Face masksFace Masks Cut Disease Spread in the Lab, but Have Less Impact in the Community. We Need to Know Why
In controlled laboratory situations, face masks appear to do a good job of reducing the spread of coronavirus (at least in hamsters) and other respiratory viruses. However, evidence shows mask-wearing policies seem to have had much less impact on the community spread of COVID-19. Why this gap between the effectiveness in the lab and the effectiveness seen in the community? The real world is more complex than a controlled laboratory situation. The right people need to wear the right mask, in the right way, at the right times and places.
In controlled laboratory situations, face masks appear to do a good job of reducing the spread of coronavirus (at least in hamsters) and other respiratory viruses. However, evidence shows mask-wearing policies seem to have had much less impact on the community spread of COVID-19.
Why this gap between the effectiveness in the lab and the effectiveness seen in the community? The real world is more complex than a controlled laboratory situation. The right people need to wear the right mask, in the right way, at the right times and places.
The real-world impact of face masks on the transmission of viruses depends not just on the behavior of the virus but also on the behavior of aerosol droplets in diverse settings, and on the behavior of people themselves.
We carried out a comprehensive review of the evidence about how face masks and other physical interventions affect the spread of respiratory viruses. Based on the current evidence, we believe the community impact is modest and it may be better to focus on mask-wearing in high-risk situations.
The Evidence
Simply comparing infection rates in people who wear masks with those who don’t can be misleading. One problem is people who don’t wear masks are more likely go to crowded spaces, and less likely to socially distance. People who are more concerned often adhere to several protective behaviors — they are likely to avoid crowds and socially distance as well as wearing masks.
That correlation between mask wearing and other protective behaviors might explain why studies comparing mask-wearers with non-mask-wearers (known as “observational studies”) show larger effects than seen in trials. Part of the effect is due to those other behaviors.
The most rigorous, but difficult, way to evaluate the effectiveness of masks is to take a large group of people and ask some to wear masks and others not to, in a so-called controlled trial. We found nine such trials have been carried out for influenza-like illness. Surprisingly, when combined, these trials found only a 1 percent reduction in influenza-like illness among mask-wearers compared with non-mask-wearers, and a 9 percent reduction in laboratory-confirmed influenza. These small reductions are not statistically significant, and are most likely due to chance.
