Not in COVID, but it's been demonstrated in other viruses. You can't do a challenge study with COVID, so the quality of evidence in COVID is going to be less.

Not in COVID, but it's been demonstrated in other viruses. You can't do a challenge study with COVID, so the quality of evidence in COVID is going to be less. https://www.latimes.com/california/story/2020-07-21/masks-help-avoid-major-illness-coronavirus https://www.latimes.com/california/story/2020-07-21/masks-he... Variolation is much safer than being exposed to smallpox in the normal way: https://en.m.wikipedia.org/wiki/Variolation https://en.m.wikipedia.org/wiki/Variolation