Covid-19: New evidence on face masks

A few months back I wrote an article about the state of the evidence on face masks. At that point, there were no good studies looking at the effectiveness of face masks in preventing the spread of covid-19 specifically, but there was a systematic review that looked at all randomized trials that had been done on face masks for the prevention of respiratory infections more generally. That review found that surgical face masks reduced the probability of getting a respiratory infection by around 4% in absolute terms (17% in relative terms).

My conclusion was that, considering how infectious covid is, face masks were unlikely to have an effect on the spread of the virus on a population level, although I thought it made sense to use them in hospitals and nursing homes, where you want to do everything possible to minimize the risk of spread to people who are at high risk of a severe outcome.

Now we finally have a randomized controlled trial that has looked at the ability of face masks to protect wearers against covid-19. It was published in the Annals of Internal Medicine. The study was carried out in Denmark, and was funded by a charitable foundation that is connected with a company that owns supermarkets (I’m not sure whether that means they wanted the study to be a success or a failure, or just wanted to know the truth).

In order to be included in the study, participants had to be over the age of 18 and and they had to spend at least three hours per day outside the home. People were not allowed to take part in the study if they had current or prior symptoms that could indicate covid-19 infection, or a previously confirmed diagnos of covid-19. All potential participants had an antibody test performed at the beginning of the study, and if it was positive, then they were excluded from taking part. Participants were recruited through adverts in media and through direct contact with companies and other organizations.

In total 6,024 people were recruited in to the study, and of these 4,862 (81%) followed through to the end. That is a nice big number, which should be able to detect a meaningful difference, if there is one. The average age of the participants was 47 years. Half the participants were randomized to wear a face masks at all times when outside the home, and half were randomized not to. For obvious reasons, this study was unblinded, since it’s hard to create a situation where people are unaware of whether they’re wearing masks or not.

Participants in the intervention group were given 50 disposable surgical masks. This actually increases the probability of the study showing a meaningful effect compared with the reality in most countries where masks are currently being used in public. Why?

Because in most real world situations, people are wearing (and repeatedly re-wearing) non-disposable cloth masks, which are likely much less effective than disposable surgical masks that are only used once. In the systematic review I wrote about in my previous article, the little data there was on cloth masks suggested that they were completely ineffective.

Participants were followed for one month, and at the end of the month an antibody test and a PCR test for covid were carried out. If participants had symptoms suggestive of covid at any point during the month, a PCR test was also performed at that time point. All participants received written and video instructions on how to use the face masks properly. If outside the home for more than eight hours at a time, they were instructed to change to a new mask, so that a single mask was never used for longer than eight hours.

Both an intention-to-treat and a per-protocol analysis was done of the results. What that means is that they looked at what the results were, both if all participants involved in the study were included (intention-to-treat), and if only participants who reported wearing the masks as instructed a high proportion of the time were included in the analysis (per-protocol).

In general, it is considered good form to do an intention-to-treat analysis, and bad form to do a per-protocol analysis. The reason for this is that a per-protocol analysis will tend to make the results seem better than they are in the real world (in the real world, not everyone does as they’re told – annoying, right?!). In this case, however, I think it’s reasonable to also do a per-protocol analysis, because we want to know what effect, if any, masks have when used as instructed.

So, what were the results?

We’ll start with the intention-to-treat analysis. In the face mask group, 1,8% developed covid-19 over the course of the study. In the control group, 2,1% developed covid-19. That is a 0,3% difference in favor of face masks, but it is not even close to being statistically significant.

Ok, let’s look instead at the per-protocol analysis, which in practice means that the 7% of participants who often didn’t follow the mask wearing instructions properly are excluded from the analysis. In the face mask group, 1,8% developed covid-19, and in the control group, 2,1% developed covid-19. So, interestingly, the result was the same regardless of whether you look only at those who wore the masks as intended, or look at everyone, including those who didn’t follow the instructions. This in itself suggests that mask wearing doesn’t make a big difference, since the results don’t change when you only look at people who have been good at wearing their masks as intended.

