Danish study finds face masks provide limited protection to wearer

https://www.reuters.com/article/us-health-coronavirus-facemasks-idUSKBN27Y1YW

by pseudolus • 6 years ago

29 70 6 years ago

Knufen• 6 years ago

Dane here, so I've read the study. One of the important aspects here is of course the sample size, the timing of the study (during a downward trend in Denmark) and more importantly, it doesn't tell us if it protects others (which other research points to).

comicjk4• 6 years ago

This is the key fact. Only 2% of the subjects tested antibody-positive at the end of the study (low enough that it's near the false-positive rate of the home antibody tests). The mask group had fewer cases (by PCR, they had zero). But the numbers are too small to say. If you ran the study in the US in the same time period, you would have had a better shot.

xyzal• 6 years ago

"The findings are consistent with previous research. Health experts have long said a mask provides only limited protection for the person wearing it, but can dramatically reduce the risk to others if the wearer is infected, even when showing no symptoms. Preventing the spread to others is known as source control."

Mouse47• 6 years ago

In the study, which was carried out in April and May when Danish authorities did not recommend wearing face masks, 6,024 adults were divided into two groups, one wearing face masks and one control group.
After one month, 1.8% of the people wearing masks had been infected, while 2.1% of the people in the control group had tested positive, Copenhagen University Hospital said in a press release.
The study does not confirm the expected halving of the risk of infection for people wearing face masks
Participants reported their own test results; mask use was not independently verified, and users may not have worn them correctly.

So the control group had no masks, and the mask group were only told to wear a mask, and we have no data for how often they wore it. If their behavior is anything like what I'm seeing in the U.S., they're wearing it into the grocery store and going and seeing friends normally without a mask. The fact the study still produced a (likely statistically insignificant) 15% reduction in risk (1.8/2.1) is surprising under these circumstances.

frumiousirc2• 6 years ago

the mask group were only told to wear a mask

They were asked if and how they followed instructions and the 7% that said they did not were removed from the analysis. Only half of the remaining said they followed instructions. I also doubt the veracity of the replies. This is a weak point of the procedure.

happytoexplain4• 6 years ago

I'm confused as to why they removed that 7%. Are they or are they not trying to test the effect of advisement? If that really was the goal, then the veracity of replies wouldn't matter, and the study would make sense - but then they also wouldn't have omitted that 7%.

Symmetry2• 6 years ago

It isn't clear that the people in the control group didn't wear masks. Few Danish people were wearing masks then but I'd imagine that people who went and volunteered for this study would be more likely to be the sort of people who would go and get their own masks if they weren't given them.

me_me_me2• 6 years ago

0.3% better chance for wearing a piece of cloth over a face - for a mild inconvenience. I'll take it anytime still.

I've seen accounts of maraton runners not able to run a 1K months after recovering.

I'll take 0.3% over nothing, thanks.

Also noteworthy, that is 10% less of a chance of getting infected. If you were to get infected.

frumiousirc2• 6 years ago

Here's the actual study.

https://www.acpjournals.org/doi/10.7326/M20-6817

The intervention being tested was merely how a RECOMMENDATION to wear a mask (and other advice and a pack of 50 surgical masks) may affect infection rates.

Of the mask wearers, and based on asking them and accepting their answers as truthful, 46% wore the mask as recommended, 47% "predominantly" as recommended, 7% not as recommended. The 7% were excluded from the results.

The infection in the area at the time was 2%. This gives relatively fewer chances for the efficacy of masks to be tested and so it is not surprising that the results are inconclusive. The benefit masks bring in this case is keeping the infection rate low and only actively do their job during the relatively rare times when an infected individual is near a healthy one.

The low infection rate also leads to an inflation of any systematic differences between the two samples. A very small difference applied to the expected 98% healthy subsample can lead to a strong contribution to the the 2% infected subsample. For example, mask wearing at the home was not covered and a few reported covid infections at the home.

Testing in places where infection rates are higher would have a better chance to show an effect. Of course that would be stymied by sample bias (if "random" self-selection of mask-wearers and anti-mask idiots were allowed in the procedures) or it would be faced with unethical practice of forcibly withholding masks from individuals in a non-mask sample.

