[cyphar]

> Right. No one has ever studied contagions among the community at large and the effect of masks. When you hear people say there is no evidence, this is what they are referring.

Actually, they have[1,2]. According to one study, surgical masks helped cut down the community spread of SARS in Beijing by 70%[3]:

> Always wearing a mask when going out was associated with a 70% reduction in risk compared with never wearing a mask.

But if you're specifically referring to randomised trials on mask efficacy during an on-going pandemic, you're right that there are no such studies -- because it would be unethical. So effectively all of the relevant studies are retrospective (which does introduce recall bias), but these are basically the best studies we can do ethically.

[1]: https://medium.com/better-humans/whats-the-evidence-on-face-... [2]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6993921/ [3]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3322931/