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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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    1. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      5. On average each infected person will then transmit disease to 2 × 0.25 = 0.5 susceptible others. At this rate, the infection quickly fizzles out. That's the idea behind herd immunity. Vaccinate enough people that on average each case generates fewer than one subsequent case.

      3 replies 31 retweets 351 likes
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    2. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      6. Q: Some people are immune because they were vaccinated; others because they had the disease and recovered. Do both count toward herd immunity? Yes. What you are trying to do is get the number of immune people up to the "herd immunity threshold".

      6 replies 26 retweets 323 likes
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    3. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      7. While it's safer to get there by vaccination, "natural" and vaccine-induced immunity both have the effect of reducing the number of susceptible people in the population.

      4 replies 20 retweets 286 likes
      Show this thread
    4. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      8. Q: Roughly what it is the herd immunity threshold for COVID-19? A: That depends on the basic reproduction number R0 that everyone talks about. R0 is the number of people infected, on average, by a single contagious person in a wholly susceptible population.

      5 replies 18 retweets 247 likes
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    5. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      9. As a rule of thumb, the herd immunity threshold (HIT) is reached when at least a fraction (1-1/R0) of the population are immune. So if R0 is 2, we need at least 1/2 of the population immune. If R0 is 3, we need at least 2/3 immune. And so forth.

      5 replies 33 retweets 299 likes
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    6. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      10. There are various refinements on this rule of thumb depending on exactly how people in the population contact one another, and other details. For COVID-19, R0 is probably around 2.5 to 3.0, though it may be higher for some of the new variants of concern such as B.1.1.7.

      5 replies 26 retweets 269 likes
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    7. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      11. This puts the herd immunity threshold somewhere in the range of 60-70% for the earlier SARS-CoV-2 strains. It could be 80% or even higher for some of the new highly transmissible variants including B.1.1.7 and the likes.

      4 replies 64 retweets 340 likes
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    8. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      12. Q: So have we reached herd immunity yet? A: If we are talking about herd immunity in a population where no one takes any precautions, probably not. Right now cases are declining in some places, but this is in part because of closures, mask-wearing, social distancing, etc.

      6 replies 40 retweets 317 likes
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    9. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      13. Q: Can we reach herd immunity at all for COVID? A: In principle, yes. If we can reach a point where 70-80% of the population is immune, we have a good chance to achieve herd immunity.

      7 replies 33 retweets 298 likes
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    10. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      14. Q: Once we reach herd immunity, can we relax all control measures and go back to life-as-in-2019? A: Not quite. Herd immunity will be necessary for living like 2019 (if we want to do so without COVID around, anyway). But it's not sufficient.

      2 replies 43 retweets 300 likes
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      Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

      Carl T. Bergstrom Retweeted Carl T. Bergstrom

      15. Before we can relax all our controls we also want to have a low number active cases. Recall that herd immunity is the point at which a new outbreak cannot start from scratch. It is not the point that an ongoing outbreak is over. I address that here 🧵:https://twitter.com/CT_Bergstrom/status/1378741102476337155 …

      Carl T. Bergstrom added,

      Carl T. BergstromVerified account @CT_Bergstrom
      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.
      Show this thread
      9:38 PM - 5 Apr 2021
      • 112 Retweets
      • 504 Likes
      • Cate Robert Weiss Alex Heeney Zoe McKelvey Michelle Nickel Gretchen Lindquist #COVIDVaccinesWork Christopher Thomas Shana "It's Shay-nuh" Carp 🐟🐠🐟 #NiUna- y #Nunca+💚
      2 replies 112 retweets 504 likes
        1. New conversation
        2. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          16. If we continue control measures once we reach herd immunity, the number of cases will rapid decline and we can soon relax. If we relax as soon as we reach herd immunity, cases will only very slowly decline and many unnecessary infections will occur.

          7 replies 117 retweets 505 likes
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        3. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          17. Q: What about escape variants? Won't they prevent us from reaching herd immunity? A: I'm optimistic that we'll be ok on this front. I haven't seen any compelling evidence that the current variants of concern (P1/2, B.1.351, etc) escape the US-authorized vaccines.