My personal conclusion is that these results are not conclusive but will be hugely appropriated for whatever sick fantasy the outgoing US administration is orchestrating.

mvcalder• 6 years ago

I did not read the article so maybe this was answered. But if the study was to see the effect of the recommendation + small supply of masks, I don't see why the non-compliant 7% would be censored. In one ear and out the other is one possible recommendation outcome.

belorn• 6 years ago

The recommendation here in Sweden, who has a similar infection rate to Denmark, is to use mask in places where the infection rate is high and no general recommendation for places with low infection rate.

The reason for that is basically identical to what you wrote. Studies done in low infection areas give relatively fewer chances for the efficacy of masks to be tested, resulting in inconclusive data. Data from high infection rate areas is more conclusive and thus the recommendation is to use mask there where the data shows a conclusive result.

mark_l_watson• 6 years ago

I thought that it was accepted that the main benefit was in protecting other people. I wear a mask and a transparent face shield together. I believe that this combination offers me some protection and a lot of protection for people near me, so it seems like an obvious thing to do.

BTW, I find cloth masks not so comfortable, but the plastic face shield is no real additional discomfort at all.

BrandoElFollito• 6 years ago

For me the discomfort is really about my glasses that get fogged.

It is much better with the FFP-3 masks than with the typical surgical ones.

eatbitseveryday• 6 years ago

Glasses don’t get as fogged if you have a surgical mask with a metal wire for the nose clamp. Fits better than a plastic insert and does not let air shoot out the top into your eyes when you exhale.

microtherion• 6 years ago

I recently learned that a microfiber cloth (resealed in a ziploc bag between applications) helps a lot with the fogging.

heyoo• 6 years ago

I also thought this was already general consensus. I.e. that face masks are used to prevent you from spreading disease, if you are infected but without symptoms.

nottorp• 6 years ago

Yep, they're there to prevent spreading germs to others.

Surgeons don't wear them to protect themselves from the patient, but so they don't breathe over their open body.

Did that need a study?

raziel2p4• 6 years ago

I can't find a source right now but I remember reading that people were more likely to wear a mask if they think it protects themselves rather than those around you, so there's some humanitarian value in pushing that narrative even if it's not technically true.

Tyrek• 6 years ago

I appreciate the intention, but this is the sort of approach that leads to a long-term distrust of what 'the scientists' say - if we let politicians dictate the scientific message and consensus, then there's no room to criticize when the scientific message gets politicized and ignored.

cocoa19• 6 years ago

Technically wearing a mask helps "protect themselves" by second order effects.

We should never encourage lying or deceiving people (pushing not technically true narratives). This just erodes trust in society.

J-dawg4• 6 years ago

That is an absolutely horrible idea, and is exactly what fuels widespread mistrust of public health initiatives.

Tell the damn truth. If something is uncertain, tell the damn truth about how uncertain it is.

jacknews2• 6 years ago

A more honest narrative is to shame people not wearing masks as selfish irresponsible dunces.

Symmetry2• 6 years ago

Yes, wearing a mask doesn't protect you against infection from droplets in your eye. And aerosols tend to shrink a bit through evaporation between when they might be exhaled and someone else inhales them. So there's some asymmetry between masks protecting yourself and protecting other, but it isn't clear how much.

rchaves• 6 years ago

Mr Taleb comment on it: https://twitter.com/nntaleb/status/1329775819342934035

pjc503• 6 years ago

I've been thinking: every one of us who's been on a plane is told how to wear an oxygen mask, every single time. I've never needed to do this, and in situations where its necessary then survival is already looking a bit dicey. There have been about 30,000 deaths from air crashes globally ever.

I wear a mask every time I go to the shops. I've never been given an official briefing on how to do it. There have been about 1,300,000 coronavirus deaths worldwide.

happytoexplain4• 6 years ago

There have been about 30,000 deaths from air crashes globally ever.

Wow, while this is obviously a very low number in relative terms, I'm still surprised how big it is intuitively. I would have guessed an order of magnitude less based on the few times I've perused Wikipedia lists of aircraft fatalities. Where did you find this? Sorry to be off topic.

geofft• 6 years ago

I think it was immoral for the scientists to publish the study in the way they did, because they either knew or should have known the media (especially anti-mask propaganda outlets) would miss the following nuances:

- The point of wearing a mask is primarily to protect others, not yourself. Since we know covid has high asymptomatic infection rates, it helps control the spread of the disease for everyone to wear a mask even if they think they have no reason to stay home.

