Thread by @apoorva_nyc

You may have already moved on from yesterday's controversy over masks. But given the surge pretty much everywhere, it's important for us all to understand what we know about masks' usefulness, and what we don't.

Here we go: 1/7

nytimes.com/article/corona…

First of all, among public health experts, there is near-unanimous endorsement of universal mask mandates to shield people from the virus and slow the pandemic. That's not in question. 2/7

But let's be precise about what we mean by masks, because they're not all equal. N95s are best, surgical masks are great, but the avg person doesn't need either. In fact, in some studies, well-made cloth masks did as well as surgical. Plus cloth masks are green/recyclable 3/7

No question masks protect people around an infected person--this is why surgeons have traditionally worn them to prevent their germs getting into patients. But for Covid, mask mandates have significantly slowed the virus in U.S. states and in health care settings. 4/7

The controversy, if you will, is over benefit to the wearer. This is hard to study, so scientists sometimes use mannequins. And yeah, the Danish study was a randomized control trial, but that kind of trial is not at all suited to behavioral non-pharmaceutical approaches 5/7

(Also that Danish study was hella flawed, as tons of people have pointed out). Bottomline: we don't know exactly how beneficial masks are to the wearer, but when everyone wears masks, the cumulative effect of masked wearer + masked people around = very effective 6/7

Please read the article for more details on what we know (and what we don't know) and how we know it and also *when* we knew it. 7/7

feat. John Brooks of @CDCgov (!), @Smogdr @linseymarr @DrLeanaWen and Julian Tang

nytimes.com/article/corona…