Let's talk about that Danish mask study--DANMASK.
https://www.acpjournals.org/doi/10.7326/M20-6817

A bit of background... I've been digging into misinformation and disinformation this year for an investigative project. So I've been seeing posts about this study for a couple of months from conservative influencers, who have been using its existence to say "masks don't work."

That's not what the study found, but more on that in a minute... Bottom line, you should still wear a mask. Please wear a mask. It's important, especially now.

How did these posters know about it? The study authors did a pretty responsible and transparent thing. They published their trial design in a scientific journal. Too often studies get buried before they report their results. https://pubmed.ncbi.nlm.nih.gov/32829745/ 

According to the conspiracy theories, the study found masks don't work (again, this isn't true) but no medical journal dared publish the results. Is that true? I asked the lead study author and the medical journal editor who published it this week.

Both said no. The study author, a cardiologist who has run clinical trials on drugs, said he followed a pretty common practice in scientific publishing--submitting to the top tier medical journals like the NEJM and Lancet first before trying others.

Scientific publishing is extremely competitive. It's like applying to an Ivy League college. The top journals only take a fraction of the papers that are submitted to them. Plenty of good studies don't make it. Researchers always try though. This is common, typical practice.

The study author told me he'd gone this route, too. His paper wasn't accepted at the first few journals he tried. But it was accepted at @AnnalsofIM, which is a great journal. If not Ivy League, it's certainly Ivy League adjacent.

I asked @CLaineMD, the editor-in-chief of Annals, if she had been afraid to publish the study. She said not afraid to publish, because it was a good study (and it was), but they were concerned about how it would be interpreted during a very fractious political moment.

She took the unusual step of running the study with two editorials. She said they worked really hard to explain the study findings as plainly as they could. One of these she co-authored. Give it a read here: https://www.acpjournals.org/doi/10.7326/M20-7448

Why was the study good? It was a randomized controlled trial. That's the gold standard of scientific evidence. You take two groups of people, roughly equal in size and general characteristics like age and sex. Half do the thing you're testing. Half don't. Then you compare them.

We haven't had enough of these during the pandemic. That's why there's so much confusion about what works, what doesn't, and how much good something may do or not do.

There's other good stuff about the study, too, which I will pick up on after I get my kids some breakfast and off to school for the day. Stay tuned...

OK... back to DANMASK. Lots of people opining that publishing the study was a mistake and that it is fueling a certain narrative about masks. That's wrong. Long before the study came out, anti-maskers were using it to claim that masks don't work.

From the things I'd already seen about it, I expected the study to be Plandemic-level crazy pants. It's not. It's a well done, real-world study with an important message: You still need to wear a mask, but don't rely only on your mask to keep you safe from COVID-19.

DANMASK is the only randomized controlled trial to test whether surgical masks protect the person who wears them.

Remember, surgical masks were not designed to protect surgeons, but patients. They're meant to catch coughs and sneezes and keep them from infecting wounds. That's called source control. The study didn't test source control.

The personal protection you may get from a mask comes from filtration, or it's ability to filter particles out of the air before they reach your nose and mouth.

The study didn't test filtration, either. It couldn't. Instead, it tested advice. The study authors advised people to wear masks. In many ways, that's more practical that testing filtration with machines in a lab. A study like that captures what people actually do.

It turns out, *surprise* people don't wear masks perfectly all the time. I know you're shocked to hear this. Half the people assigned to wear masks in the study admitted that they didn't always use them as directed.

But even when study authors restricted their analysis to the people that said they did wear their masks as directed, they didn't find the effect they were looking for. And just what was that effect?

Even with 6,000 study participants, the study only had the statistical power to detect a big effect from wearing a mask--at least a 50% reduction in the risk of infection. The researchers told me they went into the study thinking they would find a big difference. They didn't.

The study found a smaller difference, about a 14-16% average decrease in risk, depending on the group analyzed. But that number wasn't statistically significant, so we don't really know. The findings ranged from about a 40% decrease in risk to an increase in risk of about 20%.

A mask expert reached at the CDC said that's about right. That other studies are showing that any kind of protective effect to the mask wearer is likely to be on the order of about a 40% decrease in the risk of infection.

But because the study didn't have enough statistical power, we don't actually know what the effect wearing a mask was, based on this study.

Other limitations: There was a relatively low incidence of COVID in Denmark at the time the study was done. That may have influenced the results. It may be that masks are more protective when there's more virus around.

This study isn't the only evidence we have on masks, and that's important. That evidence is summarized nicely here: https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html

I asked the CDC if they would change the newly updated guidance on masks based on this study. They said absolutely not. Masks work. Wear a mask.

But here's the critical message of the study that may get lost in the debate. Masks--at least the way most people use them--aren't as personally protective as we think they are. Even good quality surgical masks.

You can't put on a mask and ignore other safety steps... say them with me now... such as social distancing, handwashing, avoiding travel and large gatherings of people... and keep yourself safe. Layer your safety.

Wearing a mask also protects other people. The more we all wear them, the safer we will all be. Thanks for listening. Here's my story: https://www.webmd.com/lung/news/20201119/how-much-does-wearing-a-mask-protect-you