- There was no recommendation for the control group not to wear a mask.

- Very few people, percentage-wise, get covid at all, and many people are staying home, avoiding crowded events, etc. anyway. You could run a similar study demonstrating that over a month or so, a group told to wear seat belts did not get injured or die in car crashes at a higher rate than the control group, just because so few of them got in crashes at all. It would mean nothing for whether seat belts are worthwhile.

People are going to die as a result of the propaganda citing this study. I wonder if the scientists are okay with that.

There may also be an argument that it was immoral to use research money to conduct this flawed of a study in the first place.

cletus• 6 years ago

In other news, wearing seat belts does little to protect people and animals hit by the car you're driving.

Masks are about reducing the spread of the virus to other people, not about protecting the wearer.

giarc• 6 years ago

Regardless of the study and how well it was conducted etc, masks have never been the panacea. Taking the phrase from the airline industry, masks are part of the 'swiss cheese' model of protection. When you stack a bunch of protective measures together (mask, clean hands, distancing, staying home when sick, clean environment etc) you limit the ability of the virus to go from source to yourself. None of the measures are perfect, but when working together they hide each others flaws.

samfisher83• 6 years ago

It is if both people are wearing it.

https://www.cdc.gov/coronavirus/2019-ncov/more/masking-scien...

It lowers transmission by 80%. Masks help more if the person who has it wears it since they won't be transmitting the virus.

mrfusion4• 6 years ago

To those promoting masks to mitigate asymptomatic spread to others, I came across this study showing asymptotic spread is almost zero (1).

Is this something we have missed are there other studies that found the opposite?

(1) https://www.acpjournals.org/doi/10.7326/M20-2671

happytoexplain4• 6 years ago

I'm also curious about this. Unrelated: Note that the phrase "inconvenient fact" is usually used as a bad-faith attack in ideological arguments, rather than just being used literally, and you may catch flak from people misinterpreting you.

comicjk4• 6 years ago

If people with no current symptoms couldn't spread COVID-19, that would be an extremely convenient fact for controlling spread! But it doesn't look like we're that lucky.

mrfusion4• 6 years ago

You’re absolutely right. I’ve edited my comment.

comicjk4• 6 years ago

Only 6% of the subjects in that study were asymptomatic by their definition. I think this is a confusion of asymptomatic and pre-symptomatic: most cases will have a few days of pre-symptomatic infection regardless of the symptoms they have later.

mrfusion4• 6 years ago

I think it might be a good point but I’m not quite understanding.

Why does 6% matter if the statistical significance works out? Also I can’t seem to see how you’re saying the study has asymptomatic and presymptomatic confused.

(I don’t have a great background in all this so I could def be missing this.)

fiblye• 6 years ago

Wearing masks, like hand washing, has limited benefits if most people aren't doing it. Vaccines are also far more effective if a large number of people get them.

Studies comparing populations with almost 100% mask wearing and countries with almost no masks would make for a better comparison.

mfer2• 6 years ago

I wonder how this relates to the general spread of the disease in a region. Denmark has a much lower rate of spread than many places in the US.

The article doesn't say when this was carried out. Was it when there were few active cases[1]. If this was carried out during the time with few active cases it would be like testing an umbrella by walking around with it open on a day with a low chance of rain. Doesn't do a good job of testing the umbrella.

There is other research showing better protection like that at Beaumont health system[2]. This was in environments with varying levels of exposures (including nurses treating COVID patients).

I wish there were more details.

[1] https://www.worldometers.info/coronavirus/country/denmark/

[2] https://www.freep.com/story/news/health/2020/11/16/beaumont-...

splitrocket• 6 years ago

It is not "all or nothing".

it's defense in depth, and if masks, which are literally dirt cheap and only a minor inconvenience are even somewhat effective, that adds to the overall mitigation efforts and should be mandated.

It's not hard folks.

"Turning off SMTP relay only prevents other people from getting spam. I'm not going to do that, even though it's a one line configuration change and doesn't really impact my use case. Moreover, the default should be open SMTP relay"

threatofrain• 6 years ago

It’s also a matter of relative cost efficacy with reasonable effect sizes. Studies like this begin to empirically place magnitudes into different ballparks.

