Thread by @Bob_Wachter

I’m not doing indoor dining, and I still wear a mask in crowded indoor spaces. While most in US have chosen to be less careful, in this 🧵 I’ll review the logic & math behind my decisions, hoping that some of you will find them useful in navigating today’s Covid landscape. (1/25)

Here’s my bottom line (in case this 🧵is TL;DR):
I’ll begin eating inside and removing my mask in most indoor spaces when the local reported case rate falls below 5/100,000/day.

You can find your local rate of cases/100K/day here @nytimes: nytimes.com/interactive/20… (2/25)

US cases have ⬇by 30% in past month, but they're still well above 5/100K/day. Today, the US is at 28. In SF it’s 19; in Marietta, GA (where I was yesterday, visiting my son) it’s 25 (Fig). Barring a bad new variant, I’m guessing we’ll be <5/100K/d in the next 1-2 months.(3/25)

Clearly, many will find my threshold too conservative; others too risky. Now that mandates are gone, we all need to make our own choices.
I won’t go into detail about why I’m still trying to avoid Covid– it's mostly Long Covid, as I explained here: washingtonpost.com/outlook/2022/0… (4/25)

How did I settle on this threshold of 5/100K/day, you ask? It's complicated.
Your odds of getting Covid from a given encounter (an airplane trip, for example, or an indoor meal) are determined by a few factors; most of them remain pretty stable over time. (5/25)

First is your immune status: it's ⬆if boosted (4 shots better than 3; etc.); ⬆if boosted recently (relatively little protection vs infection – though great protection vs severe infection – if last boost was >2 months ago); ⬆ if hybrid immunity (vax/boost & prior Covid). (6/25)

2nd factor that determines risk is virus's infectiousness. BA.5 is most infectious variant yet, so an exposure now is riskier than one with a prior variant, all else equal.
But what are the chances of becoming infected from a single exposure? That # is tricky to pin down. (7/25)

I estimate that the chances of getting Covid from a moderate exposure to an infected person (sitting in my row on a plane or a nearby table in a restaurant) is ~5-10%. That's a rough guess – & would vary depending on things like exposure time, ventilation, distancing, etc. (8/25)

Whatever odds you use, the main point is this: your risk of getting Covid is a product of your immune status AND the riskiness of an exposure & virus AND the odds that someone near you has Covid & is infectious. Since I can’t do much to change my immune status, and the … (9/25)

… setting (eg, indoor dining, 3-hour flight, etc) is what it is, the main variable subject to significant variation is the likelihood that a nearby person has Covid.
But it's not just one person – if you go maskless indoors, you’re likely to be exposed to several people.(10/25)

It seems logical to consider people within 10-20 feet (on a plane: passengers within a few rows; restaurant: tablemates plus waiter & folks at nearby tables) as posing the greatest risk. So I think about the odds that, in a group of 10 people, one of them will have Covid.(11/25)

Unfortunately, this number is not look up-able, so we’re forced to do some extrapolation. Here goes:
As you know if you’ve been following me, I am fond of using @UCSFHospitals' asymptomatic test positivity rate (ATPR) as a measure of the odds that someone (in SF)… (12/25)

... who feels fine actually has Covid. During the last two years, that number has been as high as 10% (1-in-10) during surges, and as low as 0.2% (1-in-500) during lulls. Unfortunately, the ATPR is not reported publicly at any hospital or city I know of (including mine). (13/25)

While the ATPR isn’t readily accessible, it correlates fairly well with a # that IS available – namely local cases/100K/day.
Let’s take today as an example to see how these numbers sync up. But before we discuss reported case rates, let’s answer the question that I’m… (14/25)

… sure is on your mind: “What about home testing?”
You’d be right to ask – today's publicly reported case numbers represent a small fraction of all positive tests. The best studies say that we should multiply reported case rates by 5 to get an estimate of actual cases.(15/25)

So SF’s current number of 19/100K/day is, to my mind, the equivalent of ~95/100K/day in the pre-home testing era.
Ergo, when I say my threshold is <5/100K/day, I’m really saying <25/100K/day based on pre-home testing standards. You may also recall that I used to cite… (16/25)

… a threshold of <10 cases/100K/day. I’ve raised it (& thus accepted a bit more risk) to a higher # (20-25/100K/day) because vax, boosting & Paxlovid have markedly lowered the chances of a severe case (hospitalization/death) & mildly lowered the probability of Long Covid.(17/25)

Today’s ATPR @UCSF is 2.9%, meaning that 1 in 35 people who feel fine would test + for Covid. On its face, 1-in-35 doesn’t sound like super-scary odds.
But remember, I’m interested in the probability that ANY of the ~10 people in aerosol distance of me might have Covid.(18/25)

If 2.9% of people test positive for Covid, in a crowd of 10 people, at least one person will be positive 25% of the time. 1-in-4: that’s too high for me to feel comfy ditching the mask.
What if the Covid case rate was 1/4th of today’s 19/100K/day– in other words, a... (19/25)

...reported case rate of 4.8/100K/day (remember, this is really a rate of 23.8/100K/d after we multiply by 5 to account for home tests).
Let’s assume that the ATPR would track with the case rate; ie, also 1/4th of today’s rate, so 0.73% instead of 2.9%. At that rate: (20/25)

In a crowd of 10, there’s now a 7% chance that at least 1 person is positive (see chart below).
Given my age, vax status, & risk tolerance, I’d accept a 1-in-14 chance that someone near me was positive & would resume indoor dining & remove my mask in most indoor settings.(21/25)

As you see, these are arbitrary cut-offs. Even today, a maskless person is unlikely to get Covid from a single encounter. (But the risk adds up with each contact.) And even when the case rate is below my threshold, the risk is never zero, just low enough for me to accept.(22/25)

I was asked this week by my 8 poker guys (all vaxxed/boosted) if I was up for playing inside. My response: yes, if everybody tested. Reasoning: in a crowd of 8 people in a city w/ an ATPR of 2.9%, there’s a 21% chance that one would have Covid, a bit high for my taste… (23/25)

… But if all 8 had a neg. pre-game test, odds would ⬇ by 80-90% (some false negatives, of course), bringing the risk below my threshold for a maskless indoor activity. So testing is a way to make a riskier setting safer than case #'s would imply. vox.com/even-better/23…
(24/25)

With no variant much more infectious than BA.5 on horizon, & a probable⬆in boosting soon w/ new bivalent vax, there's a good chance we’ll see continued ⬇in case rates & ATPR this fall. If so, I’ll gladly join the ranks of the maskless – as long as the rates stay down!(25/end)