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Ten months into this pandemic there is still discussion about whether or not wearing a facemask helps prevent the spread of COVID-19. There is also some “mask-denial” which may just be poorly informed skepticism or ideologically driven. Masks have become, unfortunately, a political symbol. The evidence is also pretty clear that wearing facemasks helps protect against the spread of COVID-19. But the evidence is complex enough to feed (unjustified) denial, and warrants periodic review.

We need to first recognize that the question – do masks work? – is comprised of many subquestions. There are different kinds of masks, including cloth masks (and in fact there are different kinds of cloth masks), surgical masks, and masks designed specifically to filter out viruses, such as N95 masks. Compliance is an important variable – how consistent are people in wearing the mask, how well does it fit, and do they keep it dry? There are also different situations in which masks can be worn – in a hospital or other high-risk setting, in the presence of people known to be infected with COVID-19, or in the general community (which can be further divided by the rate of infection in the community). Does mask wearing protect the wearer or other people? Does it prevent infection and/or reduce the severity of infection? No one study is going to address all these questions.

There are also different kinds of evidence we can use to get at these questions. We can look to see how well masks prevent droplet spread, or even specifically viral spread. We can look at the risk of individuals for contracting the disease. We can look at epidemiological evidence to see if mask-wearing policies affect the spread of COVID-19. We can also look at the effect on spread of other viruses. Again, no single study is going to be definitive. We need to look at the pattern of evidence to see if it supports a beneficial role for wearing masks, what kinds of masks, in what settings, and with what benefits. Mask wearing as a measure also needs to be disentangled from other prevention methods, such as distancing, hand washing, and contact tracing.

With all that in mind, what is the current state of the evidence for wearing masks as a public health measure to fight the pandemic? The short answer is – the evidence is very strong that masks are beneficial, but let’s dive into the details.

A recent review found that all masks filter out droplets and viruses, but vary significantly based on mask type. Cloth masks were not as effective as hospital masks which were not as effective as N95 masks. One thing to point out here is that there is a clear “dose response” effect – the better masks were in fact better at filtering virus. This dose-response also holds up when comparing the spread of virus to people.

At this point the evidence is very clear that hospital masks and N95 masks are effective, but the evidence is equivocal for cloth masks. This is likely because of variables in cloth mask design. The two most significant variables seem to be the fit over the face, and the number of layers. Having a well-fitting mask with three layers of cloth works well (but still not as well as a hospital mask).

Epidemiological studies provide perhaps the best evidence for the efficacy of masks, or more specifically, mask policies. One study comparing mask-wearing policies in different states in the US found that after states initiated mask-wearing policies, the spread of the virus decreased. Another study found that the risk of a spreading event was far greater in “mask-off” social settings than “mask-on” settings. Comparing countries with different mask-wearing policies also finds a good correlation with reduced spread.

So masks reduce the spread of droplets and to some extent virus itself, they show a dose-response curve with mask quality, and epidemiological studies show a strong correlation between mask-wearing and reduced spread of COVID. This is more than sufficient evidence to justify mask-wearing policies, and to educate the public about what kinds of masks work and how to properly use them.

What is lacking is randomized controlled trials, mostly because of the practical and ethical hurdles of randomizing people to not wearing a mask, especially given the current state of the evidence and the fact that a deadly pandemic is still raging. There was one RTC of mask wearing, however – the DANMASK trial. The strength of this study is that it was randomized, people were instructed with good pandemic hygiene with one group advised to wear and mask and the other not. This was an intention to treat model. The results were statistically negative.

But there are significant limitations to this trial that caution against broad interpretation. The study was only powered to see a 50% reduction in infection. The rate in the community was already very low. Compliance with mask wearing in that group was <50%. Infections were self-reported. The dropout rate was high – 16%.

At most what this study shows is that simply recommending mask wearing in the community where the rate of infection is low will not produce a dramatic difference in infection rates – but even that conclusion is affected by the limitations of the study. This is certainly not strong enough evidence to counter the epidemiological evidence, which is more extensive and more compelling.

As a side note, there is also the incidental evidence that all the measures taken during the pandemic (not just mask wearing, but distancing as well) has caused all other respiratory virus infection rates to plummet. The CDC has been tracking the flu, cold viruses, and other viruses, and the spread of these illnesses over the last year has flatlined. Whatever we are doing, it is working to prevent infections by all droplet-spread viruses. They also warn these viruses are likely to rebound with a vengeance once pandemic measures are relaxed.

We always have to make medical decisions with imperfect evidence. The evidence for mask-wearing is imperfect and complex, but there is a strong signal in this data that wearing a quality mask properly and consistently protects the wearer and especially protects others. This is more than enough to justify universal mask-wearing in public until vaccination and herd immunity bring an end to this pandemic.

In fact, this evidence and the experience of this pandemic may change public health practices in the future. Hopefully people who may have the flu, or were just exposed, will feel more comfortable distancing and wearing a mask to reduce spread to others. These tools work to prevent the spread of respiratory viruses and we should use them.

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Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.