting recommendations on the use of N95s throughout the pandemic. The U.S. Centers for Disease Control and Prevention recommended N95s for routine care of patients with COVID-19, while the World Health Organization and the Public Health Agency of Canada first recommended surgical masks.

Later in the pandemic, recommendations changed such that the World Health Organization recommended either surgical masks or N95 respirators while the Public Health Agency of Canada recommended N95s.

The study comes as low and middle-income countries are still struggling to procure N95 masks due to their high cost. Loeb said many of these same countries faced an acute shortage of N95s throughout the COVID-19 pandemic.

An accompanying editorial in the AIM journal by Roger Chou, a physician of Oregon Health & Science University, highlights that this trial provides the best evidence to date on comparative effectiveness of mask types in preventing COVID-19 infection in health care workers providing routine patient care.

External funding for the study was provided by the Canadian Institutes of Health Research, the World Health Organization and the Juravinski Research Institute.