Bob Wachter Profile picture
Jun 13, 2021 25 tweets 7 min read Twitter logo Read on Twitter
Covid (@UCSF) Chronicles, Day 453

1/ I know everybody’s sick of playing 3-dimensional Covid chess. Sorry, but the Delta variant forces us back to the chess board. Ergo, this 🧵.
If you’re fully vaxxed, I wouldn’t be too worried, especially if you’re in a highly vaxxed region.
2/ If you’re not vaccinated: I’d be afraid. Maybe even very afraid.
Why? Let’s start with the things that we know about the current situation, then layer in new information about the Delta (aka, Indian) variant, B.1.617.2 Then we’ll end with what I’d suggest you, and we, do.
3/ Current U.S. situation is good. Cases, hospitalizations, & deaths are falling fast, largely due to our fabulous vaccines. In SF – w/ 70% of people aged >12 fully vaxxed – we’re nearly in a post-Covid world. Everybody’s opening up, including (on Tues) CA. That’s fine…for now.
4/ Sadly, ~50% of the U.S. (>age 12) remains unvaccinated, and in certain states (mostly southern & right-leaning), it’s more like 2/3rds. We know that a small # of vaccinated people will get breakthru infections, but they’ll rarely get very sick & will almost never die of Covid.
5/ Unvaccinated people remain at high risk of getting Covid... if they’re exposed to it. In April, I argued @washingtonpost that this is the most dangerous time for unvaccinated people (without prior Covid), since their bodies are no better ……
6/ …at fighting the virus than they were in 2019, & the virus they’re likely to see (back then, it was Alpha [UK] variant) is better at infecting people. But unvaxxed folks had 2 advantages: 1) they tend to be younger, so less likely to get sick, & 2) they were less likely…
7/ …to be exposed to virus since community rates had fallen so much (due to kindness of vaccinated strangers). This remains true, which is why unvaxxed may well do OK for a while, probably through summer.
But fall/winter now scares me a lot, much more than it did 2 weeks ago.
8/ Reason: Delta.
From the start, we said the 3 things to know about variants are: a) are they more infectious? b) are they more serious? (ie, are you likelier to get very sick) & c) are they vaccine (or prior Covid-based immunity) resistant? Alpha was ~40% more infectious.
9/ There’s still debate over whether Alpha is more serious (some studies say yes, others no). But there’s no question that the vaccines (at least the 3 approved in the U.S.) work just fine against it. This meant that – even though it became the dominant virus in Israel, the UK…
10/ … & now the U.S. (here, it’s 69% of current cases…) – relatively high vaccination rates in these countries prevented it from becoming much of a problem (although unvaccinated people ARE at higher risk of catching it and dying).
11/ Why is Delta scarier? First, it appears to be even more infectious than the Alpha variant, probably by a similar amount (~40% more infectious than Alpha.)(h/t @EricTopol for @BBC table below) This means the same exposure that a person might have had last year is now about… Image
12/ …twice as likely to result in Covid. Second, it's looking like it is more serious, though we need more data to be sure. Third – and this is the big one – it does appear to be somewhat immune resistant. Before getting too freaked out about this, it’s worth noting that the…
13/ …data are reassuring, in a way: the efficacy of 2-doses of Pfizer is 88%, only a smidge lower than the 95% we’re used to, and still great.
But the first dose data is concerning. Normally, a few weeks after dose 1, you’re about 80% protected (this was the argument for…
14/ …delaying 2nd doses when jabs were scarce). But for Delta, Pfizer dose 1 is only ~33% protective.… This creates 2 problems: 1) People stay vulnerable until after shot 2 (& many let guard down earlier); 2) Loss in efficacy for dose 1 points to some…
15/ …degree of vaccine/immune escape. And – though this isn’t proven – I’d worry that a fully vaccinated elder, or someone whose immunity stems from an infection >12 mths ago, won’t be sufficiently protected over time, if immunity falls below a threshold needed to thwart Delta.
16/ What to do? For a vaccinated person, watch the Delta % in your region (currently ~6% in the U.S. & rising) & Covid cases in your community.… If you’re seeing more cases & more Delta, I’d restore some precautions (esp. if you're high risk) – at least…
17/ … indoor mask wearing (if you’ve stopped; BTW, I haven’t) in places w/ unvaxxed/unmasked folks.
If you’re unvaxxed, get your shots! You may be reassured by a low a local case rate , but don’t be: it may be due to summer plus the still-low Delta fraction. Both will change.
18/ As you can see, Delta has made me nervous: I’ll now bet we’ll see significant (incl. many hospitalizations/deaths) surges this fall in low-vaccine populations due to combo of seasonality, Delta’s nastiness, & “back to normal” behavior. If you’re unvaxxed, you’ve made a bet…
19/ …that Covid is yesterday's news, & that now seems like an awful bet. Moreover, I worry that a right-leaning Governor who opened up early & proudly (& has gotten away with it so far) will be stubbornly unwilling to reverse course, even in the face of surging cases & deaths.
20/ And there’s the matter of timing. If your strategy was “I’ll consider a shot if I see an uptick in cases,” that’s also a loser, since once a surge begins, you won’t be well protected against Delta for 4-6 weeks after shot #1. By that time, you may be very sick, or dead.
21/ Is the threat real? UK has been a step ahead of U.S. on vaccinations, & cases/deaths have fallen faster in UK than in U.S. But look at recent UK case # s – they’ve doubled in past 2 wks, as Delta became dominant strain (now 91% of UK cases). Reassuringly, deaths remain low… Image
22/ ...since unvaxxed are mostly younger & less likely to get sick. But don’t be surprised if we see upticks in hospitalizations/deaths, esp. if Delta proves to be more serious (it’s looking that way), and if it begins to dodge immunity in those with low or waning protection.
23/ We’ve been lulled by the amazing efficacy of the vaccines & by fact that prior variants haven’t been all that nasty (esp. w/ regard to immune escape). Delta should ring the alarm & spur action. We need FDA to fully authorize our vaccines, to step up research on boosters,…
24/ …& faster approvals for kids. We also need > outreach to unvaxxed (incl. info on Delta) & vaccine mandates in risky settings (healthcare, nursing homes).
For me, Delta adds to my resolve to keep mask on indoors when unvaxxed/unmasked people may be around. And I'm getting…
25/ …psychologically prepped for some restrictions to return in fall (tho highly vaxxed places like SF will likely do fine). Nobody wants that, but the virus doesn’t care what we want.
For unvaxxed, I wish you well but my sympathy is flagging. Your bad choice is looking worse.