J-dawg4• 6 years ago

I guess I could say I'm "mask skeptical" even though I do wear one to conform to social pressure.

I wish the conversation would move on from "do masks work?" to "do they work well enough to be worth the cost?"

Because I think the costs are being significantly underestimated. We are social creatures. All of the joy has been taken out of routine daily activities. Putting on a mask at the supermarket entrance as if you're about to enter some contaminated danger zone. Being unable to give a friendly smile to people as you pass them. I am 100% sure it's affecting my mental health, and I can't be the only one. In online discussion, these concerns get brushed aside as if they are the complaints of a spoiled child: "stop being a Karen and put a damn mask on..."

I'm not some science denier. It seems logical that a barrier over the mouth and nose must limit how far droplets can travel. As I understand it, empirical evidence also supports this (when the mask is worn by an infected person). There may be some (weak) evidence that masks also protect a non-infected wearer, despite this study.

But the size of the effect can't be that big. Here in the UK mask compliance is high, yet we're well into our second wave. So in the real world, masks can't be doing all that much. (Or, most of the infections are happening in mask-free places such as between family members in the home).

So maybe I'm just another "Karen", but I want more evidence that I'm enduring the costs of mask-wearing for a good reason.

In the meantime, I'll keep wearing my mask because it seems to keep everyone else happy, but I'm continually disappointed by the lack of nuanced discussion about this.

happytoexplain4• 6 years ago

Ignoring the people who are obviously just complaining about masks for ideological/"Karen" reasons, I have to say I've been totally caught off guard during this pandemic by the existence of people like you (not in a critical way). I can relate to it feeling a little awkward at first wearing a mask in public in a culture that doesn't usually do that, but, despite being a social person and sympathizing with the social isolation pain of staying at home, the idea that "all of the joy has been taken out of routine daily activities" due to the masks for psycho-social reasons was outside my realm of imagination. I genuinely feel bad for people with that piece of personality.

J-dawg4• 6 years ago

This might make you even more surprised about my existence, but I'm not entirely unsympathetic to the "ideological" types.

Let me put it another way: why weren't you wearing a mask during flu season every year? Or, given that Covid will probably exist in some form for many years to come (even with a vaccine), when will you stop wearing one?

Because you could make a case for mandatory mask wearing at all times, forever, using the same logic that's currently being employed: "it's only a minor inconvenience and if it saves even one life, it's worth it".

Presumably you won't wear a mask forever: you'll make a personal decision, based on data or gut feeling, that the "effect size" of mask wearing is no longer worth the inconvenience. You were already making that calculation, implicitly, when you chose not to wear a mask during flu season last year.

That calculation will be different for every individual. Which is why I don't think any of us is really in a position to judge another person when the parameters of the calculation are different for them. Ultimately, the differing parameters of that calculation are what we call "ideology".

To return to my original point, it's also why we vitally need an open discussion on the "effect size" of wearing a mask. If it's large (I suspect it is not), then it gives more weight to the idea that all the Karens should just shut up and wear one. If it's small, there's a lot more room for personal choice.

hedgehog2• 6 years ago

There's plenty of evidence mask campaigns slow transmission. It's not fun but I see it as a minuscule sacrifice for me to wear a mask to the grocery store if it saves even one person a trip to the hospital. The sad reality is the grocery store _is_ a contaminated danger zone, and will be until we get this thing under control.

The sliver of good news is that lowering the growth rate just slightly from above 1.0 to below will cause the epidemic to shrink dramatically. It's not like we need to reduce transmission by 100%.

Not that you asked for it but my advice to you is to find more sources of joy in your daily life that aren't so affected. If you're bored shoot me an e-mail, I have plenty of extra projects :)

J-dawg4• 6 years ago

There's plenty of evidence mask campaigns slow transmission

By how much do they slow transmission?

It's not fun but I see it as a minuscule sacrifice for me to wear a mask to the grocery store if it saves even one person a trip to the hospital

People go to hospital with flu every year. You or I might have unknowingly spread it around and caused someone to go to hospital before. Will you wear a mask every flu season, forever? It's worth it if it saves even one person a trip to the hospital, right?

pjc503• 6 years ago

most of the infections are happening in mask-free places such as between family members in the home

Yes, and certain workplaces. But that's the distribution you'd expect if masks had a positive effect.