As an interesting aside, the researchers didn’t just look at covid, they also looked at 11 other respiratory viruses. In the face mask group, 0,5% tested positive for one or more other respiratory viruses. In the control group, 0,6% tested positive. That is a 0,1% difference, and again, it was nowhere close to being statistically significant.

What can we conclude from this?

Wearing face masks when out in public does not meaningfully decrease the probability that the mask wearer will get covid-19. It’s possible that there is a small reduction in risk, but if there is, it is so small that it was undetectable in a study where almost 5,000 people were followed for a month.

It is worth nothing here that the effect seen in studies is usually better than the effect seen in reality. The reason for this is that study participants usually try harder than people who aren’t part of a study, and they get better instruction. In this case, they also had better masks than most people are using at present in reality, and changed to new masks on a regular basis. So, if no meaningful difference was seen in this study, then I think it’s safe to say that no meaningful difference exists in reality.

One thing that is good about this study is that it is the first randomized controlled trial that comes close to mimicking the present reality in many countries, where people are wearing face masks in public, but not at home.

One interesting result of the study was that 52 people in the face mask group and 39 people in the control group reported another individual in the home having covid-19 during the course of the study. Yet of those, only 3 actually developed covid. People sharing a home with someone with covid were really no more likely to get covid than people who weren’t. This suggests that most covid infections happen outside the home, and is in itself something that would be an interesting avenue for further research. It also suggests that most people with covid are not themselves very infectious, giving support to the hypothesis that most infections happen through a small group of highly infectious ”super spreaders”.

The main thing lacking with this study is that it only looked at risk to the person wearing the face mask. It says nothing about the risk that the person wearing the mask will infect another person. That is an equally important parameter, and at present there are no high quality studies looking in to it, so before we can truly say that masks fill no function, we need another large randomized controlled trial that looks at the ability of face masks to prevent the mask wearer infecting other people.

You might also be interested in my article about whether fever lowering drugs are a good idea when you’re sick, or my article about how accurate the covid tests are.

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Author: Sebastian Rushworth, M.D.

I am a practicing physician in Stockholm, Sweden. My main interests are evidence based medicine, medical ethics, and medical history. I frequently get asked questions by my patients about health, diet, exercise, supplements, and medications. The purpose of this blog is to try to understand what the science says and to translate it in to a format that non-scientists can understand.

39 thoughts on “Covid-19: New evidence on face masks”

  1. I would love to see some research on whether mask wearing by infected people decreases or increases the probability that they will infect others. Whilst masks may capture viral droplets, what I would question is what happens then when a contaminated mask is touched and then the wearer touches surfaces or other people. There is also the false sense of security issue. ‘I know I have Covid but I wont give it to anyone if I go out because I am wearing a mask’.

  2. Hi Sebastian,
    Thank you for this article. I think that settles the usefulness for my own protection.
    As we are told we would protect others! by wearing a mask it would be really interesting to see how much
    of protection the mask (in most cases the cotton/cloth mask) would actually give.
    I look forward to finding that out.

    Hanna Back

  3. Thank you for summarizing this study. It does conform with what we already from earlier studies and other viruses. On the point to properly studying the effect of mask wearing on others I am doubtful how this could actually be achieved with a large enough number of subjects. We would have to “demask” a whole town and compare to the rest but even then the pre-distribution of the virus compared to other towns might skew the numbers too much to be meaningful.

    I actually asked my local government on what scientific grounds they mandated mask wearing and the information I got essentially went “well it might work therefore it is better to use masks than not”. For me wearing a mask seems more and more an article of faith. They surely did not stop the winter resurgence.

  4. We have decades of randomized controlled trials showing that masks do not reduce the transmission of viral respiratory diseases. Moreover, we can see that Sweden, where virtually no one wears masks, has a smaller rate of COVID infections than many countries with strict mask rules.

    It is basic science that when no correlation exists between two variables, it is invalid to conclude that those variables are causally related. Sweden proves that masks cannot make a difference with regard to COVID transmission.

    Masks are nothing more than a forcible reminder to remain afraid. They are the equivalent of having state-sponsored media literally “in your face” constantly broadcasting to you that the air and the people around you are dangerous and accordingly that the lockdowns and other deprivations of civil liberties are justified. It’s the same as telling a prisoner the same thing over and over again until that person finally believes it. It’s brainwashing.

