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