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1. So much of the mistrust swirling around mask recommendations from WHO and other authorities seems to have arisen out of confusion between receiver protection (your mask protects you from others) and source control (your mask protects others from you).
2. A properly fitted N95 provides effective receiver protection, but fitting is very difficult and not easily achieved by members of the public without training and equipment. This thread provides further detail:
3. A basic surgical-type face covering provides effective source control. A study out this morning in @NatureMedicine of non-COVID coronaviruses provides good evidence that you are less likely to shed virus in aerosol/respiratory droplets when wearing a mask.
4. Given this and related studies it strikes me that there is a strong case for advocating use of surgical-type masks, even home-made masks, scarfs, other face coverings in public. None of this will necessarily protect you much—but it will protect others from you if you are sick.
5. My feeling is that given what we know now, we should recommend that people wear masks to reduce transmission when out in public. Of course this is not a substitute for staying home as much as possible, nor for keeping a 2m distance from others when out. But it could help.
6. One thing I've observed in the US is that people feel there is stigma about wearing a mask because it indicates you are sick.

This is something we should be able to rapid reverse with adequate public health messaging, especially at this point in time.
7. People are of course primed to attend to such messaging right now, so the uptake should be rapid.

We simply need to stress that we wear masks in public because we are each doing our part to #crushthecurve, not because we are sick. (If we were sick, we'd be isolating at home.)
8. Messaging should stress the pro-social (as opposed to self-protective nature) of mask wearing, given the risk of pre-symptomatic transmission. I'd imagine we would reach a tipping point where the stigma rapidly reverses from being against those wearing to those not wearing.
9. @BerkOzler12 has provided an important, thoughtful caveat to my thread. The thread here is well worth a close read.

10. Lots of replies suggesting that an improperly fitted
N95 may be good enough for a grocery store run. This seems entirely plausible to me.

If it gives you even a 50% reduction in risk that could help a lot to keep you safe, and reduce R0—even if far from perfect.

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