    It is only by constantly reinforcing fear that governments around the world can continue the path toward totalitarianism. There is no other explanation for why we have the complete upending of society for a disease that is virtually harmless for the vast majority of the population.

  5. Regarding this:
    “One interesting result of the study was that 52 people in the face mask group and 39 people in the control group reported another individual in the home having covid-19 during the course of the study. Yet of those, only 3 actually developed covid. People sharing a home with someone with covid were really no more likely to get covid than people who weren’t. This suggests that most covid infections happen outside the home, and is in itself something that would be an interesting avenue for further research. It also suggests that most people with covid are not themselves very infectious, giving support to the hypothesis that most infections happen through a small group of highly infectious ”super spreaders”.”

    Don’t we actually have two possible ways to explain this, either as an effect of how the virus spreads (e.g. “super spreaders”) or as an effect of how it infects? Could one explanation be that those who are predisposed simply will be infected while others will not, as the virus spreads so effectively that almost everyone gets it?

  6. thedullchannel You´re right about research regarding research on what is call “source control” in the study. It would mean ethical problems to set up to be “achieved with a large enough number of subjects” etc. Isolate if sick or suspected sick and while waiting for a test answer. Nice hints about low virus spreading in household in the study as well.

  7. What I’ve noticed in my large factory workplace (where we have all worked through this event) is that our risk assessments say that we should not work closer than 2m to each other, but that if we do we should wear a mask (Of any sort, usually home made woven fabric).

    For the first 5-6 months most people stayed in their small teams of 3-6 people and didn’t worry much about masks, but generally did stay apart. Maybe 200-300 in the factory, split over 2 separate shifts. No covid infections, but two cancer diagnoses.

    But over the last 6 weeks people have started wearing masks, probably 50-75% of people maybe 50% of the time. People now use it as an excuse to get closer, to mix teams, to move areas. Now in the last 2-3 weeks we are getting more people testing positive (2-5 per week) and several teams are isolating, totalling maybe 50 people at a time.

    Why am I not suprised? It’s almost as if strapping what is in effect a dirty handkerchief across your face and actively ventilating the contents as you walk around the place isn’t a good idea.

    Great article as usual, thanks!

  8. I mean, this is really interesting stuff, but the observation should render two possible core tracks of investigation. Either is this observation due to some characteristic in how the virus spreads, where the “super spreaders” hypotesis is one such idea, or it is due to some characteristic in how the virus infects. We know from studies that overall health (I would say metabolic health) seems to be strongly correlated with the risk of getting sick from this, maybe the individual predisposal plays a highly significant role (that possibly makes all attempts to control the spread rather uesless)??

  9. Previous comment was a response to Bengt Hanssons reply to my first post, I didn’t consider the comments section is non-threaded. 😮

  10. Gavin Rea-Davies, are you seeing more people actually getting sick? Or is it just TestDemic->CaseDemic…????

  11. Are you intentionally missing the point or did it escape your notice that people wear masks in order to prevent them spreading the virus, not because wearing a mask protects them (their protection comes from everyone else wearing a mask – as well as maintaining distance and washing hands etc).
    This study and your analysis of the results seems to be completely meaningless as we don’t know whether the people they came in to contact with were wearing masks, and we don’t even know whether the people who contracted covid-19 did so via airborne droplets or via touching contaminated surfaces etc.

    1. Hi Tim,
      This study shows that masks don’t protect the wearer. It never claims to show anything else, and neither do I. As to your belief that people only wear masks to protect others, not themselves, I think you are very wrong about that.

  12. Tim W, Rushworth wrote as his penultimate paragraph the following, which you can verify for yourself and potentially prove me wrong about:

    “The main thing lacking with this study is that it only looked at risk to the person wearing the face mask. It says nothing about the risk that the person wearing the mask will infect another person. That is an equally important parameter, and at present there are no high quality studies looking in to it, so before we can truly say that masks fill no function, we need another large randomized controlled trial that looks at the ability of face masks to prevent the mask wearer infecting other people.”

