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CT_Bergstrom's profile
Carl T. Bergstrom
Carl T. Bergstrom
Carl T. Bergstrom
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@CT_Bergstrom

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Carl T. BergstromVerified account

@CT_Bergstrom

#BlackLivesMatter Information flow in bio, society, & science. Book *Calling Bullshit*: http://tinyurl.com/y7ekfkhx  I love crows and ravens. he/him

Duwamish Lands (Seattle)
ctbergstrom.com
Joined June 2015

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    Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

    1. In the op-ed pages of newspapers and on cable news shows, I'm seeing frequent confusion over what it means to reach herd immunity—and whether we can relax COVID precautions once we do so. It will take a few posts, but let me try to explain.

    9:07 AM - 4 Apr 2021
    • 2,112 Retweets
    • 4,496 Likes
    • David Pine Paul Nuki David Knight John Anstey 💙 🌹 Alfonso de la Torre Nick Mellor Yannish Naik Lawyerlinguist69🔶🏴󠁧󠁢󠁳󠁣󠁴󠁿 🏴󠁧󠁢󠁥󠁮󠁧󠁿🇪🇺👩‍🏫 #FBPA #FBPE
    93 replies 2,112 retweets 4,496 likes
      1. New conversation
      2. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        5 replies 411 retweets 1,820 likes
        Show this thread
      3. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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

        19 replies 322 retweets 1,476 likes
        Show this thread
      4. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        7 replies 138 retweets 1,150 likes
        Show this thread
      5. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        9 replies 156 retweets 1,085 likes
        Show this thread
      6. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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

        3 replies 187 retweets 1,224 likes
        Show this thread
      7. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        7 replies 234 retweets 1,344 likes
        Show this thread
      8. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        7 replies 302 retweets 1,574 likes
        Show this thread
      9. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        14 replies 265 retweets 1,522 likes
        Show this thread
      10. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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

        4 replies 210 retweets 1,106 likes
        Show this thread
      11. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        13 replies 405 retweets 1,645 likes
        Show this thread
      12. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        4 replies 77 retweets 805 likes
        Show this thread
      13. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        8 replies 112 retweets 970 likes
        Show this thread
      14. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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.

        12 replies 210 retweets 1,252 likes
        Show this thread
      15. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 4

        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_Bergstrom
        1. 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 thread
        49 replies 176 retweets 1,666 likes
        Show this thread
      16. End of conversation

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