Health Blog

Do Face Masks Actually Help Stop Viral Infections?

Dec 12 • 8 minute read

The curious case of the Danish mask study

BMJ 2020; 371 doi: (Published 26 November 2020) Cite this as: BMJ 2020;371:m4586  Kamran Abbasi, executive editor


DANMASK-19, the first trial of mask use during covid-19, was “negative.” Masks didn’t work. We knew this before the trial was published because we were told so on social media. The authors were reported by the media to be struggling to find a major journal for their trial.1 Journals weren’t proving brave enough to publish the study, said the authors, and they didn’t make a preprint available.

When the mythical trial was finally published last week in the Annals of Internal Medicine we didn’t need to read it. We already knew its damning verdict on mask wearing. Social media told us as much. Eminent professors of evidence based medicine, Carl Heneghan and Tom Jefferson, confirmed this in an article for the Spectator.2

Except that if you read the published paper you find almost the exact opposite.345 The trial is inconclusive rather than negative, and it points to a likely benefit of mask wearing to the wearer—it did not examine the wider potential benefit of reduced spread of infection to others—and this even in a population where mask wearing isn’t mandatory and prevalence of infection is low. This finding is in keeping with summaries of evidence from Cochrane.

A disagreement among experts, especially about interpretation of a study, is a common occurrence. It is the usual business of science. Only, Facebook didn’t see it that way. The social media platform that allows statements about injecting bleach to prevent covid-19, as well as calls to behead the leading US expert on pandemics,67 decreed that Heneghan and Jefferson should be censured for misinformation after they reposted their Spectator article on the site.

It is possible to disagree with Heneghan and Jefferson about the robustness and interpretation of the DANMASK-19 trial—which I do—and still believe it is wrong that their opinion of it was marked as “false information.”8 It seems 2020 is Orwell’s 1984, where the boundaries of public discourse are governed by multibillion dollar corporations (in place of a totalitarian regime) and secret algorithms coded by unidentified employees. Where is Facebook’s accountability for the lies and damaging misinformation910 that it has peddled on controversial topics such as mental health and suicides,11 minorities,12 and vaccines1314?

The problem is less that Facebook and other social media decide what is published on their platforms,15 just as The BMJ’s editors decide what is published on Sacha Baron Cohen and Carole Cadwalladr, among others, have argued that this is exactly what these tech giants should do.1617 It is more that Facebook in particular purports to allow freedom of speech on its platform but acts selectively, seemingly without logic, consistency, or transparency. That is how control of facts and opinions furthers hidden agendas and manipulates the public.

You might expect a national body like Public Health England to offer the best advice on mask wearing, but PHE is no more after seven years of controversy.18 Its proximity to government and industry, together with funding cuts to local authorities, essentially set it up to fail and now to become the government’s fall guy for a flawed pandemic response. PHE is being replaced by a national institute for health protection, which may well be closer still to government and industry. When accountable national organisations move in the direction of political and commercial interests, public trust is eroded, and the power of unaccountable and self-serving social media platforms only grows.

This article is made freely available for use in accordance with BMJ's website terms and conditions for the duration of the covid-19 pandemic or until otherwise determined by BMJ. You may use, download and print the article for any lawful, non-commercial purpose (including text and data mining) provided that all copyright notices and trade marks are retained.


1.       Why won’t anyone publish the Danish mask study? 17 Nov 2020.

2.       Heneghan C, Jefferson T. Landmark Danish study finds no significant effect for facemask wearers. Spectator2020.

3.       Alex Nowbar’s research reviews—28 November 2020.

4.       Brophy JM. Covid-19: controversial trial may actually show that masks protect the wearer. BMJ Opinion. 24 Nov 2020.

5.       Bundgaard H, Bundgaard JS, Raaschou-Pedersen DET, et al. Effectiveness of adding a mask recommendation to other public health measures to prevent SARS-CoV-2 infection in Danish mask wearers: a randomized controlled trial. Ann Intern Med2020. doi:10.7326/M20-6817. pmid:33205991


7.       Smith A. Facebook: Steve Bannon’s call for Anthony Fauci to be beheaded do not break its rules, Zuckerberg says. Independent. 13 Nov 2020.

