COVID Prevention, Masks, and the Danish Study

Nov 24, 2020


There has been much discussion on social media about a recent mask study performed by a team of cardiologists in Denmark ( 

Details about the study: 

  • 6,000 individuals were included in the study; half were instructed to wear a mask and half were not.
  • Participants in the mask-wearing group were provided with 50 surgical masks. These types of masks are considered by most medical authorities to be more effective than many of the cloth masks approved by the CDC for use by the general population. Participants in the study were not surveyed to see if masks were being worn properly.
  • 53% of participants instructed to not wear masks were subsequently found to have coronavirus antibodies. 42% of participants instructed to wear masks tested positive for coronavirus. The authors concluded this difference did not reach the level of statistical significance.  
  • Physical distancing was encouraged but was not tracked during the study.
  • The authors concluded: The recommendation to wear surgical masks to supplement other public health measures did not reduce the SARS-CoV-2 infection rate among wearers by more than 50% in a community with modest infection rates, some degree of social distancing, and uncommon general mask use. The data were compatible with lesser degrees of self-protection.

Limitations of the study:

  • As is typical in medical publications, authors state what the limitations are of their study.
  • In the Danish study, the authors mentioned the limitations, including:
    • Inconclusive results
    • Missing data
    • Variable adherence
    • Patient-reported findings based on home tests
    • This was not a “blind” study (i.e., researchers and participants knew which of the two groups each participant was in).
    • No assessment of whether masks could decrease disease transmission from mask wearers to others.

My comments about this study:

  • This study suggested there was not a statistically significant difference in the level of protection for participants who wore masks versus those who did not.
  • This study did not consider whether wearing a mask prevents passing the virus to others.
  • This study was conducted in a population that did not have high levels of COVID-19 at the time of the study, so it does not provide information about communities with higher rates of COVD-19 prevalence.
  • This study did not make sure people were well fitted for their masks.

My conclusions:

  • The results of this study should not be over or under represented. 
  • As with many studies, the limitations presented by the authors must be taken into account.
  • Without knowing if masks were worn right, I think it is really hard to draw conclusions.
  • There are other studies that have been performed that show masks provide protection to individuals as well as the community.
  • No single study should be used to draw conclusions. I am interested in seeing if other studies provide similar results.
  • In totality, the Danish study does not provide a sufficient level of evidence to conclude that masks do not make a difference.

The most important point of all is not addressed in this study…

The only way to be certain to not contract SARS-CoV-2, is to not be exposed to people who have this virus. Remember, approximately 50% of people who are not symptomatic can still spread the virus.

As our families move into the holiday season, which in more typical times includes gathering with friends and family, we should be mindful of the following factors, which increase the likelihood of transmission:

  • Being in crowded indoor spaces
  • Being in spaces (indoors or outdoors) that have limited fresh air and poor filtration and circulation
  • Spending increased lengths of time in close proximity with others who unknowingly (or knowingly) have the virus.

To review our COVID-19 prevention and treatment protocols, please click here:

I hope that everyone has a wonderful, meaningful Thanksgiving. I encourage everyone to take time to reflect on how much we have to be thankful for, even in such crazy times.

Take care,

Dr. David