A new study out of Denmark tried to measure the benefits of masks. It didn't go so well.

Social media has been abuzz this week over a new study out of Denmark about the effectiveness of masks on the risk of getting Covid-19. Depending on what news source you looked at, you might have heard that masks might not protect the people wearing them, or that wearing masks doesn’t prevent the spread of the virus. You might also have read the near-immediate backlash in which scientists pointed out all the evidence that masks really do work.

I read the study. It doesn’t prove anything, as even its own authors admit.

Let’s dig into the actual results just a bit to see what all the fuss is about.

The study was conducted in Denmark in April and May of this year, and what it tried to do (not very effectively) was to measure the effect of a recommendation to wear masks. That’s right, they weren’t really measuring the benefits of masks directly at all!

The study enrolled about 6000 volunteers, and for half of them they recommended wearing masks whenever they went outdoors. They also provided masks to those volunteers. For the other half, they didn’t do anything. At the time (March and April), Denmark was recommending social distancing, but universal mask wearing wasn’t recommended.

Quite a few people dropped out, so in the end they only had 4862 people in the two groups, about 2400 per group.

What did they measure? Well, they did an antibody test (far from perfect, but let’s not digress) at the beginning of June to see whether or not people were infected with SARS-CoV-2, the virus that causes Covid-19.

(Note that they DID NOT measure how well the masks might have protected anyone else in the community. They were only measuring whether a mask might protect the wearer.)

And the results? 42 people in the mask-recommendation group were infected, and 51 people in the no-recommendation group were infected. (That’s 1.8% versus 2.1% of each group.) So there was a small reduction, but it was not statistically significant, which means we really can’t say if the mask recommendation helped prevent infection.

The study authors admitted this themselves, writing: “the findings are inconclusive ... compatible with a 46% decrease to a 23% increase in infection.” In other words, the results could mean that masks reduce self-infections by 46% or increase them (bizarre as that sounds) by 23%.

In other words, this experiment doesn’t tell us much. If it weren’t about Covid-19, I doubt that the Annals of Internal Medicine would have published it. (A commentary by my colleagues at Hopkins and Stanford suggested that Annals was right to publish it, as long as scientists “carefully highlight the questions that the trial does and does not answer.”)

Now some big caveats. First, in the mask recommendation group, only 46% of the participants wore masks as recommended. Second, the study didn’t ask if anyone in the no-recommendation group wore masks. Third, the study relied on self-reporting to determine who was actually wearing their masks consistently–that is, they simply asked the participants to tell them how often they wore their masks.

What? Imagine if you were studying the use of seat belts to reduce injuries, and only 46% of the people you told to wear seat belts wore them properly. On top of that, imagine that you relied entirely on self-reporting to determine who was actually wearing the seat belts. This study design is nearly worthless if you want to know the true benefits of seat belts.

So the Danish mask study was inconclusive, as its own authors report. Therefore it would be a huge mistake, scientifically speaking, to take this non-result and conclude that masks do not protect you. It would be an even bigger mistake to conclude that the study showed that masks don’t benefit the community. Unfortunately, that didn’t stop these two Oxford University scientists from jumping to exactly that conclusion. They claimed, in an article in The Spectator, that the study showed that “wearing masks in the community does not significantly reduce the rates of infection.” This is dead wrong. The study wasn’t even measuring community rates of infection.

(For an excellent Twitter-take on what the study does not prove, see this thread from The Health Nerd.)

And as the CDC has documented, multiple studies have already shown that masks are highly effective at limiting the spread of Covid-19. And a study last summer pointed out that increasing the use of masks by just 15% “could prevent the need for lockdowns and reduce associated losses of up to $1 trillion” in the U.S. alone. So yes, it’s a good idea to wear a mask.

It’s also simply common sense that if you wear a mask, the amount of virus that you breathe in or out will be reduced. You’re not just protecting yourself, you’re also protecting everyone around you.

Until we can get this pandemic under control, we all need to wear masks in public. It’s utterly ridiculous that this has become a political issue, as it has in the U.S. To those who think mask wearing somehow limits their personal freedom: get over it. When you see a red light at a busy intersection, do you race through it because you need the “freedom” to drive like a crazy person? No. Civilized society requires everyone to follow some basic rules to protect each other, and during a pandemic, wearing a mask is one of them. And to those who think that wearing a mask somehow shows their civic virtue? No, that’s wrong too. To use the same example, stopping at a red light doesn’t prove that you’re virtuous.

It’s just a mask. It’s not a political statement. Get over it.

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