Thread by Bob Wachter

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There are so many Omicron #'s floating around, it’s overwhelming. In today’s 🧵,I’ll try to simplify things with a couple of rules-of-thumb that I’ve found useful. I’ll be interested in hearing if this helps or confuses more – it might hinge on whether you’re a chess fan.(1/25)
Chess? We’ll get to that in a bit. For several of the big picture Omicron issues, my go-to is not a chess board, but rather the Number 2.
Why 2? Because many of Omicron’s key variables are either just about twice, or approximately half, of what they were with Delta.(2/25)
Two key variables for which “2” is a useful rule-of-thumb:
a) How much more infectious is Omicron than Delta? About twice. (And Delta was about twice as infectious as Alpha was.)
b) How much less serious (measured by hospitalizations) is Omicron than Delta? About half. (3/25)
Note that – across a population – these variables kind of cancel each other out: if there are twice as many cases & each is, on average, half as severe, the # of hospitalizations will be about the same as expected. And that, in fact, is my guess as to what will happen… (4/25)
…nationally: we’ll see similar hospital #s (& ultimately deaths) than had this been a Delta surge. But it won’t be uniform, since the level of protection from immunity will be the deciding factor. Sticking with the Number 2, the avg level of immunity-related protection… (5/25)
…in the U.S. is ~half (50%) (if 100% is if we were 100% vaxxed w/ 100% effective vaccine). We’ll parse these levels of immunity (w/ chess analogy) soon, but for now suffice it to say that 50% is my rough mash-up of the strata of immunity multiplied by approx % of people...(6/25)
...in each category. This is a very rough rule-of-thumb, but I find it useful. To illustrate, if everyone had gotten a 50% effective vax, we'd be at 50% protected. As it happens, in U.S. 62% have had 2 shots (~50% effective), ~33% of them boosted (~75% effective), & so on. (7/25)
I find this rule-of-thumb useful since, while U.S. may overall be ~50% functional immunity, that 50% is made up of regions (like SF) that are ~70%, & others more like 30%. Those places will have very different experiences handling their (inevitable) Omicron surges.(8/25)
Another “2”: vax cuts by 50% the level, & probably length, of infectiousness w/ Covid (at least w/ Delta; needs to confirm w/ Omicron). It’s partly why it made sense to shrink isolation from 10d to 5 in asymptomatic people (tho I’m more confident in vaxxed than unvaxxed). (9/25)
One final “2” before we move to chess (I know you can't wait):
Avg. length of a surge thus far = 2 months, & that seems to be what South Africa is showing us w/ Omicron. So if you are hunkering down with the current surge, it’s likely we’ll be coming out of it by March.(10/25)
OK, on to chess!

To play well, it’s crucial to know the value of the different pieces. Most experts assign these values:
Queen♛: 10 (very high)
Rook♜: 5 (high)
Bishop♝ (or knight): 3 (modest)
Pawn♟️: 1 (low)
(11/25)
www.chess.com/terms/chess-piece-value
A surprising number of Covid (& now Omicron) variables comport with these chess values, making it a useful rule-of-thumb (to me at least). In each case, my convention is that the queen is best (highest protection or lowest risk); things go downhill from there. Here goes: (12/25)
Levels of immunity, pre-Omicron:
Queen: 3 mRNAs (better: 3mRNAs + infection)
Rook: 2 mRNAs, OR 1 J&J plus mRNA, OR infctn plus 1 vax
Bishop: 1mRNA OR 1 J&J OR infctn in last few months
Pawn: 1 vax-only or infctn >6 mths ago
No protection: neither vax nor prior infection.(13/25)
That was with Delta. With Omi, I move each down by a piece in terms of protection vs infection. So the protection from 3 shots vs symptomatic infection is now rook-like, not queen-like.(If you like #’s, it fell from ~95% protected to ~70%). Same for the other categories. (14/25)
In terms of protection vs. SERIOUS infection (hospitalization, ICU, death), I move UP a piece. Ergo, while 3 shots now offers only rook-like protection vs symptomatic infection, it still offers queen-like protection against getting seriously ill. Get your booster!(15/25)
Let’s turn to masks. Since Omicron is 2x as infectious, an encounter that you could have tolerated for Delta may well infect you w/ Omicron. Knowing this, it’s worth upgrading the protection you get from your mask, by at least one chess piece. Here’s the mask hierarchy: (16/25)
Queen: N95, KN95, or equivalent
Rook: Well-fitting cloth on top of surgical mask
Bishop: Surgical mask alone
Pawn: Cloth mask alone
No protection: no mask (this includes mask worn on chin).
(17/25)
The safety of an indoor encounter? Chess helps here too:
Queen: Small group, all 3x vaxxed, all feel fine
Rook: Small group, all vaxxed, some no boosters
Bishop: Larger group, some w/ only 1 shot or prior Covid
Pawn: Larger group, some w/ no prior vax or infection. (18/25)
Can you make a pawn- or bishop-like encounter queen-like safe? (Crucial if some at higher risk; see below.) Yes, adding layers of protection can bump you up a piece. Outdoors, ventilation if indoors, & especially testing can render it far safer. So, of course, can masking.(19/25)
Let’s think now about levels of personal risk – of course whatever it is is massively modified by your level of immunity, mostly via vaccination/boosting. But for two people with similar vaccination status, then:
(20/25)
Queen: Young (say <65, he says at age 64) with no co-morbidities (most protected)
Rook: Young but with medical co-morbidities
Bishop: Elderly (>65) but generally healthy
Pawn: Elderly (>65) plus co-morbidities (eg, diabetes, heart disease, obesity)
(21/25)
And then there's the risk of exposure. I tend to think in terms of cases/day/100K in a region, so:
Queen: <10/d/100K (with Omicron, nobody’s going to be queen-like for long)
Rook: 10-25/d/100K
Bishop: 25-100/d/100K
Pawn: >100/d/100K
(22/25)
All these categories interact w/ one another. They are, in essence, strata you can use as you apply so-called Swiss cheese model of pandemic protection – creating multiple overlapping layers of protection, particularly if there are vulnerabilities. www.nytimes.com/2020/12/05/health/coronavirus-swiss-cheese-infection-mackay.html (23/25)
While I'm sure I've exhausted many of you (or tried your patience), I hope some find these mental model/rules-of-thumb helpful in developing a rough sense of the relative risk and relative benefits of various conditions and situations, and how they’ve changed with Omicron.(24/25)
And, for those who play, I hope I haven’t ruined chess for you forever!

Stay safe.
(25/end)
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