(I would really like good league tables on where all the transmission is happening, including workplaces, schools, and universities, but that seems to be beyond the capacity of the UK state).

The cost of not wearing masks is transmitting the disease, which, even if you and the person you transmit it to, do not experience symptoms, prolongs the pain. If we had 100% effective transmission prevention this would all be over by now. In some countries it is. The sooner people improve their anti-transmission behaviour the sooner they can stop doing it and we can all go back to the pub.

MagnumOpus• 6 years ago

I would really like good league tables on where all the transmission is happening, including workplaces, schools, and universities, but that seems to be beyond the capacity of the UK state

PHE has that league table, as widely reported in the press. E.g.

https://news.sky.com/story/covid-19-supermarkets-most-common...

Upshot: a huge amount of infections happen in schools where kids and teachers don't wear masks.

stateofnounion• 6 years ago

Glad everyone's so quick to pick out the issues with methodology here. Now go back to all the "wear-a-mask" studies and apply the same criticisms.

christophilus• 6 years ago

Yes. They all have problems. Small sample sizes, or non randomized groups, not double blind, etc. But, it’s all we have. The preponderance of data (imperfect as it may be) suggests that a public mask policy is prudent. Given the extremely low burden of wearing a mask, and the weight of the evidence in its favor, I really can’t understand anti maskers. Most of my friends are anti mask, and I’ve listened and tried to understand, but I always come away thinking they’re simply uninformed. Most of them get their information from sources that border on being conspiracy theorists.

ahamdy88• 6 years ago

I think you should read the study yourself instead of taking some journalist conclusions who really loves flashy titles

https://www.acpjournals.org/doi/10.7326/M20-6817

From the study: Inconclusive results, missing data, variable adherence, patient-reported findings on home tests, no blinding, and no assessment of whether masks could decrease disease transmission from mask wearers to others.

sleepysysadmin• 6 years ago

I guess every mask has 'limited protection'. N95 is 95%. The limit being 95%.

The masks most people are wearing are 'surgical masks' but those still tend to be in the 75% range. Which if both sides are wearing 75% masks, then the effectiveness is still relatively high. An effectiveness above 90%.

The article says:

“The results could indicate a more moderate degree of protection of 15-20%, however, the study could not rule out that face masks do not provide any protection.”

That's just not in line with what we do know. Mask technology isn't new. This seems overly broad as well, how did they account for say hand washing? Everyone had the same hand washing regime? I doubt they controlled for that.

Here's the thing, if you don't trust in the level of masks. You are completely free to do better. This isn't an argument to go backwards in protection.

cashsterling• 6 years ago

I am going to disagree with the study based upon (1) the study didn't perform a proper experiment and are misinterpreting their results... and (2) basic particle/aerosol science. Finally I give a "close to home" ancedotal story.

(1) It only takes one non-masked, close-contact exposure to coronavirus to become infected... that it, mostly. how it is spreading. The study does not control for the exposure risk profile of the sample populations so... there you go. This study is next to worthless...

(2) There have already been numerous studies, recently, that cloth masks are effective at filtering out particles, even down to 1-5 microns in some cases. Most cloth face masks are very effective and filtering larger particles (50-250 microns). Particle mass/volume scale with the particle diameter to the third power (i.e. V=1.33PIR^3 or 0.166PID^3). So a 100 micron particle contains 1000 times more mass/volume or viral load than an 10 micron particle and a million times more than a 1 micron particle. Thus is most important to filter the big particles in reducing viral load expelled by infected individual and breathed in by others. Cloth face masks, and surgical masks are pretty good at doing this. N95 and KN95 are already well tested in this area. So while masks can't filter out all virus... they are pretty good a significantly reducing the viral load which is important.

(3) I know people at work who became infected at a party and came to work for days while they were unknowingly infected. But thus far, there have been no "at work" coronavirus transmission cases because we wear good masks. Thus far this is true at our other sites as well. I know a local state epidemiologist who remarked to me that the overall rate of transmission in the workplace, for places with good mask protocols, is remarkably low.

jacknews2• 6 years ago

I thought it was now already established that the point of mask-wearing is to protect everyone else from the wearer - who may not know they are infected.

Danish study finds face masks provide limited protection to wearer

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