The CDC is asking vaccinated people to continue to mask and social distance:
Q: Do I need to wear a mask and avoid close contact with others if I have gotten 2 doses of the vaccine?
A: Yes. Not enough information is currently available to say if or when CDC will stop recommending that people wear masks and avoid close contact with others to help prevent the spread of the virus that causes COVID-19. ... We also don't yet know whether getting a COVID-19 vaccine will prevent you from spreading the virus that causes COVID-19 to other people, even if you don't get sick yourself.
This does not make sense. They're saying we should act as if vaccinated people are no less likely to infect others, but if that were the case then vaccines would not be helping us get to herd immunity.
In the absence of specific studies showing whether these particular covid-19 vaccines reduce infectiousness, we need to make the best predictions we can based on the information we do have. A good starting point is the measured efficacy of the vaccine, which for the Pfizer and Moderna vaccines, the only ones I'll be talking about in this post, is ~95% (1/20th) two weeks after the second shot. [EDIT: this guess should likely be adjusted down, given that we already know asymptomatic spread is common.] This predicts that an interaction between two vaccinated people is ~0.25% (~1/400th) as likely to transmit covid as if they were both unvaccinated.
At this point, to figure out what activities makes sense, it's useful to bring in some numbers. We can use the idea of a "microcovid" to represent a one in a million chance of getting coronavirus:
An activity that's 20,000 microcovids is very unsafe, as you have a 2% risk of getting covid every time you do it. An activity that's 20 microcovids is relatively safe, as you could do it every week for a year and still have only about a 0.1% chance of getting covid.
Until recently, our house's typical weekly covid risk, given current covid rates in the Boston area, looked like:
- One housemate working in person three [EDIT: two] days a week in a well-fit kn95: ~600 microcovids
- Each of seven housemates spending about half an hour a day outside with other people around: ~100 microcovids
Comparing microcovid for individuals and groups is awkward, since you need to take into account that if one of us gets it the others might as well. A risk of 700 microcovids total, for a seven person house like ours is ~1.5% yearly individual risk.
The housemate who works in person got their first shot recently, and two weeks after their second shot I estimate we'll be at:
- One vaccinated housemate working in person three days a week in a well-fit kn95: ~30 microcovids (1/20th of 600)
- Each of six unvaccinated housemates spending about half an hour a day outside with other people around: ~86 microcovids.
This is much better, at a ~0.24% yearly risk.
Then imagine we agree to invite a random friend to dinner, inside. (Not something we've done since before the pandemic.) And say they're generally careful, at a 1% individual yearly risk. The dinner risks our household ~200 microcovids; too risky. If instead our guest were vaccinated, that would be ~10 microcovids; not bad.
We could do this every week and our yearly risk would rise from ~0.23% to ~0.26%. As the rest of us start getting vaccinated, the risk continues to fall.
If you are otherwise careful, limited socializing as/with a vaccinated person is likely a small enough risk that it's a very reasonable thing to spend your risk budget on.
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