    The study is not meaningless, and you’re being hyperbolic. Understanding comes from assessing the perspectives of multiple viewpoints using various kinds of observers using various kinds of techniques. This means we need all the various pieces and the ways for collecting them, and approaching scientific and general understanding this way — no one piece stands alone in our understanding as the totality; no one model can ever represent completely that which it models — allows us both curiosity for what we haven’t yet learned and appreciation for the mistakes we made to become ever more curious. Calm down and breathe, practice charity and gratitude, and we’ll listen to you even more.

    So, no, Rushworth was not intentionally missing the point nor failing to notice something.

  13. Most interesting regarding not catching it from infected people in the same home as you would think this would be the best way to catch it – Elsewhere it has been proposed that all virus travel around the World on natural air currents and their pattern reflects somewhat the pattern of international infection – This route would explain why Lockdowns don’t work even where severely enforced in Peru etc – And in Spain where the Mask is severely enforced infection rates are the same as not so enforced countries – The virus delivered by international air currents direct to the Lockdown location and right through the Mask which is no more significant than a hair net to the tiny air borne virus – This would explain why Lockdowns and Masks are only good for demoralising the population and destroying the economy – Governments want to instil sufficient fear to maintain support for their policies until the virus fades away, as they all do, and then claim victory – You can so see why the conspiracy theories thrive to give meaning to policies everyone can now see from their own experience and observation don’t work and just cause damage which will likely get much worse and endure long after this virus is as accepted as all the others – The enforced ‘cure’ from a pandemic where the total deaths are average and most have to be tested to know they are infected so mild are the symptoms is worse than the disease – This event will likely be looked back on as the worst self inflicted wound of all time – (Unless of course it is used to enforce vaccination and on the back of that international health passports to facilitate the Great Reset etc in which case we will all be re educated to love and praise it?)

  14. Great to see an RCT finally on this topic. Anecdotally, I had heard it was challenging to get this published, and it seems there may be some credence to that considering the time from endpoint date to publication for a study of this type, especially in comparison to the many other clinical studies on COVID, including treatments & vaccines, that have been published almost in real-time (often with many issues and biases slipping through the cracks at the cost of expediency).

    Interestingly, and in complete opposition to the results presented in this RCT on mask wearing, the Airlines with the help of a few high profile institutions have put out this little gem:
    Check out the figure showing how wearing a masks correctly keeps you safe, but not covering your nose offers “no protection. And where the exclaim in bold letters: “In summary, face masks offer an important line of protection against the spread of SARS-COV-2, by reducing potentially infectious exhaled respiratory particles and providing some barrier protection against inhaled particles. Therefore, despite differences in design and material, face masks of any kind without exhalation valves or vents help prevent the spread of respiratory infectious disease and should be worn by all people in public places as a mechanism to protect public health during the COVID-19 global pandemic”
    This seems actually to be potentially quite dangerous as it provides a major false sense of security where even prior to this study, there was literally no data to demonstrate efficacy of protection to one’s self via mask wearing, and even the WHO’s NPI guidance for influenza stated that masks offer no protection from contracting the virus. .

  15. “we need another large randomized controlled trial that looks at the ability of face masks to prevent the mask wearer infecting other people” – no, we don’t. Masks do not stop a pathogen such as SARS-CoV-2.

    The virion has a diameter between 50 and 200 nm. The highest class of commercially traded masks, FFP3, can’t stop a single virion from permeating the mask.

    Claims that carriers of viable SARS-CoV-2 can be asymptomatic are anecdotal, not a single peer-reviewed study exists that proves this, and it’s highly unreasonable to assume that this were true when it isn’t for all other known coronaviruses.

    Instead of making masks compulsory for everyone, including the vast majority of the population who are not carrying the virus, containment should focus on 2 symptoms: fever, and coughing. Send every household a thermometer in the mail. If someone is caught with fever and coughing outside their home and not on their way to a doctor, send them into isolation, and a hefty fine.

