2) Lower infection fatality rate (IFR) After we have an estimate of the true infections, we can compute the implied infection fatality rate by factoring in deaths from approximately 4 weeks later. We can do this for the US nationally, or on a state-by-state basis.
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The case fatality rate and implied infection fatality rate have all decreased significantly over the past few months (US is currently at ~0.25% IIFR). I believe the largest contributor is lower median age of infection. There's also some elements of improved treatments.pic.twitter.com/uVN11fGrgZ
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Using CDC's COVIDView data, one can see that the proportion of confirmed cases for individuals above age 65 dropped significantly from April to June. Once we factor in test positivity rate to get true prevalence, the difference is even more pronounced. https://www.cdc.gov/coronavirus/2019-ncov/covid-data/covidview/07312020/commercial-labs.html …pic.twitter.com/vyzFGO5RxN
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Since the IFR of those age 65+ is 20-50x higher than those under age 50, it's no surprise that we've seen a steep reduction in the overall IFR across the US. This is further helped by improved treatments and earlier detection. One caution is that this trend is slowly reversing.
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3) Lower herd immunity threshold (HIT) Infections are now declining in almost all heavily-impacted states, despite no clear policy interventions. Estimating the HIT based on the current effective reproduction numbers results in a ~10-35% effective HIT. (Formula: HIT = 1-1/Rt)
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Replying to @youyanggu
In this context, I would say that HIT is the wrong term to use. HIT implies (long-lasting) immunity, whereas this is a potentially transient result of behaviour shortly after reopening. In many places, these behaviours have not lasted and thus Rt climbs
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Replying to @youyanggu
Honestly, I'm not sure. Immunity implies, well, immunity, which this isn't exactly. HIT is really only used traditionally in the vaccination context, although I know everyone wants it to be applicable to COVID-19
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Replying to @GidMK @youyanggu
Perhaps Epidemic Reduction Threshold or similar? If the measures are transient - i.e. no long-lasting immunity - then this conveys the fact that cases stop spreading but it may also reverse if behaviour changes
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Because it doesn't really make sense. Herd Immunity Threshold at time t is theoretically weird - it's odd to imply immunity that varies over a time period. The word immunity itself just doesn't really fit with the whole idea imo
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Replying to @GidMK @youyanggu
hmm. it's pretty intuitive to me, as you're just replacing R0 in the equation with Rt, but perhaps there's a better term.
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