8.       Heneghan C. Twitter. 20 Nov 2020.

9.       Morgan M. Matt Morgan: The two pandemics—covid and lies. BMJ2020;371:m4516doi:10.1136/bmj.m4516.

10.      Salisbury H. Helen Salisbury: Careless communication costs lives. BMJ2020;371:m4383doi:10.1136/bmj.m4383.

11.      John A, Pirkis J, Gunnell D, Appleby L, Morrissey J. Trends in suicide during the covid-19 pandemic. BMJ2020;371:m4352. doi:10.1136/bmj.m4352 pmid:33184048

12.      Khunti K, Routen A, Pareek M, Treweek S, Platt L. The language of ethnicity. BMJ2020;371:m4493. doi:10.1136/bmj.m4493 pmid:33229395

13.      Will AJ. covid-19 vaccines be cost effective—and does it matter?BMJ2020;371:m4491.

14.      Mahase E. Covid-19: What do we know about the late stage vaccine candidates?BMJ2020;371:m4576doi:10.1136/bmj.m4576.

15.      Oliver D. David Oliver: Should we bring the curtain down on NHS social media performances?BMJ2020;371:m4497.

16.      Baron Cohen S. Read Sacha Baron Cohen’s scathing attack on Facebook in full: “greatest propaganda machine in history.” Guardian. 22 Nov 2019.

17.      Cadwalladr C. If you’re not terrified about Facebook, you haven’t been paying attention. Guardian. 26 Jul 2020.

18.      Vize R Controversial from creation to disbanding, via e-cigarettes and alcohol: an obituary of Public Health England. BMJ2020;371:m4476. doi:10.1136/bmj.m4476 pmid:33218977

Carl Heneghan & Tom Jefferson published in “The Spectator”

Landmark Danish study finds no significant effect for facemask wearers 19 November 2020, 12:45am


Do face masks work? Earlier this year, the UK government decided that masks could play a significant role in stopping Covid-19 and made masks mandatory in a number of public places. But are these policies backed by the scientific evidence?

Yesterday marked the publication of a long-delayed trial in Denmark which hopes to answer that very question. The ‘Danmask-19 trial’ was conducted in the spring with over 6,000 participants, when the public were not being told to wear masks but other public health measures were in place. Unlike other studies looking at masks, the Danmask study was a randomised controlled trial – making it the highest quality scientific evidence.

Around half of those in the trial received 50 disposable surgical face masks, which they were told to change after eight hours of use. After one month, the trial participants were tested using both PCR, antibody and lateral flow tests and compared with the trial participants who did not wear a mask.

In the end, there was no statistically significant difference between those who wore masks and those who did not when it came to being infected by Covid-19. 1.8 per cent of those wearing masks caught Covid, compared to 2.1 per cent of the control group. As a result, it seems that any effect masks have on preventing the spread of the disease in the community is small.

Some people, of course, did not wear their masks properly. Only 46 per cent of those wearing masks in the trial said they had completely adhered to the rules. But even if you only look at people who wore masks ‘exactly as instructed’, this did not make any difference to the results: 2 per cent of this group were also infected.

When it comes to masks, it appears there is still little good evidence they prevent the spread of airborne diseases. The results of the Danmask-19 trial mirror other reviews into influenza-like illnesses. Nine other trials looking at the efficacy of masks (two looking at healthcare workers and seven at community transmission) have found that masks make little or no difference to whether you get influenza or not.

But overall, there is a troubling lack of robust evidence on face masks and Covid-19. There have only been three community trials during the current pandemic comparing the use of masks with various alternatives – one in Guinea-Bissau, one in India and this latest trial in Denmark. The low number of studies into the effect different interventions have on the spread of Covid-19 – a subject of global importance – suggests there is a total lack of interest from governments in pursuing evidence-based medicine. And this starkly contrasts with the huge sums they have spent on ‘boutique relations’ consultants advising the government.

The only studies which have shown masks to be effective at stopping airborne diseases have been ‘observational’ – which observe the people who ordinarily use masks, rather than attempting to create a randomised control group. These trials include six studies carried out in the Far East during the SARS CoV-1 outbreak of 2003, which showed that masks can work, especially when they are used by healthcare workers and patients alongside hand-washing.

But observational studies are prone to recall bias: in the heat of a pandemic, not very many people will recall if and when they used masks and at what distance they kept from others. The lack of random allocation of masks can also ‘confound’ the results and might not account for seasonal effects. A recent observational study paper had to be withdrawn because the reported fall in infection rates over the summer was reverted when the seasonal effect took hold and rates went back up.

This is why large, randomised trials like this most recent Danish study are so important if we want to understand the impact of measures like face masks. Many people have argued that it is too difficult to wait for randomised trials – but Danmask-19 has shown that these kind of studies are more than feasible.

And now that we have properly rigorous scientific research we can rely on, the evidence shows that wearing masks in the community does not significantly reduce the rates of infection.

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