8. But that does not mean we've reached the herd immunity threshold for a world in which everyone returns to how things were in 2019. In other words, as you open up, the herd immunity threshold shifts higher.
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9. The other thing to recognize is that implementing non-pharmaceutical interventions (masking, social distancing, gathering restrictions) around the herd immunity threshold is a *very* efficient way to reduce the total size of the epidemic, by reducing the overshoot.
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10. We can see the basic principle in a simple SEIR model. Implementing aggressive controls for even a short period around the herd immunity threshold reduces the overshoot and prevents many cases that would have occurred without controls.pic.twitter.com/EyL0jeNgFT
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11. For this reason, it's a big mistake to open up right when you're reaching the herd immunity threshold but before the number of current cases—the momentum of the pandemic, so to speak–is low. This will generate a larger overshoot and lead to many preventable infections.
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12. In the above I have used simple SEIR models to illustrate the basic concept of herd immunity during an ongoing pandemic, and the notion of overshoot. In practice, we're dealing with a complex situation where these lessons hold but the dynamics are more complicated.
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13. In particular, we have a) ongoing vaccination as well as natural immunity b) ongoing behavioral changes c) highest-risk groups already vaccinated d) increasing cases due to variants of concern that spread at higher rates than the previous strains. These all interact.
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14. So I'm not aiming to predict exactly what the next couple of months will look like. My main point: Reaching herd immunity is necessary but not sufficient to relax control measures. To do that, you also want to have a low number of cases to avoid overshoot.
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Carl T. Bergstrom Retweeted Carl T. Bergstrom
15. Finally, I'm already being accused of moving the goalposts. For the record, the goalposts were always here. Below, a thread I wrote nearly a year ago explaining these same points and stressing the need for controls around the herd immunity threshold.https://twitter.com/CT_Bergstrom/status/1252075528711860224 …
Carl T. Bergstrom added,
Carl T. BergstromVerified account @CT_Bergstrom1. I believe that if#SARSCoV2 is allowed to spread uncontrolled through an entire nation, it will be an unprecedented humanitarian disaster. Every possible measure should be taken to prevent this from happening. Yet in some countries this may be unavoidable.Show this thread48 replies 175 retweets 1,658 likesShow this thread -
Replying to @CT_Bergstrom
I tried to explain both the overshoot and how remaining large numbers of unvaccinated people can interact with local outbreaks in a recent piece. My experience is that many people think of herd immunity like a switch rather than a population-level dynamic. https://www.theatlantic.com/health/archive/2021/03/fourth-surge-variant-vaccine/618463/ …pic.twitter.com/878bClZquB
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That article is full of good news as well.
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Herd immunity is good news :)
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Replying to @zeynep @CT_Bergstrom
I loved your article. The NYT piece today makes everything sound so bleak. It’s hard to know where to land.
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