Danish face mask study did not show that masks were ineffective at reducing spread of COVID-19; study was underpowered and results were inconclusive
Public health authorities like the World Health Organization and the U.S. Centers for Disease Control and Prevention recommend that the general public use face masks primarily to protect other people from an infected person who is wearing a face mask (source control), rather than protecting the wearer from becoming infected (prevention). Several scientific studies have shown that wearing face masks helps reduce the release of infectious droplets and aerosols into the air, thereby reducing the spread of COVID-19 from infected to uninfected people.
Misleading: The Danish study was designed to determine whether recommending that people wear face masks reduces their risk of COVID-19 infection by 50%. The study only evaluated the ability of face masks to protect the wearer. Hence, the study does not address whether wearing face masks are effective at reducing the spread of COVID-19 from an infected to uninfected person.
Fails to grasp significance of observation: The authors of the Danish study stated that their findings were inconclusive and they were not able to determine whether face masks are effective at protecting the wearer from COVID-19 infection.
Lack of context: The study has several important limitations, such as the lack of statistical power and low adherence to mask recommendations by participants.
Several reports on the findings of a face mask study conducted in Denmark, which was published in the scientific journal Annals of Internal Medicine, went viral on Facebook within a day of publication, receiving a total of more than 150,000 interactions on social media platforms.
The study was a randomized controlled trial that aimed to determine whether recommending that people use face masks would reduce the infection risk for the wearer by at least 50%. The researchers included more than 6,000 participants, among whom 3,030 participants received a supply of masks and were recommended to wear a mask, while 2,994 participants did not receive a recommendation to wear a mask.
At the end of one month, both groups were tested for COVID-19. The researchers reported that:
“Infection with SARS-CoV-2 occurred in 42 participants recommended masks (1.8%) and 53 control participants (2.1%) […] the difference observed was not statistically significant”
The lack of a statistically significant difference indicates that the results are likely due to chance alone, and that the intervention, in this case recommending the use of face masks, did not influence the infection rate for the wearers.
Some incorrectly interpreted this study’s findings to mean that masks are of no use at all in controlling the spread of COVID-19. Examples of this inaccurate interpretation can be seen in this article by writer Sharyl Attkisson and social media posts by writer and former New York Times reporter Alex Berenson, television show host Buck Sexton, and activist Tom Fitton.
Firstly, it is important to recognize that wearing face masks can make a difference to the spread of a disease in more than one way. One is by protecting others from the wearer in the event that the wearer is infected (source control), and the other is by protecting the wearer from becoming infected by others (prevention). Public health authorities like the World Health Organization and the U.S. Centers for Disease Control and Prevention recommend the general public uses non-medical face masks primarily as a means of source control, rather than as prevention.
Secondly, as explained above, the aim of the Danish study was to determine whether recommending that people wear face masks would protect the wearer from COVID-19 infection. The findings in no way relate to the effectiveness of masks as source control, which is the primary purpose of community use of face masks during the COVID-19 pandemic. In fact, the study’s authors clarified this point in the study itself:
“Although masks may also have served as source control in SARS-CoV-2–infected participants, the study was not designed to determine the effectiveness of source control.”
Therefore, oversimplified and blanket statements that masks don’t work, made by Attkinson, Berenson, Sexton and others, including U.S. Representatives Anthony Sabatini and Andy Biggs, as well as Ron Paul, are inaccurate and misrepresent the study’s findings. Although the study found no statistically significant difference in infection rate between the group that received the mask recommendation and the group that did not, the study provides no information about the effectiveness of masks as source control. Numerous studies have demonstrated that masks are effective at reducing the spread of viral respiratory illnesses by minimizing the release of infectious droplets into the air, as explained in this review by Health Feedback.
The headlines of some news reports on the study are also misleading, for instance in these articles by the Daily Mail (“Masks DON’T stop the spread of Covid”), The Federalist (“Major Study Finds Masks Don’t Reduce COVID-19 Infection Rates”) and Washington Examiner (“Face masks ‘did not reduce’ coronavirus infections with ‘statistical significance’”). None of these headlines signal to the reader that the study’s infection rates solely reflected the effect of receiving a recommendation to wear a mask on the wearer’s infection risk. The results did not relate in any way to the COVID-19 infection rates of others around the wearer, which is related to the effectiveness of masks as source control.
