4 ms·

Would't any preexisting immunity be accounted for by any decent estimate for R0 and the IFR?

bluGill 6y ago
Not really. If there is existing immunity then the measured R0 will be lower than the actual R0, but we don't know how to measure the true R0 in absence of the current population with their immunity so we can never know what the true R0 might actually be. Note that true R0 is a concept I made up on the spot. I think it is clear what I mean but experts in this may have better terms.
tripletao 6y ago
A first-order SIR model with unmodeled pre-existing immunity will be accurate initially, but get gradually more wrong with time. For a simplified, non-representative example with fake numbers, let's say half the population already had perfect sterilizing (they don't get sick, and the virus doesn't replicate at all so they can't spread it to anyone else) immunity. Then an apparent R0 of 2.5 would imply an actual R0 of 5. But R0 = 2.5 would imply you need 1 - 1/2.5 = 60% of the population infected for herd immunity, while in that hypothetical case you'd need only 30% infected (so the 50% initially immune plus that 30% make 1 - 1/5 = 80%). The above still assumes a well-mixed population, but with heterogeneous susceptibility (using the terminology from [1]). There's also heterogeneous connectivity (i.e., the fact that some people have more social contacts than others), which further reduces the herd immunity threshold. To be clear, the above is a fake example. We know that almost no one has perfect pre-existing sterilizing immunity to the novel coronavirus, because we've seen highly-connected populations (homeless shelters, slums, boats, etc.) where almost everyone tested positive. But that doesn't mean everyone is equally susceptible! Immunity is a continuum, not a binary thing, and the 20-50%[2] of people with reactive T cells very likely get less sick given the same initial viral dose (or, stated in the other direction, would need a greater viral dose to get equally sick). They also might spread the virus less, though that's more speculative. Finally, I'll note that the herd immunity threshold is the point where R becomes < 1, and the number of people currently infected starts to shrink instead of growing. People still get infected on the downslope, so the fraction of people infected can overshoot higher (and potentially much higher, especially if no precautions are taken to slow the spread). 1. https://www.medrxiv.org/content/10.1101/2020.07.23.20160762v2 https://www.medrxiv.org/content/10.1101/2020.07.23.20160762v... 2. https://science.sciencemag.org/content/early/2020/08/04/science.abd3871?ijkey=a3504aae1c4fe757a654a7b4c742d55a3ebc5828&keytype2=tf_ipsecsha https://science.sciencemag.org/content/early/2020/08/04/scie...
- argonaut 6y ago
  > We know that almost no one has perfect pre-existing sterilizing immunity to the novel coronavirus, because we've seen highly-connected populations (homeless shelters, slums, boats, etc.) where almost everyone tested positive. We don't actually know that. Even if some significant % of the population has pre-existing sterilizing immunity, then by chance we'd still expect to have many groups of people where everyone is vulnerable.
  - tripletao 6y ago
    Fair point. I still think pre-existing sterilizing immunity is unlikely, since (to my non-biologist's understanding) antibodies tend to be specific, and T cell immunity is broader but not usually sterilizing. The existence of those unlucky groups isn't conclusive evidence in itself, though, especially if pre-existing immunity is distributed non-uniformly (which it probably is, if there's some genetic basis, or if different strains of common cold coronavirus swept through different regions last year and primed people's T cells differently). Of course non-sterilizing immunity could still be incredibly valuable. They're speculating that the current vaccine candidates may not cause sterilizing immunity[1], but immunity that turns a fatal illness into a bad cold (or a bad cold into a couple of sniffles) is great, whether naturally pre-existing or from a vaccine. 1. https://www.statnews.com/2020/05/22/the-world-needs-covid-19-vaccines-it-may-also-be-overestimating-their-power/ https://www.statnews.com/2020/05/22/the-world-needs-covid-19...

Not really. If there is existing immunity then the measured R0 will be lower than the actual R0, but we don't know how to measure the true R0 in absence of the current population with their immunity so we can never know what the true R0 might actually be. Note that true R0 is a concept I made up on the spot. I think it is clear what I mean but experts in this may have better terms.

tripletao 6y ago

A first-order SIR model with unmodeled pre-existing immunity will be accurate initially, but get gradually more wrong with time. For a simplified, non-representative example with fake numbers, let's say half the population already had perfect sterilizing (they don't get sick, and the virus doesn't replicate at all so they can't spread it to anyone else) immunity. Then an apparent R0 of 2.5 would imply an actual R0 of 5. But R0 = 2.5 would imply you need 1 - 1/2.5 = 60% of the population infected for herd immunity, while in that hypothetical case you'd need only 30% infected (so the 50% initially immune plus that 30% make 1 - 1/5 = 80%). The above still assumes a well-mixed population, but with heterogeneous susceptibility (using the terminology from [1]). There's also heterogeneous connectivity (i.e., the fact that some people have more social contacts than others), which further reduces the herd immunity threshold. To be clear, the above is a fake example. We know that almost no one has perfect pre-existing sterilizing immunity to the novel coronavirus, because we've seen highly-connected populations (homeless shelters, slums, boats, etc.) where almost everyone tested positive. But that doesn't mean everyone is equally susceptible! Immunity is a continuum, not a binary thing, and the 20-50%[2] of people with reactive T cells very likely get less sick given the same initial viral dose (or, stated in the other direction, would need a greater viral dose to get equally sick). They also might spread the virus less, though that's more speculative. Finally, I'll note that the herd immunity threshold is the point where R becomes < 1, and the number of people currently infected starts to shrink instead of growing. People still get infected on the downslope, so the fraction of people infected can overshoot higher (and potentially much higher, especially if no precautions are taken to slow the spread). 1. https://www.medrxiv.org/content/10.1101/2020.07.23.20160762v2 https://www.medrxiv.org/content/10.1101/2020.07.23.20160762v... 2. https://science.sciencemag.org/content/early/2020/08/04/science.abd3871?ijkey=a3504aae1c4fe757a654a7b4c742d55a3ebc5828&keytype2=tf_ipsecsha https://science.sciencemag.org/content/early/2020/08/04/scie...

argonaut 6y ago
> We know that almost no one has perfect pre-existing sterilizing immunity to the novel coronavirus, because we've seen highly-connected populations (homeless shelters, slums, boats, etc.) where almost everyone tested positive. We don't actually know that. Even if some significant % of the population has pre-existing sterilizing immunity, then by chance we'd still expect to have many groups of people where everyone is vulnerable.
- tripletao 6y ago
  Fair point. I still think pre-existing sterilizing immunity is unlikely, since (to my non-biologist's understanding) antibodies tend to be specific, and T cell immunity is broader but not usually sterilizing. The existence of those unlucky groups isn't conclusive evidence in itself, though, especially if pre-existing immunity is distributed non-uniformly (which it probably is, if there's some genetic basis, or if different strains of common cold coronavirus swept through different regions last year and primed people's T cells differently). Of course non-sterilizing immunity could still be incredibly valuable. They're speculating that the current vaccine candidates may not cause sterilizing immunity[1], but immunity that turns a fatal illness into a bad cold (or a bad cold into a couple of sniffles) is great, whether naturally pre-existing or from a vaccine. 1. https://www.statnews.com/2020/05/22/the-world-needs-covid-19-vaccines-it-may-also-be-overestimating-their-power/ https://www.statnews.com/2020/05/22/the-world-needs-covid-19...