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
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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
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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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Thank you! This has become so hard to discuss on social media. 
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Plaudits for your perseverance in continuing to discuss it on social media.
0 replies 0 retweets 1 likeThanks. Twitter will use this to make your timeline better. UndoUndo
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And other hard-hit cities like London and... Stockholm.
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End of conversation
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