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The Evidence Shows Masks Work

They aren’t perfect, but they certainly help curb airborne transmission of viruses.

This article was originally posted in the Montreal Gazette.

As we see cases of multiple respiratory viruses rising in Canada and around the world, we now have to deal with not just COVID-19, but also respiratory syncytial viruses (RSV) and influenza. As a result, overcrowded ERs are functioning so far beyond capacity it beggars belief and the health-care system is struggling to provide people with the optimal care they deserve.

One of the easiest and most cost-effective ways to prevent airborne virus transmission is to wear a mask, ideally a well-fitted N95. Hence the Public Health Agency of Canada’s reiteration to do so and Sunday’s suggestion by Quebec’s Collège des médecins that everyone wear masks in public spaces.

Some people oppose masks for purely political reasons. However, some oppose their use because they claim that either they do not work or, the more subtle argument, that there is no good evidence that they do.

At the beginning of the pandemic, universal mask use for the general public was unheard of. That, and the fact that we collectively under-appreciated how important airborne spread was in viral transmission essentially wrong-footed the conversation around masks from the get go.

However, claims that there is no data supporting mask use are just untrue. Even prior to the pandemic, lab studies showed masks prevent the spread of infection, though the studies were in the context of influenza. During the pandemic, it became clear that masks were effective. Data from Massachusetts found that COVID rates rose sharply in March 2020, but stabilized when universal masking was implemented for health-care workers at the end of March, and started decreasing when implemented for patients at the beginning of April. In fact, many other reviews of the published data have found that wearing masks reduced the risk of being infected.

Critiques of this data focus on two points. First, that studies are mainly observational, that is, not randomized, and they did not look at the general population. Critics point to a randomized study from Denmark showing that masks don’t work. But they neglect to mention that what was studied was not mask use, but a government recommendation to wear masks. Unsurprisingly, asking nicely isn’t usually enough to get people to change their behaviour. Fewer than half the people in the study actually wore the mask as directed. A more relevant study was a cluster randomized trial from Bangladesh where villages with community-level masking saw a decrease in COVID-19 infections.

Another recent study in the New England Journal of Medicine shows the impact of mask-wearing in Boston-area schools. Using a difference-in-difference statistical design, that is, comparing the difference before with the difference after changes in mask policy, researchers were able to look at how different schools fared when they lifted mask mandates at different times. The researchers found that lifting mask mandates resulted in an extra 45 cases per 1,000 students and staff, or an extra 12,000 cases, which translated into just under 30 per cent of all infections in that time period.

I am at a loss as to how some people (including to my great surprise some physicians) can look at the data and doubt the efficacy of masks in preventing respiratory viral transmission. I can only speculate as to their reasons. Some may indulge in the kind of therapeutic nihilism that classifies everything as either perfect or useless because, sadly, masks are not perfect and indeed they only work if people actually wear them. But in life, things aren’t always clean and perfect. When deciding if masks work, we have to look at the pre-pandemic data, simulations with aerosols in labs, data from inside hospitals and the research from the community at large. Look at it all, and clearly masks help reduce the spread of viruses. They aren’t perfect, but they help. And in a pandemic where hospitals are overwhelmed, every little bit helps a lot.


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