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
It’s an important caveat to be aware of but I don’t think it will be that important in the fatality estimates. Time from infection to death is relatively short (esp when compared to smoking & ca), but assumption is that no longer term risk exists
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Replying to @naomhgallagher
Gotta say I disagree. One Iranian study calculated IFR from the deaths on the day their seroprevalence data was collected - if they had used deaths from a week later, the IFR estimate would've doubled
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Replying to @GidMK
Ok I was assuming some lag between collecting data and mortality figures used (same day seems very efficient!). The importance will still be greater in a study using diagnostic test, as most pos seroprevalence will be over their infection. Cohort study best if possible
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Efficient is definitely one way to describe it!
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Replying to @GidMK
Yea probably not the best word to use
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