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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There are probably a bunch of people who HAVE the disease on day x who are counted in our sample and will die but haven't yet!
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So, what we SHOULD do in cases like these is either: a) use a statistical model to account for this issue b) wait a few weeks and use different death estimates to correct for potential right-censoring
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Instead, most people just take the proportion immune on day x and divide by deaths on the same day This will almost certainly underestimate the 'true' infection-fatality rate, and is a big worry
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Replying to @GidMK
This example of right-censoring is accurate if you are looking at PCR but not as accurate for antibody tests, since it takes 2-3 weeks to reliably produce antibodies the fatality/seropositive case ratio comes closer to the true IFR than fatality/PCR positive case ratio does - no?
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Replying to @BonellC
From my understanding, it depends on the antibody tested, but generally speaking people will develop antibodies progressively over time from a few days after infection, so you'll have some people relatively recently infected testing positive
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...as well as people who were infected weeks ago. The crudest way to account for this is to take the deaths from some point in the future (say, x+7 days), but you can also estimate the impact in a much more sophisticated way
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There's another variable: lag between conducting the antibody test and reporting/publishing them in a study. I'd imagine this takes about a week, perhaps more.
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