Lots of confusion about what Provincetown outbreak & CDC guidance mean
Mainly: how to think about spread among the vaccinated
And as importantly, what we know and what we don’t
So a short thread
First, the P-town outbreak unusual
Many thousands of people (some unvaccinated) showed up to celebrate July 4
Leading to packed bars, clubs, and lots of mixing of vaccinated and unvaccinated folks in tight quarters
Ideal conditions for COVID spread
And COVID did spread
And there were a lot of infections, including among vaccinated folks
We’ve seen this before — Singapore airport, Yankees clubhouse
Key issue is: did vaccines fail to
1. Prevent infection?
2. Prevent spread?
3. Prevent severe illness?
1. Did vaccines prevent infections?
Based on CDC’s estimate, Pfizer/Moderna prevent about 75-85% of symptomatic infections from Delta
So if no one had been vaccinated, initial case loads would have been about 5 times higher
Did vaccines prevent spread among the infected?
Don’t know
Study examined Ct values of symptomatic vaccinated and unvaccinated
Ct values were comparable
Driving news headlines
But single set of Ct values only loosely correlated with transmissibility
So we don’t know
Did vaccines prevent severe illness?
Only 4 fully vaccinated folks ended up in hospital
None died
Those are very low rates
Consistent with theme that vaccines prevent severe illness
So yeah, they seem like they did
So what do we not know?
Because Ct values are rough surrogate of transmissibility
We don’t know if actual spread among vaccinated people compare to unvaccinated
And big one: we don’t know if vaccinated asymptomatic folks spread
This is key for mask guidance
Bottom line?
P-town outbreak would’ve been a nightmare if no one was vaccinated
Initial case #s would have been 5X
And past experience says such outbreaks fuel larger regional outbreaks
Instead, this one is fizzling out
I suspect because vaccines are working
End
Originally tweeted by Ashish K. Jha, MD, MPH (@ashishkjha) on July 31, 2021.
Also, this:
Leaked CDC slides on delta variant got you concerned?
I find the slides insightful & largely reassuring
Bottom line
Delta variant is a problem
Vaccines prevent vast majority of infections, transmission
And nearly all hospitalizations, deaths
So let’s talk about what’s in the slides about the delta variant
First, it is really, really contagious
Like more contagious than Ebola, Spanish Flu and probably chicken pox
Really contagious
Second, it appears to cause more serious disease if you get infected
I’ve been worried this is the case but certainly not sure
The data here is suggestive but not definitive
So reasonable to say delta probably more severe
But the vaccines are working.
Let me repeat, the vaccines are working
80-90% effectiveness against symptomatic infection
90-95% effectiveness against severe disease
That’s what the data from other countries say
But what about data from our country??
Glad you asked
Right now, about 35K vaccinated Americans having breakthrough infections weekly
Sound high?
Actually, probably 300K unvaccinated Americans having infections weekly
And given 50% of Americans are vaccinated
That’s a rough vaccine effectiveness of around 88%
Very rough
Oh — and protection against severe infection well above 90%
So yeah, vaccines are working fine
But what about this whole breakthrough infections and spreadability?
Can vaccinated folks with breakthrough spread?
Yes. And this is the bad news
CDC cites several studies (and I’m aware of even more data) showing vaccinated folks with breakthrough infections have similar viral loads to unvaccinated folks with infections
These are viral loads, not actual evidence of similar transmission
But a reasonable assumption
Then CDC has modeling that says universal masking is critically important
But they assume no other behavior change like some folks avoiding large gatherings, etc.
But you know what their model also says would work great?
Near universal vaccinations
So bottom line?
Yeah, delta variant is bad. Like really bad
Our vaccines are good. Like really good
Breakthrough infections happen
Sometimes they may spread to others
But if enough people get the shot
The pandemic does come to an end
Fin
Originally tweeted by Ashish K. Jha, MD, MPH (@ashishkjha) on July 30, 2021.