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DFisman's profile
David Fisman
David Fisman
David Fisman
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@DFisman

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David FismanVerified account

@DFisman

Air: it’s the new poop. 💨💩😷 "An alarmist seeking the spotlight" —@MNewmania

Chelm
dlsph.utoronto.ca/faculty-profil…
Joined February 2019

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    1. David Fisman‏Verified account @DFisman 5 Feb 2021

      I keep getting asked about the possibility that sars-cov2 will be with us for a long time (many of the questions are driven, I think, by Chris Murray’s statement that we won’t simply “put this behind us”). I do think it is likely to become a seasonal, endemic virus

      93 replies 1,025 retweets 3,139 likes
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    2. David Fisman‏Verified account @DFisman 5 Feb 2021

      At least for a long time to come. That’s because (a) the virus has show an ability to move from humans to animals and back again (mink) and (b) eradication is hard, even with good vaccines and disease w/o animal reservoirs (polio, measles).

      8 replies 54 retweets 450 likes
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    3. David Fisman‏Verified account @DFisman 5 Feb 2021

      And yet I’m optimistic about a non-reckless return to normalcy over the medium turn. Why?https://en.m.wikipedia.org/wiki/You_Are_Old,_Father_William …

      4 replies 25 retweets 372 likes
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    4. David Fisman‏Verified account @DFisman 5 Feb 2021

      As noted previously, pandemics have a beginning, middle and end. Even in the pre-microbial era, they end. Remember the great pandemic of 1925? No, because there wasn’t one. But by around 1922 the terrifying pandemic of 1918-19, thought to have killed around 40 million: over

      9 replies 50 retweets 419 likes
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    5. David Fisman‏Verified account @DFisman 5 Feb 2021

      Where did influenza H1N1 disappear to? It didn't go anywhere. The descendants of devastating 1918 H1N1 continued to circulate as seasonal influenza until 1957. An H1N1 strain threatened to cause a pandemic in 1976, and we had a mild pandemic of a novel H1N1 in 2009.

      3 replies 45 retweets 397 likes
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    6. David Fisman‏Verified account @DFisman 5 Feb 2021

      Co-circulation of H1 strains with H3 strains has been common since the 1990s. H1 is the "mild" flu virus.https://pubmed.ncbi.nlm.nih.gov/15321847/ 

      1 reply 14 retweets 247 likes
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    7. David Fisman‏Verified account @DFisman 5 Feb 2021

      [NB severity of flu is a spectrum, and as a clinician I have seen tragic situations, including respiratory failure and death, from H1 and influenza B. Get your flu shot!]

      3 replies 24 retweets 326 likes
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    8. David Fisman‏Verified account @DFisman 5 Feb 2021

      But the reason 2009 was a mild pandemic? Those most likely to die conditional on infection (older people) were born pre-1957, and had substantial immune protection from early life exposure to related H1N1 strains (that were dominant when they were kids)https://pubmed.ncbi.nlm.nih.gov/19907052/ 

      2 replies 39 retweets 371 likes
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    9. David Fisman‏Verified account @DFisman 5 Feb 2021

      Which brings us to the difference between a pandemic and a seasonal epidemic caused by a respiratory virus. Might influenza have attenuated by 1919? It seems mortality in wave 4 of that epidemic (autumn 1919) had declined. But the major change was in population susceptibility.

      1 reply 20 retweets 266 likes
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      David Fisman‏Verified account @DFisman 5 Feb 2021

      We tend to be very focussed on *the bug* with pandemics. What really creates a pandemic is widespread susceptibility. Does the discarded cigarette butt start the forest fire? Sure. But what makes the forest fire a conflagration is having a long drought and a lot of dead trees

      4:19 AM - 5 Feb 2021
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      11 replies 61 retweets 484 likes
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        2. David Fisman‏Verified account @DFisman 5 Feb 2021

          A discarded cigarette butt in a forest that's wet from rainfall doesn't have the same impact. Same with us and pathogens. When a novel pathogen is introduced into a totally susceptible population, and it has an R0 > 1 (each old case makes > 1 new case), we are dry tinder.

