2. The key thing to note is that the herd immunity threshold is the point at enough people are immune (by vaccination or previous infection) to prevent a new epidemic from starting from scratch. It is *not* the point at which an ongoing epidemic disappears.
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3. When you reach herd immunity, a pandemic is far from over. In fact, in a basic SEIR model, an ongoing epidemic is at its *peak* when the herd immunity threshold is reached.pic.twitter.com/7pnHqeRYqU
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4. The problem is that while a new epidemic can no longer start from scratch once you reach herd immunity, when we reach this point for COVID we'll still have the old epidemic underway—and epidemics have something akin to momentum.
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5. All of the people who are currently infected will continue to transmit disease after you reach the herd immunity threshold. They just will infect fewer than one additional person, on average. The people they infect? Those will infect others downstream. And so on.
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6. This gives rise to what we call "overshoot"—additional cases above and beyond the herd immunity threshold.
@nataliexdean and I wrote an OpEd about this almost a year ago. https://www.nytimes.com/2020/05/01/opinion/sunday/coronavirus-herd-immunity.html …pic.twitter.com/psva23mwk0
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7. Another thing to keep in mind is that the herd immunity threshold depends on the transmissibility of the disease—which depends on behavior. For example, in many places we could already be reaching the herd immunity threshold for a masked, socially distant world.
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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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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 …
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