    And to those people who are so fond of pointing out how China went into lockdown to contain the virus: no, they didn’t, instead they checked every single resident of Wuhan within a single week (not a month or 5 months of lockdowns), going from door to door, taking temperature, inspecting for coughing symptoms, and those that were symptomatic were taken into hospital, the rest stayed at home, until they had checked all residents in the region. A few weeks later life went back to normal. China never did mass-testing with RT-PCR or antibody kits. But they did introduce fines for violating the above rules: report in sick when you have fever and/or coughing.

    It’s really that simple.

  16. I wish we had doctors like you in Canada. The mask mandates here are getting ridiculous. Do you think that will ever happen in Sweden? I was hoping to come there soon to get away from all this but there seem to be more and more restrictions there as well.

    1. Hi Louise,
      The politicians in the Swedish government seem to be panicking as a result of the recent increase in cases and are pushing through increasingly populist measures. How that will end, I can’t say. Anders Tegnell has continued to reiterate that the evidence for masks is weak, and as this study shows, it is getting weaker, not stronger. So I doubt there will be a general recommendation that everyone wear a mask in public. At the same time, there are a number of prominent people aggressively lobbying for face masks, so who can say what will happen.

  17. Sebastian – you probably already know that this study was cited, ahead of publication, as an example of apparently rigorous piece of work rejected by at least three major journals. (my source is but that can easily be verified from other sources, I think.)

    it would be quite interesting to have the authors of the papers publish the reviews and editorial decision letters they have received from Lancet, NEJM and JAMA. I really wonder what their reasons were, and in which specific way they necessarily would clash with the acceptance decision by the ACP AIM. (which has an IF of about 21 by the way, so not bad at all.)


    1. AIM is one of the ”big five” major journals in medicine, so getting an article published there is no mean feat. This is a well done, high quality study – the highest quality evidence on face masks that exists so far. I think it’s obvious that they didn’t want to publish because the results weren’t politically palatable. And even though AIM did publish, they also published a highly negative (and scientifically nonsensical) commentary to go with it, so that no-one would be under the impression that they support this study. I think, in the end, they felt that they had to publish, because the study is high quality and to not publish would be clear scientific censorship.

  18. This synopsis piece in Healio straight up, without any shame, falsely claims in the most blatant terms, both in the body of the text and the title (“Study: COVID-19 risk slightly lower for mask wearers”) that this Denmark Mask study actually shows masks are effective at reducing risk of COVID. Per the author of the article (a cardiologist): ““The identified potential positive effect of the mask for prevention — although small — might still be of interest considering COVID-19 is a very serious disease,” he added.”… How is the happening, how are these publications allowing this? This isn’t the NYT, this is a publication that is read mainly by other healthcare professionals. This is incredibly irresponsible on the part of the physician as well. I truly believe that this should be grounds for suspending medical licenses; as I’ve said this before about Eric Topol and others of his ilk spewing nonsense fearmongering MSM pieces on Twitter and elsewhere as if they are scientific fact.

    The author of this Healio article completely ignores the concept of statistical significance, then draws the conclusion that works best for his narrative, ignores everything else, and then does backflips in an attempt to discredit and highlight the problems and deficiencies of the study. I wish these same folks would put this much effort into their critical assessments of the many other studies coming out, especially the pre-prints they and the media love to champion whenever the results can be used to drive more fear and panic. It’s really becoming exhausting, and I am truly concerned for the future of science in our world, where scientists and healthcare professionals are willing to cast aside objectivity. The determent to the public’s trust in science & medicine over the long term is no longer a hypothetical, we are now entering the dark ages of the 21st century.

    Link to Healio article

  19. I shared your article on Facebook and also one on the same subject from The Spectator. The latter one has been taken down as ‘false information’. Your article is still up. I wonder for how long. Thanks for all your writings….a sane voice amongst the idiots.

  20. Well, currently people here in Bristol can only get a test if they have typical symptoms, so those who tested positive all went sick first. I say ‘all’, the number is still only the usual couple of percent of the factory population. Just funny how it has only happened in the last few weeks, as opposed to the last 5 months without masks.

  21. Good point you make. We are asked to wear masks so as not to infect other people. There is no effective evidence on this – it’s based on common sense, plus the fact that wearing one isn’r really any hardship.

