Negative Antibody Test Post-COVID Vaccination
May 20, 2021 4:36 AM   Subscribe

I tested negative for COVID antibodies about two months after my second Pfizer shot. Should I be concerned?

I received shot 1 (Pfizer) on 2/17 and shot 2 on 3/10. Last week, without asking, my internist gave me the Quest Diagnostics "SARS CoV 2 SEROLOGY (COVID19) AB (IgG,IgM), IA" test and both IgG and IgA came up negative.

This concerns me because it leads me to one of two conclusions, neither of them good:

(1) (if the test only tests for antibodies arising from an actual infection) - He is routinely giving a test to vaccinated people that will sow doubt about their level of protection.

(2) (if the test tests for both) - I am not fully protected against COVID.

Have others had this experience? Should I be concerned? I will definitely call my doctor to ask, but he is pretty hard to reach.
posted by redondo77 to Health & Fitness (12 answers total)
Response by poster: Sorry, I meant to say both IgG and IgM tested negative (not IgA, which was not tested)
posted by redondo77 at 4:37 AM on May 20, 2021

I think you can’t make too much conclusion in any direction. The currently available serology tests are not intended to be used to test vaccinated people. FDA made a specific call out of this on their weekly town hall yesterday and sent this notice.
posted by Tandem Affinity at 4:53 AM on May 20, 2021 [14 favorites]

Best answer: More specifically, this snippet from the notice may apply to your situation (or there are other possibilities):
“ Be aware that vaccines trigger antibodies to specific viral protein targets. For example, currently authorized COVID-19 mRNA vaccines induce antibodies to the spike protein and not to nucleocapsid proteins that are likely detected only after natural infections. Therefore, COVID-19 vaccinated people who have not had previous natural infection will receive a negative antibody test result if the antibody test does not detect the antibodies induced by the COVID-19 vaccine. ”
posted by Tandem Affinity at 4:57 AM on May 20, 2021 [17 favorites]

Best answer: You should not be worried. Yesterday the FDA put out a statement that antibody tests should not be used to test people who received the COVID vaccine. The available tests were not authorized for this purpose and vaccinated people can test negative.
Well, you should be concerned about #1 in that your doctor is giving people a test they don't need and isn't accurate and presumably billing their insurance for it.
posted by emd3737 at 4:57 AM on May 20, 2021 [11 favorites]

I mean, if the blood draw was for diagnostics, wouldn’t it be helpful to know if the patient was experiencing any long term COVID symptoms? My understanding is the presence of antibodies is reliable (the person had COVID even if asymptomatic). But a negative test does not have the same conclusiveness.
posted by amanda at 6:29 AM on May 20, 2021

As noted above, this is a new area of science and clinical practice, and it's not yet known if the antibody tests are helpful.

There some possibility that your immune system is not working properly due to some unknown disease, some medication you've been treated with, or whatever. Your doctor should have taken a moment to interpret that chance in the light of any recent physical exam, other test results, and symptoms you may have reported. These would be things like unexplained weight gain or loss, swollen lymph nodes, out-of-range blood tests. There is no particular reason why he would talk to you about this sort of mental check unless you ask.

In your place, I'd keep up the anti-Covid avoidence measures like masks and avoiding crowds until the situation is more clear and/or the disease disappears from your community.
posted by SemiSalt at 6:48 AM on May 20, 2021 [1 favorite]

My understanding is that the vaccine teaches your body to make neutralizing antibodies, which are not tested for in the commonly available lab tests.

I’m sorry I haven’t read more deeply into this but it may provide you another path to finding information.
posted by bilabial at 7:13 AM on May 20, 2021

Response by poster: Thanks everyone! It sounds like the most likely explanation is #1, based on reading about the test more, the FDA statement, and speaking to a few other people. Still strange that these tests are being run without indication of prior infection, given that at this point the number of people who have been vaccinated is quite high. Seems likely to cause more problems (concerns about vaccine efficacy) than it addresses (not clear what it is accomplishing in this context).

Still being cautious about being indoors with others, but going to continue to assume that I am indeed vaccinated.
posted by redondo77 at 8:59 AM on May 20, 2021

My understanding is the presence of antibodies is reliable (the person had COVID even if asymptomatic).

If you have had the vaccine, an antibody test may be positive even if you hadn't had Covid--it could pick up the vaccine generated antibodies but depends what kind of vaccine you had, and what kind of antibody test it is.

The false positive rate in antibody tests is relatively high. Maybe a few percent IIRC. If you haven't had symptoms or any other reason to think you're infect but get a positive, this could translate into maybe a 25% chance that the result is wrong and you never had Covid.
posted by mark k at 9:19 AM on May 20, 2021

Still strange that these tests are being run without indication of prior infection

Except that asymptomatic COVID rates appear to have been very high, and we have no idea yet what it means to have had COVID unknowingly as far as long-term health. Also, it would let you know you are a candidate for antibody-positive blood donation, which there are still calls for in some regions. Plus, it's data, and data is good right now. I don't know that there's an overarching recommendation for standard antibody testing but it might be in the guidance from your city/county/state at this time.
posted by Lyn Never at 9:33 AM on May 20, 2021 [1 favorite]

Best answer: Antibody geekery follows, but the Quest test fitting that description is unsuitable to post-vaccination.

Your listing says IgG+IgM. Their tests: the IgG+IgM panel is IgG nucleocapsid + IgM spike. From two months ago, your IgM would be long gone, and you never saw nucleocapsid. What you presume you have is IgG to spike, and that's not in this test. (I'm curious why they test that particular combo.)
posted by away for regrooving at 10:03 AM on May 20, 2021 [1 favorite]

In addition to the above (correct) answers, the covid vaccine should also have taught your T cells and B cells how to recognise and deal with the virus, which will last for some time after the antibodies are gone. When you hear talk of vaccine booster shots perhaps being needed, that's partly in case that protection wears off. So it's being studied, and they're getting booster shots under way, ahead of knowing whether they'll be needed.

If you're immunocompromised, and worried about lack of antibodies for that reason, there is a T cell test that the FDA has authorized recently.
posted by joeyh at 4:07 PM on May 20, 2021 [1 favorite]

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