Thread by @ashishkjha

If you're fully vaccinated, should you wear a mask indoors?

@WHO, LA Health Dept say yes

CDC says no

Who is right?

Well, it turns out, its nuanced

It depends largely on where you live...and your risk tolerance

My latest op-ed in @BostonGlobe

bostonglobe.com/2021/07/07/opi…

Think of getting vaccinated like getting a great hockey goalie

Like Gerry Cheevers, who blocked 90% of shots on goal

That's like Pfizer: prevents about 90% of infections against Delta variant

So is that good? Yeah

But 90% isn't 100%.

So question is: 90% of what risk??

2/5

If you're in N England, where vaccination rates high, infection rates low

You're unlikely to encounter the virus

But if you do, 90% protection against a small risk is a tiny risk

And you can skip the mask if you want

But if you're in SW Missouri....

3/5

If in SW Missouri, where vax low, infection high

You'll encounter the virus a lot

And 90% protection against a large number is still a sizable risk

So you're much more likely to have a breakthrough infection

Ah you say,

but do you care about breakthrough infections?

4/6

You might care about breakthrough symptomatic infections

They can be miserable
You probably won't get Long COVID but we don't know for sure

You may not care about breakthrough because simply:

You are VERY unlikely to get particularly sick and that's what matters most

5/6

And of course, there is another wrinkle:

Indoor masking can't be enforced on unvaccinated people only without vaccine verification system

So not having mask mandates in high outbreak areas means unvaccinated walking around unmasked. Only drives cases higher

Not great

6/7

So what do I do?

In highly vaxxed N England, I wear mask indoors intermittently

Based on if I'm in a very crowded place or not

If I were in SW Missouri and many other places with low vax, high cases, I'd wear a mask indoor reliably

Your preferences may differ

End