Q&A: What a major review does and does not tell us about masks and COVID-19
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An updated analysis published in The Cochrane Database of Systematic Reviews has fueled debate about the effectiveness of masks to prevent the spread of COVID-19.
The review includes 78 randomized controlled trials (RCTs) and clustered-RCTs assessing physical interventions to prevent the spread of respiratory illnesses — all but six of which were conducted before the COVID-19 pandemic. The trials included 610,872 total participants.
According to the authors, “many studies were conducted during non-epidemic influenza periods” and “in the context of lower respiratory viral circulation and transmission compared to COVID-19.”
The researchers also noted five ongoing, unpublished studies, two of which are specifically evaluating mask use against SARS-CoV-2 acquisition.
We asked Linsey C. Marr, PhD, the Charles P. Lunsford Professor of Civil and Environmental Engineering at Virginia Tech University and an expert on airborne virus transmission and mask technology, to share her thoughts on the analysis.
Healio: What does the Cochrane review tell us about the efficacy of masks against the spread of respiratory diseases?
Linsey Marr: There are lots of little details about the statistics and the exact outcome they’re looking at, whether it’s just symptoms or lab-confirmed infection, but the take-home message is that wearing an N95 or other high-quality, well-fitting mask or hand-washing is associated with a small (10% to 30%, perhaps higher) reduction in transmission of influenza and SARS-CoV-2 when considered on the population scale.
Additionally, N95s are more effective than surgical masks. There’s a bit of variability in the numbers, and in some cases, the reduction may not be significantly different from zero. Wearing a cloth or surgical mask probably has no effect, although I think it’s problematic that the researchers grouped these two types of masks together. We know that the average surgical mask is more effective than the average cloth mask, so any effect of surgical masks might have been obscured by grouping.
The hand-washing studies are more relevant for the average person because they covered schools, child care centers and homes. In contrast, many of the mask studies took place in health care settings, with their usually excellent ventilation and use of masks only when interacting with patients.
I would say that these results concur with a combination of, first: fundamental, mechanistic knowledge about how masks and hygiene work; second: lab studies on masks and hygiene; and third: observations about human behavior.
Healio: What does the review not tell us?
Marr: The review does not tell us how well these measures work on an individual basis. It doesn’t mean that masks don’t work or that hand-washing doesn’t work for an individual. The studies instructed people to wear a mask or wash their hands, and they did not actually ensure that subjects did it properly and all the time.
A major limitation is that many of the mask studies took place in health care settings, and the health care workers wore masks only when interacting with patients. The health care workers probably did not wear masks at other times, like at home or when interacting with people in the community — for example, restaurants, shopping, socializing — and they easily could have picked up infections in those other settings.
Thus, the effect of wearing a mask would be diluted, like if you told people to use condoms occasionally but not always. An individual can achieve a much greater reduction in risk than suggested by this study.
One of the authors’ key conclusions that I want to highlight is “relatively low adherence with the interventions during the studies.” This means that people didn’t actually wear a mask or wash their hands as instructed. I’m fairly certain there would have been a larger effect if adherence had been better. So we can conclude that low adherence to mask wearing means you won’t see a benefit populationwide. Surprise, surprise.
Healio: Do you think masks protect people from SARS-CoV-2 infection?
Marr: There is no question that a high-quality, well-fitting mask can reduce the risk of SARS-CoV-2 infection if worn properly and consistently. We know this from a study in California by the CDC that found that individuals who consistently wore an N95 or KN95 had an 83% lower risk of testing positive for COVID-19 compared with those who didn’t mask. Those who wore a surgical mask had a 66% lower risk, and those who wore a cloth mask had a 56% lower risk — although there was enough variability with cloth masks that the difference might not be significant [because] some cloth masks are better than others.
We know from physics that a good quality mask acts like a filter, trapping almost all the virus particles in the air that you would otherwise breathe in from the air around you. This reduces your dose of virus and reduces the chance that you might become infected.
References:
- Andrejko KL, et al. MMWR Morbid Mortal Wkly Rep. 2022;doi:10.15585/mmwr.mm71062.
- Jefferson T, et al. Cochrane Database Syst Rev. 2023;doi:10.1002/14651858.pub6.
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