I am optimistic that we will not see another big wave of COVID hospitalization & death in the USA.

Why?

Don't call it "herd immunity." But the vast majority of adults in the US have either been vaccinated or infected by now.

Thread on the US pandemic present & future...
First: What fraction of US adults have been vax'd or infected?

Estimated 1/3 infected by Dec 2020: go.nature.com/3vawFHb

20% of blood donors infected (+ 63% vax'd) by May 2021: bit.ly/3mPkr37.

So pre-delta, likely at least 1/3 of unvax'd adults had been infected.
But 10m confirmed US cases (25% of total) have occurred since May - and most were in unvax'd adults.

Need to adjust for multiple infections, low testing in early 2020, etc.

But almost certain that >5% of US adults (=13m people) were infected in delta surge. Mostly unvax'd.
So, among US adults, estimate:
66% fully vax'd
>10% more (1/3 of remainder) infected by Dec 2020
5% more infected by May 2021
>5% more infected during delta
5% more (1/3 of remainder) w 1 dose

Again, caveat of multiple infections, but likely ~90% of adults have some immunity.
Waning immunity could lead to more cases.

But for serious illness, vax effectiveness remains very high, even rising - see below from CDC (bit.ly/3oVG30s).

Similar effect likely from natural immunity - not 100% protective vs infection, but very good vs severe illness.
New variants can always emerge.

But despite 4 months of intense transmission, no new variants have been able to compete with delta - in the US or worldwide.

Virtually every COVID case is still delta.

And as transmission declines globally, fewer mutation events are happening.
A new wave can start anywhere in the world.

(And the failure to share vaccines globally is a curse on our house as humanity.)

But other than in Eastern Europe, COVID deaths are largely falling worldwide. Even in places where our global vax response has been shameful.
Also encouraging: case rates are falling fastest in states (AL, FL, TN, MS) that had the worst delta surge.

While not rising much in states where vax levels are high.

This is what we would expect if immunity were playing an important role & vax effectiveness were holding.
Caveats:
1. Nobody can predict the pandemic future.
2. Kids are still at risk (though don't get as ill: bit.ly/3iYsMAs).
3. Trends reflect current behavior. If we relax too quickly, cases & deaths can rise again.
4. Outbreaks will occur among unvax'd. Keep vaccinating!
In summary:

- More US adults were likely infected w delta than are still non-immune.
- Likely <10% of adults have no immunity.
- Immunity against serious illness is holding.
- No variants have replaced delta.

Meaning: any future waves will likely not be as bad as the last one.

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More from @davidwdowdy

28 Sep
Six months ago, 15% of Americans had been fully vax'd.

How is vaccine effectiveness (VE) holding up? I looked up VE estimates over time, state by state.

Long story short: VE has been remarkably stable over the last 2 months.

If immunity is waning, it's not waning fast.
California: VE 84% on July 31, 87% now.
bit.ly/39IiOy0
Colorado: VE 62% on Aug 1, 69% now.
covid19.colorado.gov/vaccine-breakt…
Read 21 tweets
26 Sep
A 🧵 for non-scientists on vax & boosters, using data from @KCPubHealth.

Seattle shows us "case relative risk", comparing unvax'd vs vax'd, over time (bit.ly/3ESvUqV).

Think of this as "level of vax protection".

Since late July, it's been stable, even going up. Image
Think of this number as "how many times more protected you are vs COVID if vax'd vs unvax'd."

There are 2 factors likely to affect this.
1. How long since your last shot (waning)
2. Level of exposure in the community (when exposure is intense, vax may not prevent all infections) Image
From mid-April to mid-May, "protection level" went up because:

1. People were getting their 2nd shots (upper right, dark bars = 2nd doses given).
2. Exposure levels were going down.

By May 16, vax'd folks were 20x more protected from getting COVID than unvax'd, on average. Image
Read 8 tweets
24 Sep
I applaud the ACIP for taking a nuanced stance.

But am hearing lots of concern from vax'd under-65s who are worried their immunity will be gone 6 months after their 2nd shot.

To address this, let's take a tour of 2 populations of about the same size: Israel and Virginia.
First, Israel. It's essential to debunk this myth:
"Israel vax'd its population first.
It now has high COVID rates.
Vax immunity must be waning."

This storyline has a lot of problems.

First, 50% vax coverage was only achieved 6 months ago (Mar 17), 4 mos before the surge.
So when the booster was made available in Israel on July 30, only 20% of its population had been fully vax'd for 6 months.

But at the time of the Israeli surge (July-Sept), most people had not been vax'd for >6 mos.

This surge was not caused by waning immunity.
Read 9 tweets
18 Sep
The big COVID news, dropped on a Fri afternoon:
"effectiveness vs COVID-19 hospitalization was higher for Moderna than Pfizer (and Janssen/J&J)."

bit.ly/3kjuoFR

But let's take a look at the analysis before accepting these results at face value...
1. Here are the data - on the surface, looks like Moderna effectiveness is constant at 92-93% after 120 days, Pfizer drops from 91% to 77%.

(btw, this was CDC not Pfizer funded, but what a great strategy for Pfizer - push "waning immunity" so people need a 3rd dose of your vax)
2. But note that the comparison group for all of these is the same - difference b/w numerator & denominator is always 1463 cases & 899 controls.

Meaning that people w Moderna & Pfizer (& J&J) are being compared to the same unvax'd folks.
Read 7 tweets
16 Sep
A visual representation of what we know and don't know about waning immunity.

We know, at 6-8 months:
- Partial vax not great.
- Full vax ~70-85% effective (better vs severe disease than infection).
- Booster gives short-term bump.

We don't know what happens next.
4 scenarios:
Scenario A: Booster gives long-term benefit (by increasing immune memory), and immunity to 2-dose series continues to wane.

In this scenario, boosters are the right thing to do, now.
Scenario B: Booster doesn't improve immune memory, so the added effect of booster is short-lived.

This is the worst-case, and in my mind least likely - because vax has been effective, w/o booster, for 8 months now.

Here, booster helps a bit, but we need revised vax.
Read 6 tweets
12 Sep
Dear FDA Advisory Ctte:

As you consider boosters for older Americans, ask yourselves *why*.

Is it to:
1. Slow transmission,
2. Boost long-term (waning?) immunity, or
3. Give short-term protection?

1. Will be a small effect
2. Has no data
3. May be the wrong time.

A thread...
1a. For transmission, look at the data from Israel. After giving booster shots, rates in those 60+ went down sharply.

But what was the effect on other groups? None.

ourworldindata.org/vaccination-is…
1b. This makes sense, because:
(a) in the US, people 65+ have the lowest case rates
(b) most of those cases are in not-fully-vax'd
(c) fully vax'd recover faster
(d) people 65+ have fewer contacts

Bottom line: to halt transmission, fully vax'd seniors would be lowest priority.
Read 13 tweets

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