Your Personal Covid Risk

September 14th, 2021

I’ve spent like eighteen months trying to figure out how to think about and understand this question, in a way that lets me make what seem like sensible, everyday decisions. I think I’ve gotten there, or close. I’m sharing here in case it’s helpful to my gentle readers.

I’m fully vaxxed. Here’s a typical, day-to-day question: If I go out to dinner in Seattle with some random friends, indoors, unmasked, how much of a risk is that? This Dave Leonhardt article finally gave me the numbers I needed to figure that.

His (literal) headline takeway: In the U.S., if you’re vaxxed your daily odds of getting infected are about 1 in 5,000. In low-infection, hi-vax areas like Seattle, more like 1 in 10,000. (Per Leonhardt, only three places in the U.S. even collect that data for vaxed vs unvaxed: Utah, Virginia, and King County, WA. Yay us.)

But what in the hell do I do with that number? What does it mean? He tries to help: “It would take more than three months for the combined risk to reach just 1 percent.” That three-month multiplication is well-intentioned, but it’s an odd, arbitrary choice of period.

I realized long ago: when you ask “what are my odds/chances of getting infected?” (and then etc. from that), you have to ask, your odds over what period? Otherwise it’s meaningless.

So now jumping to the best thing I’ve seen, a personal Covid risk calculator that some SF folks built.

It starts with an arbitrarily-chosen personal annual risk “budget”: “I’m willing to accept a 1% annual risk of getting infected.” (This choice is baked into the site, right down to its name: “Microcovid.”) Divide that risk budget by 12 for monthly budget (0.08% risk), 52 for weekly, whatever. They use weekly, which I also find useful.

Now compare: a 1 in 10,000 daily risk, 0.01% (which sounds super low, right?) is 3.65% annual risk. (Just multiply by 365.) So I’m like, “1% annual is kind of a ridiculously low risk budget, given the low ensuing risk of hospitalization much less death.” Especially if you’re vaccinated. Those worst outcomes are very unlikely.

So I’m like, what’s a benchmark annual risk I could compare it to? Try this: An average person’s daily risk of a home accident/injury with a doctor/ER visit is 1 in 5,000. That includes kids and elderly, who are more accident-prone.

That’s 7% annual risk. Once every 14 years. Six times in an 85-year life. Seems a decent ballpark estimate to my anecdotal experience/observations, if you include childhood/old-age injuries. Maybe a bit high. Whatever.

So, say I change my annual covid-infection risk budget to 7%. Then the calculator sez: if I eat out with four friends, indoors, nobody’s masked, restaurant has a HEPA filter running, that only consumes 5% of my weekly risk budget. You can tweak those numbers as you wish; I might do so as well. But it’s not a bad starting ballpark for me.

This doesn’t touch on risk you pose to others, community risk, risk of exponential spread in the population. Or, say, the risk to your restaurant servers (notably including my daughter). Those are things I also definitely consider. But this is a baseline of what you’d need to start with, to consider those subjects.

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