          10 replies 43 retweets 377 likes
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        4. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          18. While sera from vaccinated individuals do exhibit reduced activity against some of these strains, my guess is that the immune response remains sufficiently strong even against these variants to prevent disease.

          3 replies 16 retweets 265 likes
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        5. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          19. Even if escape variants do arise, we can readily incorporate these variants into booster shots. There is no reason why with concerted effort, we shouldn't be able to stay out ahead of any evolving escape variants with a proactive program of vaccine boosters.

          8 replies 26 retweets 288 likes
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        6. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          20. Q: Immunity from COVID infection or vaccines may not last all that long? Will waning immunity prevent us from reaching herd immunity? A: Of course we don't know yet, because we haven't had enough time to find out.

          1 reply 15 retweets 224 likes
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        7. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          21. That said, my guess is that we'll get at least a couple of years of immunity from natural infection, and at least 3-5 years from vaccination. If so, that generates a modest schedule of booster shots not that different from the tetanus vaccine.

          9 replies 30 retweets 323 likes
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        8. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          22. Q: You said "in principle" we can reach 70-80% immune. Why did you qualify that? A: Three reasons. The first two, escape variants and waning immunity, I've already addressed. The third, and I think the big one, is vaccine hesitancy.

          2 replies 23 retweets 258 likes
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        9. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          23. Without vaccine hesitancy, we'd be in really good shape. We have Pfizer and Moderna with <90% efficacy and J&J somewhere in the 66-75% range. Any reasonable mixture of these, applied across the entire population, would confer more than 80% immunity and would do the trick.

          9 replies 41 retweets 314 likes
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        10. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          24. With vaccine hesitancy, it could be close here in the US. I'm hoping that much of the hesitancy we see is really more like what @DrMelissaClarke describes as vaccine deliberation. A lot of people feel their communities have not been so well served by US healthcare.pic.twitter.com/PFnCXj68GK

          9 replies 29 retweets 273 likes
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        11. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          25. Understandably, they would like more information *that they consider trustworthy* before deciding what to do. As the vaccines prove themselves safe and effective, and as we improve health equity, those deliberating about what to do may decide to take the vaccine.

          3 replies 10 retweets 219 likes
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        12. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          26. Q: If we don't reach herd immunity, what would it look like? A: I suspect we'd see some seasonality, much as we do for influenza. Minimal cases in the summer, followed by a winter wave. With vaccination, most people will be protected from serious illness at the very least.

          2 replies 21 retweets 250 likes
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        13. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          27. Ongoing boosters or "updates" would remain essential, though. Notice also that even if we reach herd immunity in the US, disease is a global problem. Without aggressive vaccination programs worldwide, we'll face continual reintroductions into the US.

          4 replies 32 retweets 291 likes
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        14. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          28. Unmitigated spread worldwide also offers abundant opportunity for escape variants to arise. Ultimately the best way to ensure COVID safety in the US will be to take care of the US — and the same time recognize and address the global nature of the problem.

          4 replies 30 retweets 286 likes
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        15. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          29. Q: If we have a safe, readily available vaccine, who cares whether we reach herd immunity or not? If people don't want the vaccine, that's their problem. A: Recall that not everyone can safely take the vaccine, and not everyone has an immune system that will respond to it.

          12 replies 50 retweets 349 likes
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        16. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          30. One very important function of herd immunity for diseases such as measles is that by vaccinating the majority, we are able to protect a minority for whom vaccines are not safe or effective. For many diseases this includes *all* young infants.

          11 replies 86 retweets 513 likes
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        17. Carl T. Bergstrom‏Verified account @CT_Bergstrom Apr 5

          31: Q: I asked a question in the replies and you didn't answer. Do you hate me? A: Unlikely. I probably just don't know the answer, and want to defer to those who do.

          22 replies 10 retweets 325 likes
          Show this thread
        18. End of conversation
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