The Danish Mask Study (DANMASK-19, 2020)

The Danish Mask Study (DANMASK-19, 2020) was the first large randomized controlled trial to test mask-wearing against COVID-19, concluding no statistically significant protection for the wearer. However, the 95% confidence interval could not rule out up to 46% protection, leaving key uncertainty unresolved. Despite its rigorous methodology, multiple journals refused to publish the study, fueling accusations of censorship and deeply polarizing the mask debate. Compared to the average observational study that claimed 65% protection from masks, this trial's null result was an outlier, but its findings were 30% more precise than typical mask studies of the era, making the controversy as much about publication bias as it was about data.