How Effective Are Cloth Face Masks?

“Making Sense of the Available Evidence:

Taken as a whole, the available mechanistic and clinical evidence leaves substantial uncertainty as to whether, to what extent, and under what circumstances community-wide use of cloth face masks helps to reduce infection rates of SARS-CoV‑2. The voluminous mechanistic evidence clearly demonstrates that masks reduce some measures of droplet transmission, such as the distance that larger droplets travel, and it is known that such droplets can contain SARS-CoV‑2. However, such surrogates of efficacy have not been demonstrated to correlate with infection outcomes and therefore fail to show that masks reduce the true measure of interest.

Scholars who have meta-analyzed the primary data have mostly concluded that evidence of mask benefit is weak and that benefit is modest at best. Uncontrolled observational studies suggesting larger benefits are hopelessly confounded. The best available evidence — the RCT — has largely failed to demonstrate mask effectiveness, particularly of cloth masks, despite trial sizes with thousands or even hundreds of thousands of participants.

The Path Forward:

… More than a century after the 1918 influenza pandemic, examination of the efficacy of cloth masks has produced a large volume of mostly low-quality evidence that has generally failed to demonstrate their value in most settings. When repeated attempts are undertaken to demonstrate an expected or desired outcome, there is a risk of declaring the effort resolved once results consistent with preconceived notions are generated, regardless of the number or extent of previous failures. Scientists and public health officials should exercise caution to ensure that this potential bias does not lead to a cessation of research as the first studies demonstrating mask efficacy are reported.”