A: The short answer, as statistician Thomas Lumley says, is that “counting rare things is hard.”
There is much we still don’t know about antibody testing for COVID-19 (levels of detection, interpretation, length/duration/extent of immunity). One issue is the testing itself. While current active illness tests (swab/PCR) are very accurate when performed correctly and at the right time (when you can catch the virus in the nose/throat and slightly after symptoms start), you have a two in three chance of testing positive if you have early COVID-19 symptoms. Still, it is the best test we have.
Antibody tests on the other hand, have high false positive rates (even manufacturers note that the false positive rate could be as high as 2%). This leaves a confusing message in the estimated number of positive cases. People test positive because they had COVID-19 and the test was accurate or they didn’t have COVID-19 but had a positive test result (false positive). The short answer: When tests have false positive rates up to 5%, you can’t draw conclusions about the positive cases if they are also a small percentage of the population. This doesn’t mean that there isn’t a role for antibody tests, but we may need better tests with a lower false positive rate and/or more research into how testing can be applied at a population level.
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