The first-ever randomized controlled trial (4,862 participants) for mask-use effectiveness in the COVID-19 era reveals that people who wear face masks properly in public are infected only 0.1 of a percentage point less often (2.0% vs. 2.1%) than those who do not wear masks in public.
In 2018 the United States’ Center for Disease Control (CDC) cited a study that said wearing a cloth mask is only effective 1% of the time in preventing viral transmission. This is because facemasks do not tightly seal to the face and thus they “cannot prevent particles in the air from bypassing the filter.”
Image Source: CDC
In the spring of 2020, just as COVID-19 was surging throughout the world, the CDC reviewed 10 randomized controlled trials of mask effectiveness in preventing influenza-like viral infections. In each instance they found “no significant reduction in influenza transmission with the use of face masks” and the “overall reduction in ILI or laboratory-confirmed influenza cases was not significant” and “none of the household studies reported a significant reduction in…influenza virus infections in the face mask group.”
Image Source: CDC
On the 18th of November, 2020, the first comprehensive randomized controlled trial study (Bundgaard et al., 2020) assessing mask effectiveness during the COVID-19 pandemic was made available online.
The study was conducted in Denmark, a country where health authorities did not recommended universal mask use during the study period (April and May, 2020). There were 4,862 participants who completed the study in either the control group (no masks) or mask-wearing group. Only about half (46%) of the mask-wearing group admitted they wore their masks in the way they were instructed (i.e., always fully covering the nose and mouth).
When comparing the proper mask-wearing group to the no-masks control group, the authors found no statistically significant difference in the rates of COVID-19 infection: 2.0% of those who wore masks properly in public were infected with the virus compared to a 2.1% infection rate for those who did not wear masks in public.
So out of 1,000 participants, 1 more person (20 vs. 21) wearing a mask (properly) avoided infection when directly compared to 1,000 participants who didn’t wear a mask at all.