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Daily life in Marseille with the obligation to wear a mask. – LOUAI-BARAKAT / SIPA
- Danmask-19, a Danish study published in the journal Annals of internal medicine Wednesday 18 November is the subject of numerous viral publications on social networks.
- Researchers tried to figure out whether wearing the surgical mask outside the home reduces the risk of Covid-19 infection. According to their findings, 1.8% of mask wearers contracted the coronavirus, compared to 2.1% of those who did not. Proof of its uselessness for many Internet users. For the authors of this study, these very partial results are “inconclusive”.
- 20 minutes takes stock with Professor Didier Lepelletier, co-chair of the permanent working group on Covid-19 at the High Council for Public Health. According to him, the study “has too many biases to provide meaningful results”
“Stop obscurantism, long live science! “In a post on Facebook, Florian Philippot, leader of the Patriots, praised the merits of a Danish study that would have shown” the total uselessness of the mask on the outside against Covid-19 “.” All the truths are coming to light ” , exclaimed the former right-hand of Marine Le Pen about this study, which would question the French doctrine on wearing the coronavirus mask.
Similar posts have spread like wildfire on social media, based on the publication in the magazine Annals of internal medicineOn Wednesday, the results of a research project called “Danmask-19”. This Internet user, for example, tweeted about the study: “It covers 6,000 people, 3,000 with a mask, 3,000 without. No difference on Covid but an increase in infections in the group…. masks! “
However, the study authors admit that their findings are not “statistically significant”. 20 Minutes make the point.
FALSE
Danmask-19 is a study conducted by 21 Danish researchers between April and June 2020. It aims to “estimate whether wearing a surgical mask outside the home reduces the risk of Sars-Cov-2 (Covid-19) infection in a context of wearing a mask is unusual and not recommended by public health measures. ”In fact, wearing a mask was not recommended in Denmark during the period covered by the study. Health authorities, on the other hand, supported social distancing and quarantine of suspected cases, and bars and restaurants closed until May 18.
For the Danmask study, 6,000 people were selected, then tested for Covid-19. Negative people were divided into two groups, one wearing the mask and the other not wearing it. All participants were then asked to spend at least three hours a day away from home for a month, before undergoing a new battery of tests (serological and PCR). A first group carried out the experiment from mid-April to mid-May, the other for the entire month of May.
In the end, the study concludes that 1.8% of mask wearers contracted Covid-19, versus 2.1% of those who did not. First point: Contrary to what some Internet users claim, there is no longer infection in the group of masked people.
In light of these results, one might think, like Florian Philippot, that wearing a mask outside the home is useless. However, one of the authors of the study, Kasper Iversen, of the University of Copenhagen, himself said that the current recommendations on wearing a mask “are not seriously challenged by the study”.
A study with too many prejudices
To better understand Danmask-19, 20 minutes appealed to Didier Lepelletier, co-chair of the permanent working group on Covid-19 of the High Council for Public Health and professor of hospital hygiene at the university hospital of Nantes. He is also a “reviewer” for more than thirty scientific journals. “This is an interesting clinical study, published in a recognized journal, but one that has too many biases to provide meaningful results,” says the professor.
First, and as its authors indicate, the study is not “randomized”, either single or double-blind. This means that each of the parties involved in the clinical study knows the result of the draw carried out to separate the study groups.
Single blind, is the patient who does not know what he has been assigned (a drug or a placebo, in the context of a pharmaceutical study, for example). Double blind, this is also the case for researchers. The advantage of this type of protocol is to reduce the influence that knowledge of the information could have on the person being analyzed, as well as on the researcher. “Here, for example, it would have been necessary to distribute both effective masks and less effective masks,” notes the professor.
A particular context
Hence, it is the general conditions of the study that make the results difficult for Professor Didier Lepelletier to interpret. The study group tested from mid-April to mid-May, for example, conducted the experiment in full containment and in a country with an extremely low transmission rate, which greatly limits the risk of contamination. “Travel outside the home was also uncontrolled, and there is nothing to indicate that people infected with Covid-19 could not be once home, without masks,” he adds. he.
Participants were also not trained to wear masks. If they have yet to receive a user guide, the authors indicate that 46% of participants “followed the recommendations” to wear a mask, that 47% “generally respected” them, and that 7% did not. done. did not follow them.
Too wide a statistical range
Finally, the most important bias is undoubtedly statistical. In the results highlighted on social media, the detractors of the mask point to the 23% increase in the risk of contagion for the group of masked people. But the sentence published in the magazine Annals of internal medicine is considered as a whole: “Although the results are not statistically significant, the 95% confidence interval (CI) is consistent with a 46% reduction to a 23% increase in the risk of infection,” indicate the co-authors.
“That’s too wide a range!”, Explains Didier Lepelletier. I’ll spare you the biostatistics course, but neither the confidence interval (CI) nor the odds ratio (OC) allow us to arrive at conclusive results. To say that the mask at best reduces the risk of infection by 46% and that in the worst case scenario it increases it by 23% is not enough to draw conclusions from this study, and the authors readily admit this. “
“No data on the effectiveness of wearing a general mask”
The authors of the study warn about quick interpretations that could be made of their work: “These results do not provide data on the effectiveness of generalized use of the mask. [une communauté] to reduce Sars-Cov-2 infections […]. The findings also suggest that people should not abandon other security measures against Covid-19, regardless of wearing masks. “
Opinion shared by Didier Lepelletier who, in accordance with the report of the High Council for Public Health, indicates “that in the general population, wearing a mask, even by asymptomatic people, greatly reduces the transmission of Sars-Cov-2, in combination with other barrier measures. “
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