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trvrb's profile
Trevor Bedford
Trevor Bedford
Trevor Bedford
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@trvrb

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Trevor BedfordVerified account

@trvrb

Scientist @fredhutch, studying viruses, evolution and immunity. Collection of #COVID19 threads here: https://bedford.io/misc/twitter/ 

Seattle, WA
bedford.io
Joined December 2010

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    1. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      I wanted to discuss the degree to which population immunity may be contributing to curbing #COVID19 in Florida, Arizona and Texas, where recent surges have resulted in substantial epidemics. 1/16

      181 replies 1,059 retweets 2,689 likes
      Show this thread
    2. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      After increasing dramatically in June and July, daily case counts in Florida, Arizona and Texas have begun to subside. Data from @COVID19Tracking. 2/16pic.twitter.com/QLzoxUFhKA

      30 replies 73 retweets 418 likes
      Show this thread
    3. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      This corresponds to a peak Rt of between 1.2 and 1.4 in late-May / early-June and steady reductions since this point. Declining case counts correspond to Rt < 1. Rt estimates from https://rt.live/ . 3/16pic.twitter.com/LES1VNrPKd

      9 replies 38 retweets 307 likes
      Show this thread
    4. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      The recent uptick in Rt in Texas comes from increasing test positivity and the algorithm used by https://rt.live/  to convert test positivity to infections. I'm not exactly sure what's going on here, but I wouldn't ascribe too much weight to this particular estimate. 4/16pic.twitter.com/dDY4kBykHc

      7 replies 33 retweets 285 likes
      Show this thread
    5. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      Trevor Bedford Retweeted Trevor Bedford

      I (and others) have argued that the main thing curbing these epidemics have been societal responses, but I believe there is also a role for population immunity in controlling these epidemics. 5/16https://twitter.com/trvrb/status/1284255564545982464 …

      Trevor Bedford added,

      Trevor BedfordVerified account @trvrb
      A follow up to the thread two weeks ago on rising case counts and "reopening", looking at continued trajectories in states with large epidemics. 1/15 https://twitter.com/trvrb/status/1279187556106723333 …
      Show this thread
      15 replies 65 retweets 404 likes
      Show this thread
    6. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      If we take the simplest model of population immunity, we expect Rt will equal R0 ⨉ fraction of the population susceptible. This is where the usual "herd immunity threshold" comes in. If we assume R0 of 2.5 then we need 60% of the population immune to bring R0 down to 1. 6/16pic.twitter.com/kadSboUepW

      10 replies 45 retweets 301 likes
      Show this thread
    7. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      However, societal behavior has dramatically reduced Rt through social distancing, mask wearing, etc... The surge in Florida had Rt of only ~1.4 at its peak. Because of this reduction in transmission through social means, we don't need as much immunity to impact spread. 7/16

      9 replies 64 retweets 402 likes
      Show this thread
      Trevor Bedford‏Verified account @trvrb 7 Aug 2020

      Here, we expect Rt will equal R0 ⨉ fraction of the population susceptible ⨉ relative social connectivity. If R0 is 2.5 and social connectivity is 56% of normal then realized Rt will be 1.4. In this case, the herd immune threshold would be 29%. 8/16pic.twitter.com/AzMDXbIGxS

      3:15 PM - 7 Aug 2020
      • 83 Retweets
      • 403 Likes
      • huh what Jon Spaihts Infinite Possibilities Justin Bell Aymeric de kerdanet Shevin Jacob climbthathill, AA, BA, MA, MAT Gotta Get Thru This 🌸
      6 replies 83 retweets 403 likes
        1. New conversation
        2. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          We can plot out the general relationship between Rt (as red vs blue) vs population immunity (on the x-axis) vs social connectivity (on the y-axis). With R0 of 2.5, to keep Rt<1, we need either lots of immunity, very strong social distancing or something in between for each. 9/16pic.twitter.com/Jo4aXOe1Gl

          7 replies 121 retweets 532 likes
          Show this thread
        3. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          At this point, we think that a substantial fraction of the population of Florida has had COVID-19. If we use a 8:1 ratio of confirmed cases to underlying infections, we'd estimate 510k x 8 = ~4M infections in Florida or roughly 20% of the population. 10/16

          13 replies 86 retweets 396 likes
          Show this thread
        4. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          Similarly, https://covid19-projections.com/us-fl  currently estimates 21% of Florida has had COVID-19 at this point. 11/16pic.twitter.com/JG6eZMXAs7

          12 replies 66 retweets 360 likes
          Show this thread
        5. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          Assuming a large majority of infections leave enough immunity to be protected (which I believe to be the case, though correlates of protection are still being worked out), population immunity of 20% will have real impact if societal behavior has already reduced Rt to ~1.2. 12/16

          16 replies 61 retweets 369 likes
          Show this thread
        6. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          I've been thinking of this as: to get to R0 of 1.0 with no immunity we need avoid 60% of transmission events. However, if 20% of the population is immune, then we need to avoid 50% of transmission events. 13/16

          4 replies 68 retweets 394 likes
          Show this thread
        7. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          Or, with 20% population immunity, we can behave as though Rt is 1.25 and still get an epidemic that no longer propagates. 14/16

          8 replies 40 retweets 325 likes
          Show this thread
        8. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          Thus, I believe the substantial epidemics in Arizona, Florida and Texas will leave enough immunity to assist in keeping COVID-19 controlled. However, this level of immunity is not compatible with a full return to societal behavior as existed before the pandemic. 15/16

          24 replies 133 retweets 746 likes
          Show this thread
        9. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          That said, the costs for this immunity have been substantial and are continuing to accrue. We need a vaccine to achieve population immunity in a fashion that doesn't kill people. 16/16

          84 replies 128 retweets 1,025 likes
          Show this thread
        10. Trevor Bedford‏Verified account @trvrb 7 Aug 2020

          Trevor Bedford Retweeted Natalie E. Dean, PhD

          Follow up #1: My use of 20% total infected in Florida is not the point I was trying to make. I think it could easily be 10% of Florida infected at present. The point being even 10% population immunity starts to make a difference when behavioral Rt is ~1.2.https://twitter.com/nataliexdean/status/1291883840986853382 …

          Trevor Bedford added,

          Natalie E. Dean, PhDVerified account @nataliexdean
          Is there stronger evidence that 20% of the entire state of Florida has been infected? This model also estimates that over 30% of people in Miami have been infected. Given what we saw in NYC, these both feel too high for me. Other sources of evidence? https://twitter.com/trvrb/status/1291860670439280640 …
          46 replies 46 retweets 348 likes
          Show this thread
        11. End of conversation
        1. Christoph Bonitz‏ @chris_bonitz 8 Aug 2020
          Replying to @trvrb

          Is social connectivity an input variable, though, or a function of the perceived severity of the epidemic? Policy makers and the population seem to titrate their response against infection rates.

          0 replies 0 retweets 2 likes
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