That's right-censoring in a nutshell If you stop counting results too early, suddenly your study has a significant bias (often towards low/no difference)
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What does this mean to antibody results for COVID-19? Well, think about how these studies are conducted. We test a bunch of people randomly on day x to get an idea of how many people are immune to COVID-19 on that day
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Then, to calculate mortality, most people take the number of deaths on day x and divide by the denominator implied by the results
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So, if we think 4% of 100,000 people have had COVID-19, and 10 of them have died on day x, we'd say that the infection-fatality rate is 10/4000 = 0.025%
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BUT there's an issue here People don't die from COVID-19 immediately. It usually takes somewhere between 15-20 days from when they get infected The data is right-censored!
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Replying to @GidMK
People don’t seroconvert immediately either so if you are using serology you are finding out how many were infected in the past. So given, on average, it takes longer to seroconvert than to die this is actually introducing a “left censoring” effect.
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Replying to @MutantBug
For COVID-19, the published estimates I've seen are median 7-11 days for seroconversion and 15-21 days for death. Would be interested if you've seen substantially different!
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Replying to @GidMK
Matthew Avison Retweeted Florian Krammer
No much longer to make IgG. See this thread and associated paper. And quite a few others. Maybe 14 days after resolution of symptoms. But that’s at least 28 days after date of infection. https://twitter.com/florian_krammer/status/1257828922638700546?s=21 …https://twitter.com/florian_krammer/status/1257828922638700546 …
Matthew Avison added,
Florian KrammerVerified account @florian_krammer1) I wanted share some data from a manuscript that just wen online at https://www.medrxiv.org/content/10.1101/2020.04.30.20085613v1 …. This was spearheaded by the fantastic Dr. Ania Wajnberg in collaboration with many parts of the Mount Sinai Hospital and Icahn School of Medicine.Show this thread1 reply 0 retweets 0 likes -
Replying to @MutantBug
Not sure where you're getting the estimate from there, because that study performed a single test between 3 and 60 days after symptom onset
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Replying to @GidMK
The point is they showed the liklihood of positivity increased with time after symptom onset up to 28 days. And even if you don’t agree with the exact number, I’m sure you will agree that at best the use serology to define infection minimises the effect you describe.
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Not really, you'd need consecutive longitudinal samples for that, and as I said this is only for IgG. I'd mistrust any estimate that makes no attempt to account for right-censoring
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