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I'm a bit confused reading articles like this: https://www.nytimes.com/2020/04/23/nyregion/coronavirus-antibodies-test-ny.html

One of every five New York City residents tested positive for antibodies to the coronavirus, according to preliminary results described by Gov. Andrew M. Cuomo on Thursday that suggested that the virus had spread far more widely than known.

If the pattern holds, the results from random testing of 3,000 people raised the tantalizing prospect that many New Yorkers — as many as 2.7 million, the governor said — who never knew they had been infected had already encountered the virus, and survived. Mr. Cuomo also said that such wide infection might mean that the death rate was far lower than believed.

As per this previous Stackexchange question, it appears that we have no way of knowing whether someone has coronavirus antibodies or not. So I have a few follow-up questions:

  1. How do they detect whether someone has coronavirus antibodies?
  2. Does someone having the antibodies mean they were definitely infected with the novel coronavirus at some point?
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  • Note the date on the previous question you've linked to, and think a bit about how that relates to the time course of the epidemic...
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 22:45
  • Also related: medicalsciences.stackexchange.com/questions/23192/…
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 22:46
  • Thanks @BryanKrause, I looked up other questions on antibodies on this site and couldn't find anything relevant except the previous one. Didn't realize it was two months old. Am happy to update or remove the links as necessary if they don't help.
    – Sam
    Commented Apr 23, 2020 at 23:03

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How do they detect whether someone has coronavirus antibodies?

You test the blood against antigens carried by the SARS-CoV-2. Antibodies in the blood will bind to these antigens.

The information for the Cellex test says:

When a correct volume of test specimen is dispensed into the sample well of the test cassette, the specimen migrates by capillary action along the cassette. The anti-SARS-CoV-2 virus IgG, if present in the specimen, will bind to the SARS-CoV-2 conjugates. If IgG is present in the specimen, the immunocomplex will then captured by the anti-human IgG line, forming a burgundy colored G Line, indicating a SARS-CoV-2 virus IgG positive test result.

The anti-SARS-CoV-2 virus IgM, if present in the specimen, will bind to the SARS-CoV-2 conjugates. The immunocomplex is then captured by the anti-human IgM line, forming a burgundy colored M Line, indicating a SARS-CoV-2 virus IgM positive test result. Information regarding the immune response to SARS-CoV- 2 is limited and still evolving.

Does someone having the antibodies mean they were definitely infected with the novel coronavirus at some point?

It depends on the specificity of the test. If the test is 99% specific it means that it is 99% probable that you're detecting the right antibody, and not an antibody against another virus or antigen.

https://www.fda.gov/media/136625/download

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  • Looks like they tested 50 known-negative samples and had 1 show up positive. For 49/50 samples the 95% binomial CI would be [.89,.99]. You'd really think they could manage more than 50 negative controls....
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 23:10
  • Are you talking about the control vials provided with the test kits?
    – Graham Chiu
    Commented Apr 23, 2020 at 23:18
  • No, the experiments described in the link you sent, where they spiked (negative) blood with either positive serum or control. One of the negative controls out of 50 came up positive.
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 23:52
  • They also didn't do a great job of showing lack of cross-reactivity; it's hard to know whether their clinical tests that mismatched with their known status is because they've already recovered or if there is cross reactivity.
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 23:55
  • In any event, you didn't say the test is 99% specific just explained what that means, but I meant to highlight that at least from this info I don't see good evidence that it's 99% specific.
    – Bryan Krause ♦
    Commented Apr 23, 2020 at 23:56

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