I have been meaning to update the information on Covid-19 testing, since it has been such a confusing subject. This post will only cover antibody (aka serology) tests. And most of the post will be direct quotes from the FDA and CDC websites, because you would probably not believe me if I simply paraphrased the information. So here is my synthesis, followed by direct quotes from the agencies.
Boiled down, what FDA and CDC say is:
1. The presence of antibodies does not mean you have had Covid.
2. The test might be positive due to a cross reaction with antibodies from a cold you had.
3. Even if you have antibodies and did have Covid, they don’t tell us whether or not you are immune.
4. Even if you are immune today, you may not be immune tomorrow–who knows?
5. You may have had Covid and have no antibodies
6. Antibody tests are meaningless for individuals, but might give us useful information at the population level.
7. Regardless of the result, you still need to wear masks and socially distance.
8. In a population where less than 5% have had Covid, over 50% of positive results will be false positives.
Yet CDC admits that reinfection is rare. There are only 3 documented cases of reinfection in the US, so there must be good immunity after infection for most people.
Bottom line: why are we doing these tests? Why hasn’t FDA rescinded its emergency approval, since in the majority of the US, most positive results are false positives? And why don’t we have better tests? T-cell immunity is probably more accurate, but there are no commercially available T-cell tests for the public. Why are we still measuring antibodies that cross-react with those for the common cold? Is this another way the federal government is keeping us confused and locked down?
Here are the direct quotes:
“…COVID-19 antibody tests can help identify people who may have been infected with the SARS-CoV-2 virus or have recovered from the COVID-19 infection.”
At this time, researchers do not know whether the presence of antibodies means that you are immune to the coronavirus in the future; or if you are immune, how long it will last.”
“…additional data are needed before modifying public health recommendations based on serologic [antibody] test results, including decisions on discontinuing physical distancing and using personal protective equipment.
…Although serologic tests should not be used at this time to determine if an individual is immune, these tests can help determine the proportion of a population previously infected with SARS-CoV-2
…Recurrence of COVID-19 illness appears to be very uncommon
…However, it remains uncertain to what degree and for how long individuals with antibodies (neutralizing or total) are protected against reinfection with SARS-CoV-2
…in a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%. In other words, less than half of those testing positive will truly have antibodies.
…pending additional data, the presence of antibodies cannot be equated with an individual’s immunity from SARS-CoV-2 infection.
…Some tests may exhibit cross-reactivity with other coronaviruses, such as those that cause the common cold. This could result in false-positive test results. Some persons may not develop detectable antibodies after coronavirus infection.
…Serologic testing by itself should not be used to establish the presence or absence of SARS-CoV-2 infection or reinfection.”
And here is a Reuters article quoting the CEO of Roche (which makes Covid tests) claiming that some of the other available tests are worthless.