    Btw Sebastian – one of your earlier pirces said Covid-19 was over in Sweden. But 400 people in Sweden have died in the last month and your government has introduced social distancing controls. You really should revise your earlier output and/or issue an update. Maybe even an aplogy for anyone who followed your advice.

    1. Hi James,
      Thanks for your comments. I didn’t offer advice, I said that I thought the pandemic was over in Sweden, which was a reasonable interpretation of the evidence available at that point, since there was a widespread consensus that covid did not behave in a seasonal manner. Since then it has become clear the covid is highly seasonal. I did post an update at the top of my earlier article where I proposed that Sweden had reached the point of herd immunity, which you can see here:

  22. James,
    There is no “common sense” involved with the recommendation to wear masks, or many other medical interventions. Medicine doesn’t work that way. What appears to be medical “common sense” among laypeople is usually based upon ignorance. In the past, “common sense” has led to other interventions such as bloodletting, which are harmful.

    There is ample evidence that masks have zero effectiveness, and no good evidence that they have any efficacy. Mask recommendations/orders are based upon the desire of governments to perpetuate fear, nothing more.

    Wearing a mask may not seem like a hardship to you but it is a significant hardship to other people.

    If wearing a mask makes you feel good, great. But people should not be asked to do things that are contrary to the best available data. Hopefully you are open to scientific evidence.

  23. “This study shows that masks don’t protect the wearer.”
    That’s really overstating it. It lacks the power to do that.

  24. I would like to offer an aspect of mask bearing that hasn’t been discussed widely to my knowledge. In 2004 Ulrike Butz published her German PhD thesis on the subject of “Rückatmung von Kohlendioxid bei Verwendung von Operationsmasken als hygienischer Mundschutz an medizinischem Fachpersonal”

    I.e. rebreathing of carbon dioxide during use of surgical masks with medical personnel.

    It came to the conclusion, that, if correctly worn, such masks (as are being used nowadays by the masses) lead quickly to increased CO2 levels inside the mask of 22 mm Hg, ca. 3 percent (in contrast to 0.04 % in normal air).
    This means, when one wears such masks correctly one is subjected to greatly elevated CO2 levels possibly over a long time (e.g. a work day or school day).
    If looking for recommendations for CO2 levels i found (for Germany) that for levels up to 0,1 percent, no consequences can be expected. A maximum of 0.5 percent is the legal highest value (MAK) acceptable at a work place for a period of 8 hours.
    Compare this to the six times higher value (3 percent CO2 for correctly worn surgical masks) that seems to be suddenly acceptable for major parts of the population, including children in some countries over long periods of time.

  25. What is found is that this increase in ‘cases’ (with an unknown proportion of false positives and non-infectious individuals) and deaths (attributed to COVID-19, but without knowing how many were actually determined by the SARS-CoV-2 infection), not only isn’t causing excess mortality from all causes, but it remains below the baseline defined by the country’s recent historical average. Considering the demographic growth that has occurred in Sweden in recent years, and based on the available data, it is expected that the gross mortality rate in 2020 in Sweden will end in line with that of recent years, as if no pandemic had occurred. This is the most reliable indicator of the global impact of the viral pandemic and the measures to manage it, in addition to being the most relevant in health.

  26. Masks are designed for the user in all industries except in hygenic operating theatres where they stole skin and hair entering open wounds. Filtration is minimal in reverse flow especially masks with vents.

  27. People should pay attention to what Sebastian Kurz, Chancellor of Austria, said in summer when Austria introduced a mask mandate. To paraphrase from Austrian German, “Even if they don’t work, people should view them as a symbol of agreement with government policy.”

    In other words, it’s a symbol of submission.

    Should also point out that not only is Mr Kurz not a virologist or epidemiologist, but he did not learn any profession. And by that I don’t mean that he didn’t study but rather became a craftsman – he literally did not learn anything.

    And he’s 34 years old.

    A 34 year old who never worked in the private sector and never learned any profession decided that Austria must go into lockdown, and that all citizens must wear a mask as a symbol of submission to his rule.

    At least the other Austrian became a somewhat proficient painter.

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