I don't know that I see the point of this study.

First, it's not testing the efficacy of wearing a mask: it's testing the efficacy of telling someone to wear a mask. This is explicitly stated in the "limitations" section.

Second, getting a decent control group is almost impossible. Essentially no one in modern society is unaware of COVID or the recommendation to wear a mask, and essentially no one had failed to form an opinion on whether or not they will comply with that recommendation. And indeed, "variable adherence" is another factor called out in the limitations.

The summary could be reworded "having one more doctor tell someone to wear a mask does not make them less likely to contract COVID-19."

This study is very much not about the efficacy of state- or nation- wide mask mandates.

> The summary could be reworded "having one more doctor tell someone to wear a mask does not make them less likely to contract COVID-19."

That's not an accurate summary; that should be "having one more doctor tell someone to wear a mask does not have a large enough effect size on their likelihood of getting COVID-19 for this study to detect." They had 1.8% infected in the extra-recommendation group and 2.1% in the control group.

They asked about compliance:

> Based on the lowest adherence reported in the mask group during follow-up, 46% of participants wore the mask as recommended, 47% predominantly as recommended, and 7% not as recommended."

Personally, I find it easy to believe that people were in fact wearing their masks. To be in the study you it looks like you had to joined voluntarily on your own accord, indicating a participant group with an interest in it:

> Recruitment involved media advertisements and contacting private companies and public organizations

If anything, I find it easy to believe that adherence would be less in the general population than in this study. Particularly stuff like wearing masks incorrectly, eg not covering the nose, leaving gaps, etc.

How many people in the control group wore masks? It's not like they were told not to wear masks, they just weren't told to.

Edit: it actually looks like the study doesn't include that statistic at all, which is appallingly poor design.

> It's not like they were told not to wear masks, they just weren't told to.

Why do you think that? The study says "A total of 3030 participants were randomly assigned to the recommendation to wear face masks, and 2994 were assigned not to wear face masks".

Did you check the supplemental materials?

> Essentially no one in modern society is unaware of COVID or the recommendation to wear a mask

At the time the study was performed (April and May 2020), it was not common to wear masks in Denmark outside of hospitals. In fact the authorities recommended against wearing masks because there was a shortage and the available masks should be reserved for high risk groups.

>During the study period (3 April to 2 June 2020), Danish authorities did not recommend use of masks in the community and mask use was uncommon (<5%) outside hospitals (22).

No, the study would only be worthwhile if it fitted the narrative and proved the usefulness of masks. Otherwise its time to break out the excuses.

The study is not about usefulness of wearing mask during epidemic. It's limited to the usefulness of wearing surgical mask to protect the wearer.

There is lots of evidence that wearing mask protects others, that's why surgeons wear surgical masks during surgery.

Surgical masks are not intended to protect the wearer, you need medical respirator for that. The question is how effective they are in doing job they are not designed to do. It seems to be limited, maybe around 30-50% depending on the study.

> There is lots of evidence that wearing mask protects others, that's why surgeons wear surgical masks during surgery.

There is a lack of evidence for the effectiveness of masks during surgery, and they aren't always mandatory to wear:

https://www.cochrane.org/CD002929/WOUNDS_disposable-surgical...

https://www.medpagetoday.com/infectiousdisease/infectioncont...

https://ncbi.nlm.nih.gov/pmc/articles/PMC4480558/

There are obvious hygienic benefits - no-one wants to be splashed with bodily fluids - but there is a lack of evidence that they actually protect the patient.

Sounds like you're the one making excuses. Anti intellectualism is strong in this thread.

Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask, A Randomized Controlled Trial

Discussion

>a recommendation to wear a surgical mask when outside the home among others did not reduce, at conventional levels of statistical significance, incident SARS-CoV-2 infection compared with no mask recommendation.

Important however

> .. The findings, however, should not be used to conclude that a recommendation for everyone to wear masks in the community would not be effective in reducing SARS-CoV-2 infections, because the trial did not test the role of masks in source control of SARS-CoV-2 infection.

