I’ve been googling, and must not be putting my question right, because I’m not finding a specific answer.
I know that the antibody test shows if you’ve had Covid in the past. Once you’ve been vaccinated, though, wouldn’t that test be unhelpful? The vaccine made you produce antibodies, so won’t the test just detect those?
I’m trying to see if an asymptomatic case of Covid in a vaccinated person could be detected after the fact with an antibody test.
That was my understanding–once you’re vaccinated, you’ll test positive for antibodies even if you haven’t been infected. But the CDC says there’s different types of antibody tests, and some will be positive due to a vaccine and others won’t:
https://www.cdc.gov/coronavirus/2019-ncov/testing/serology-overview.html
I’m sure this has been discussed before on these boards, and explained by people better qualified than I am; but the explanation, as best as I can give it, is this.
You don’t have an antibody “for a virus”. Antibodies learn to recognize specific regions of a virus. Your body, in fighting infections, will develop and produce an array of antibodies, each capable of recognizing and acting at a different part of the virus.
To date, in the US and Europe, so far as I am aware, all vaccines were developed to train your body to produce antibodies which act specifically at the spike structures on the surface of COVID-19. (This was done deliberately because the spike structures conferred an enhanced infectiousness to the virus; mutations which changed the spike might allow the virus to evade the vaccine; but they would also result in a less dangerous virus).
If an antibody test just shows up antibodies specific to the spike, that is taken to be an indication of vaccination. If the test also detects other COVID-19 antibodies - specific to other parts of the virus - that indicates an infection BUT it does not tell you anything conclusive about vaccination status.
This:
allows you to download a PDF with a much more detailed discussion. But for your question I think the key para is.
Serology tests are widely used in sero-epidemiological studies to assess the prevalence of SARS-CoV-2 antibodies in different population groups and areas. In a population where vaccines based on the spike protein are used, serological tests can identify the proportion of vaccinated (anti-spike antibodies only) and infected individuals (presence of anti-nucleocapsid and spike antibodies).
Anyways, I think the answer to your question is: in principle yes; in practice, because antibody levels diminish with time, it isn’t always that easy.
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Ask for the antibody test for the nucleocapsid (N) protein. You can only have those antibodies if you had an infection or you had a whole virus vaccine. Moderna, pfizer, and Jensen code for the spike protein only.
Yes, essentially everyone who had covid has antibodies to the nucleocapsid protein, as long as they have antibodies at all. But the vaccines don’t expose you to that protein (except the Chinese killed-virus vaccine, i suppose) they only expose you to the spike protein.
Back when the Red Cross was doing antibody tests on donated blood products there were three possible results; Negative, Positive, and Reactive. The Reactive result was what people who had been vaccinated and had not caught Covid got.
They’ve recently started doing Covid antibody tests again, for a limited time. They started on March 7th. The plan is to use blood with antibodies to help Covid patients.