          2 replies 29 retweets 344 likes
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        3. David Fisman‏Verified account @DFisman 5 Feb 2021

          And if we don't put it out fast a conflagration occurs. If we don't change behaviour, then the R(effective) for a communicable disease is approximately R0 x S (the fraction of the population susceptible).

          1 reply 15 retweets 253 likes
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        4. David Fisman‏Verified account @DFisman 5 Feb 2021

          As we acquire immune experience, R(effective) drops, whatever else we are doing or not doing. In this pandemic, that immune experience can be acquired via prior infection, or (and hopefully increasingly) via immunization.

          2 replies 11 retweets 255 likes
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        5. David Fisman‏Verified account @DFisman 5 Feb 2021

          The epidemic peaks when R(effective) hits 1, and declines thereafter, as each old case makes < 1 new case before going away. Note that this applies under conditions where there's no control, no behavior change, etc. Very different from our current reality.

          2 replies 10 retweets 210 likes
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        6. David Fisman‏Verified account @DFisman 5 Feb 2021

          But the basic idea is that the pandemic itself seals its own doom by depleting population susceptibility, just as a forest fire ensures its own end by burning up all the fuel. And it overshoots: so by the time the pandemic is over, the R(effective) is LESS than 1.

          1 reply 15 retweets 261 likes
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        7. David Fisman‏Verified account @DFisman 5 Feb 2021

          So: game over for the virus, right? Erm, no. That decline in R is transient. Susceptibility in the population starts to reaccumulate, both because of waning immunity in infected/vaccinated individuals, and because of accumulation of newly minted baby-people in the pop

          4 replies 21 retweets 286 likes
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        8. David Fisman‏Verified account @DFisman 5 Feb 2021

          who replace the elder-people at the other end of the life span as they die. That results in repletion of susceptibility. The virus (as with flu, as maybe with sars2) can also lend a helping hand by mutating over time (we call this "drift") with flu, and becoming...

          1 reply 13 retweets 221 likes
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        9. David Fisman‏Verified account @DFisman 5 Feb 2021

          ...sufficiently different from previously circulating strains that it de facto increases population susceptibility. We see this play out every year or two with very drifty flu. It's one of the reasons we have to keep changing the flu vaccine (which has 2 different A antigens)

          2 replies 7 retweets 233 likes
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        10. David Fisman‏Verified account @DFisman 5 Feb 2021

          But that's not a pandemic, for two reasons: 1. We have a much lower R because of prior population immune experience. 2. Those who have been around longest (older people) have prior immune experience.

          2 replies 11 retweets 224 likes
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        11. David Fisman‏Verified account @DFisman 5 Feb 2021

          And are protected against these resurgences by earlier exposure. That's why the 2009 pandemic was mild. Those who would be most likely to die from a novel resp infection had prior lifetime immune experience. Not the case in 2019-2021.

          1 reply 14 retweets 243 likes
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        12. David Fisman‏Verified account @DFisman 5 Feb 2021

          So be confident. The idea of a forever-pandemic is about as plausible as the idea of a forever-forest fire. Not gonna happen.

          9 replies 85 retweets 632 likes
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        13. David Fisman‏Verified account @DFisman 5 Feb 2021

          Viruses replicate a lot faster than we do, so they always have a head start on anything that depends on mutation. It is likely that we'll challenges over time as novel variants, maybe even vaccine escape mutants, emerge.

          3 replies 14 retweets 220 likes
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        14. David Fisman‏Verified account @DFisman 5 Feb 2021

          And I would expect that even with immunization we will continue to see seasonal and off-season outbreaks of covid-19 in crowded institutional settings (and cruise ships) which have been hotspots for outbreaks long before there was a COVID-19 pandemic.https://pubmed.ncbi.nlm.nih.gov/16276954/ 

          7 replies 38 retweets 347 likes
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        15. David Fisman‏Verified account @DFisman 5 Feb 2021

          I hope that's helpful. There's more to say here, but it's coffee time. COI declaration: I study infectious diseases for my job, and I think it's dumb for people to die of communicable diseases unnecessarily.

          82 replies 71 retweets 1,486 likes
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        16. End of conversation

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