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The curious case of the Danish mask study

BMJ 2020; 371 doi: (Published 26 November 2020) Cite this as: BMJ 2020;371:m4586

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  1. Kamran Abbasi, executive editor
  1. The BMJ
  1. kabbasi{at}
    Follow Kamran on Twitter @KamranAbbasi

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.

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