" Face masks help contain respiratory droplets that can transmit SARS-CoV-2, the coronavirus that causes COVID-19, from people who do not know they have the virus."
aka they prevent the wearer from spreading it.
"
- A retrospective case-control study from Thailand documented that, among more than 1,000 persons interviewed as part of contact tracing investigations, those who reported having always worn a mask during high-risk exposures experienced a greater than 70% reduced risk of acquiring infection compared with persons who did not wear masks under these circumstances.34"