Unpacking The Mask Debate - 2020-05-21
The gist is that there's little to no scientific evidence that masks are effective for the population at large and that what protection there might be is minimal at best. Additionally, they argue that mask-wearing may create a false sense of security that leads people to relax more effect mitigation strategies like distancing and hand washing. So the net effect of mask-wearing may actually be more infections rather than fewer.
If you read the report closely however a few points emerge.
First, it's not evidence that masks are not effective – few studies really show this or demonstrate it in any clear way – but a lack of evidence for their efficacy. Second, they focus heavily on health care workers, both for available studies about what works and doesn't and for the standards we should apply for efficacy. Finally, they take a very binary approach to efficacy. They work or they don't.
These are all important points. But it is important to understand what they mean in context. One of the reasons there is no evidence that public masking works is that it's really hard to test. Without a full-blown epidemic it is hard to test the proposition in a robust and meaningful way. So the lack of evidence in this sense isn't the end of the story.