It strains credulity to believe that masks provide protection in one direction, but not the other.

The direction does matter, actually. The virus particles leaving an infected person will generally be inside droplets large enough to be caught by a mask before they get a chance to evaporate and atomize. Meanwhile, the virus encountered by an uninfected person nearby will naturally be either in smaller droplets or in airborne form. Much harder to defend against.

That’s the theory that people have traditionally advanced, yes.

Is there any evidence that capturing heavier droplets has any material effect on transmission? No. And we know that transmission is driven by aerosols now. Aerosols are not heavy droplets.

Moreover, at this point, it isn’t challenging to find cities and states with high levels of mask compliance, and spiking cases:

https://mobile.twitter.com/ianmSC/status/1329187274794225666...

https://mobile.twitter.com/ianmSC/status/1329218665187745794...

https://mobile.twitter.com/yinonw/status/1329558423604695046...

https://mobile.twitter.com/ianmSC/status/1329610060125749251...

A great many preprints were created in the the spring (when cases were on the decline in the northern hemisphere) claiming to prove that masks “work”, based on the same kinds of observations. Now that cases are increasing in the northern hemisphere, I eagerly await a revisiting of those publications.

You mention mask compliance, but your links discuss mandates - did the mandates have a material effect in changing the number of people actually wearing them?

From an aerodynamics point of view, I'd expect it to be easier to protect the wearer than others. When you breath in, a decently fitted mask sticks to your face, directing airflow through the filter. When you breath out, it's very easy for air to get out gaps on the sides.

Frankly, I think it's been a huge mistake to discourage valved masks: they're much easier to breath in and more comfortable. Better to encourage the vulnerable to wear high quality, valved N95 masks and protect themselves, than worry about those same people infecting others.

My risk from COVID is something like 1/50th that of my parents.

It's quite possible that infection is occurring via the eyes, this could explain why masks protect others but not the wearer.

Wait, do you have a source for the statement that "masks protect others but not the wearer"?

Because I've seen that claim a lot. But never actually seen a scientific study making that claim.

Isn't this very study an example of one that shows that masks do not offer significant protection for the wearer?

Whether or not masks protect others seems much harder to isolate but there does seem to be lower R^2 values for countries and areas with more normalized mask wearing. Perhaps it is just a correlation (just seeing masks reminds people of the risk and they modify their behavior).

It's a fact.

The purpose and design of surgical masks is to protect others, not the wearer. Mask catches big droplets but is not effective catching small particles. It's hard to wear tightly.

Respirator is designed to protect wearer. Buy N95 respirator and wear it correctly (trim your beard if necessary) if you want to be safe.

A respirator with an exhalation valve, protects the wearer but does not give as good protection to others as respirator without a valve.

I do wear an N95 and encourage others to wear an N95 or at least a surgical mask, but "it's a fact" is a poor argument. You may just as well say "Bruce Wayne won the 2020 US election. It's a fact".

I have personally been unable to find a single reliable study of mask efficiency, and the ones about N95 respirator efficiency that I found had the grim conclusion that they don't help much unless expertly fitted, and even health professional aren't very good to fitting them properly.

What I have found are some simulations that make unrealistic assumptions and reach the pre-supposed conclusions.

> Mask catches big droplets but is not effective catching small particles.

Well, then it's all a question of virion size vs mask hole size, then, and how long virions are still active, and what the effect of breath diffusion is. Of which the raw numbers would say that masks are mostly useless. I have not yet seen a model/study/trial that addresses this properly.

I wear an N95, and I encourage everyone else to do the same, but I do recognize that it's not a "fact"

p = 0.38?!

Nobody would take a p-value of 0.38 as evidence. There are a lot of problems with p-values in general, but even uncritical fans of p-values would ignore any study with p = 0.38.

Why research supporting the null hypothesis should be ignored?

This is negative result.