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More from @Bob_Wachter

May 23
Covid (@UCSF) Chronicles, Day 1161
With the Pub Health Emergency over, it's a good time to update you on my view of Covid & my current approach to Covid behavior – which is now based on 2 principles:
1) Is the risk worth it?
2) Can I live w/ the behavior for the long haul? (1/25)
First, let’s review today's situation. Since Omicron's emergence in 11/21, the biggest surprise has been the absence of surprises. Yes, we’ve seen a few variants that led to mild surges, but we haven’t had to interrogate our Greek dictionary for a new letter for 18 months. (2/25)
mRNAs still work great in preventing severe infection, and Paxlovid, home tests, & ventilation still work well too. Long Covid remains a concern, but we know that both vax & Pax lower its frequency, that most (though definitely not all) folks… (3/25)…
Read 25 tweets
Feb 17
Important @TheLancet systematic review finds Covid infection confers robust & long-lasting (good at 40 weeks) protection vs both symptomatic & severe Covid infection.(Weaker w/ Omicron, but still good.)…
How will results influence my behavior/thinking?(1/7)
a) I've been considering a Covid infection to be the equivalent of a booster in terms of protection against reinfection & severe disease (hospitalization/death). These study results indicate that it is at least that good, maybe even a bit better... (2/7)
b) We've been headscratching about why XBB variant didn't cause more of a surge, despite low uptake of the bivalent booster. It may be that immunity from all those 2022 Omicron infections kept it at bay (that's informed speculation – Lancet study ended before XBB spike)... (3/7)
Read 7 tweets
Jan 19
Covid (@UCSF) Chronicles, Day 1038
Some folks continue asking what I'm doing viz Covid behavior...
Answer: I'm changing my behavior. In the Bay Area, I'm now OK with indoor dining & removing my mask for small group gatherings.
I haven't changed, the risk has. Here's how: (1/25)
Specifically, I haven't changed my perspective on balancing prudent caution with everyone's (including my) desire for "normal."
But, in the Bay Area at least, the Covid risk has come down considerably, and, by my way of thinking, this allows for a more permissive approach.(2/25)
Where I'm coming from:
1) I'm 65 & haven't had Covid
2) I want to live as fully as I can, but am comfortable taking reasonable steps to avoid infection
3) I'm fully vaxxed & had bivalent in Sept
4) My main fear is Long Covid, which I peg at ~5% probability per Covid case. (3/25)
Read 25 tweets
Dec 27, 2022
I appreciate all (or, at least most) of the feedback on yesterday’s post regarding how I make decisions about Covid risks and mitigations, including my 50% Rule.
A brief follow-up thread today, with a few additional thoughts and responses: (1/12)
a) First, of course we’re entering year 4 (!) of Covid (I mistakenly said year 3 in yesterday's first tweet). Sorry.
I was hoping that by now Twitter would have added an edit function, but the new management seems focused on other changes. Hopefully at some point. (2/12)
b) A few folks asked how masking fits into my risk mitigation algorithm.
Just as I’m comfortable doing anything OUTSIDE without a mask, I’m OK in ALL indoor spaces – incl. crowded theaters, subways, etc. – wearing a KN95. I’m also OK removing my mask briefly to eat/drink. (3/12)
Read 12 tweets
Dec 25, 2022
Covid (@UCSF) Chronicles, Day 1013
As we enter Covid Year 3, it’s clear we’ll be in our current predicament for the foreseeable future. This means we all need to find our own method to weigh & mitigate risks.
Today I’ll describe my “50% Rule” & how it governs my choices. (1/25)
All of us make risk choices daily, without much thought. What is the chance of rain above which I’ll bring an umbrella? Do I buy flood or earthquake insurance? Do I take a statin for my cholesterol? In making these choices, it’s rare there’s an unambiguously “right” answer.(2/25)
Instead, we weigh the odds & badness of the thing we’re trying to avoid; how unpleasant, risky, & expensive the mitigation is; & our own risk tolerance. Since the cognitive burden of doing this for myriad choices daily is onerous, we all develop rules of thumb to guide us. (3/25)
Read 25 tweets
Dec 19, 2022
Covid (@UCSF) Chronicles, Day 1006 (!)
There’s an awful lot of Covid confusion & misinformation out there, particularly around these three topics: masks, vaccines, and home tests. Let’s look at them in today's thread. (1/24)
To start, these statements are correct:
• Wearing the wrong mask, or wearing a mask incorrectly, doesn’t work
• Vaccination/boosters don’t work as well as they used to in preventing infection
• Home tests yield more false negative results than they used to.
But these statements are unambiguously wrong:
• Masks don’t work
• Vaccines/boosts don’t work
• Home tests don’t work
This is confusing enough that it’s an easy mark for those who want to spread misinformation. Here’s how I interpret the science to inform my decisions: (3/24)
Read 24 tweets

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