Scientists took to Twitter to comment on the study. Monica Gandhi, a professor of medicine at the University of California, San Francisco, wrote in a Twitter thread that “the study team (many cardiologists in Denmark) should be commended for performing a randomized controlled trial of a non-pharmaceutical intervention (mask-wearing) in the early days of a pandemic.”
However, she also pointed out several limitations with the study, such as the fact that it is underpowered. This means that the study did not have enough participants for the level of risk reduction (50%) that they wanted to detect. In addition, self-reported adherence to mask recommendation in the study was low (only 46% of participants wore the masks as recommended), meaning that many people who received the recommendation to wear a mask reported not doing so, but were still considered as part of the “mask-wearing” group. Both of these issues mean that the protective effect of face masks may not be detectable in the study.
These same concerns were also echoed by other scientists.
The underpowered Danish randomized mask trial, w/ only 46% of those in the intervention group adherence, and only focused on the mask wearer (no insight about transmission) is published @AnnalsofIM https://t.co/xp1Nx1J2lU
✓ commentary by @DrTomFrieden and editors (why publish?) pic.twitter.com/bnrviqT2pt
— Eric Topol (@EricTopol) November 18, 2020
10/n As the authors note, compliance was pretty poor. Lots of people were told to wear masks, but didn't
Hard to say what this means for an individual wearing a mask 24/7 pic.twitter.com/2vnN12EpVI
— Health Nerd (@GidMK) November 18, 2020
"Based on the lowest adherence reported in the mask group during follow-up, 46% of participants wore the mask as recommended, 47% predominantly as recommended, and 7% not as recommended."
-adherence is likely to matter a lot pic.twitter.com/9HNAYokxdv
— ɪᴀɴ ᴍ. ᴍᴀᴄᴋᴀʏ, ᴘʜᴅ 🦠🤧🧬🥼🦟🧀 (@MackayIM) November 18, 2020
In the end, the study’s authors concluded that:
“[T]he findings were inconclusive and cannot definitively exclude a 46% reduction to a 23% increase in infection of mask wearers in such a setting. It is important to emphasize that this trial did not address the effects of masks as source control or as protection in settings where social distancing and other public health measures are not in effect.”
In summary, the Danish study examined whether recommending that people wear face masks would reduce their risk of infection by at least 50%. While it found no statistically significant difference in infection rates between the group that received a mask recommendation and the group that did not, this result simply tells us that mask recommendations don’t reduce the wearer’s infection risk “by more than 50% in a community with modest infection rates, some degree of social distancing, and uncommon general mask use.”
This result does NOT provide any information on the effectiveness of masks in reducing the spread of COVID-19 by an infected wearer, which is the primary motivation for community mask recommendations made by public health authorities. It also does not rule out the possibility of masks reducing infection rates by a degree that is smaller than 50%.
In this Forbes report, Henning Bundgaard, the first author of the study and a professor of cardiology at Rigshospitalet in Denmark, said, “Even a small degree of protection is worth using the face masks, because you are protecting yourself against a potentially life-threatening disease.”
Epidemiologist Gideon Meyerowitz-Katz explains what the Danish study’s findings tell us (and what they don’t) in this article.
This New York Times article interviews several experts on face mask studies, including the Danish study.
UPDATE (31 May 2021):
An editorial published in the Annals of Internal Medicine in March 2021 explained the journal’s rationale for publishing the study and similarly pointed out that the study didn’t examine whether masks reduce the spread of COVID-19. Instead it examined whether mask recommendations reduce the spread of COVID-19, and concluded that “the results of this trial should motivate widespread mask wearing to protect our communities and thereby ourselves while we await more definitive evidence during this pandemic.”
UPDATE (2 Dec. 2020):
After our review was published, The Blaze corrected their article by providing an accurate summary of the Danish study’s findings in the headline and including an Editor’s Note explaining the limitations of the study (see archive of corrected article).
- 1 – Bundgaard et al. (2020) 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. Annals of Internal Medicine.