Did a Danish study show mask-wearing has a statistically insignificant impact on COVID-19 infection and "basically doesn't do a damn thing"? No, that's not true: Both the study authors and an editorial in the journal where it was published clearly point out the study was designed to test if recommendations to wear a mask outside the home reduced infection-catching, but not to test its effect on infection-spreading.
The mis-statement of the findings was made in a Facebook post (archived here) published November 18, 2020. It opened: "Large scientific study of mask wearing in Denmark finally comes out" and it continued:
Mask wearing basically doesn't do a damn thing. 'Statistically insignificant' Scientists quoted in the article: "mask wearing is essential because we say so! SCIENCE!
This is what the post looked like on Facebook at the time of writing:
(Source: Facebook screenshot taken on Fri Nov 20 18:09:47 2020 UTC)
Dr. Christine Laine, the editor-in-chief of Annals of Internal Medicine, the journal that published the Danish study. Laine, who also holds a Masters in Public Health, said there are two common errors in interpretation of the study.
"Every study has limitations," Laine wrote in a November 20, 2020 email to Lead Stories. "This trial was well designed to answer the question it aimed to answer. It was NOT designed to evaluate the effectiveness of a recommendation that everyone wear masks." She wrote:
People who use this trial as evidence that widespread mask wearing does not reduce community transmission of SARS CoV-2 infection are misinterpreting the findings.
Laine said the study was only designed to examine masks' protective effects and not the effectiveness of masks in preventing infected persons, called "sources", from spreading the virus:
Source control is thought to be the predominant mechanism for reducing SARS-CoV-2 transmission as transmission can occur before symptoms develop and many infections are asymptomatic. The DANMASK trial was not designed to examine source control, but rather only the masks' protective effect.
University of Washington Prof. Barbara Richardson, a PhD biostatistician who has for 25 years served as lead statistician on medical studies, said the Annals of Internal Medicine paper has done more harm than good.
"There are so many limitations to this paper it is hard to know where to start," Richardson wrote in a November 20, 2020 email in response to Lead Stories' questions. "I understand that it was published becuase it is really the only randomized study of mask wearing, but that still doesn't mean it is good science."
Richardson cited the summary prepared by Brandon Guthrie, a PhD in Global Health, who publishes a daily review of new scientific literature about COVID-19.
"...Among participants spending at least 3 hours outside of home per day without occupational mask use and already practicing physical distancing, the intervention to recommend wearing a surgical mask when outside of home did not significantly reduce SARS-CoV-2 infection among mask wearers," Guthrie's Literature Situation Report said. And it added this clarifier:
EDITORIAL NOTE: This trial evaluated only the outcome of infections among people instructed to wear a mask, and not the effect of wearing masks on decreasing transmission to other people.