Does the 4th booster ever cause a positive test?

We got the (2nd) Moderna booster Friday. Today, Monday, Mrs. CC, older and immunocompromised, tested positive with a home test. Could that result be from getting a booster?

I’m not a doctor nor do I play one on TV, but I don’t see how an mRNA vaccine could cause a positive result on an antigen test.

I agree, I’m afraid it sounds like Mrs CC has Covid. Probably worth getting a PCR test to be certain. Hopefully she only suffers a mild dose.

Or a false positive; the newer lateral flow (‘at home’) tests have much better specificity but earlier ones were pretty poor in that regard, so it is always advisable to use multiple tests or a rtPCR (lab test) if unsure.


No, it is impossible for there to be any causal connection between an mRNA vaccination and a positive antigen test. See here for the structure of the virus:

Severe acute respiratory syndrome coronavirus 2 - Wikipedia

The mRNA vaccines code only for the S (spike) part of the virus, not the entire virus. The antigen tests detect the N (nucleocapsid) part of the virus, see here for example with reference to the Abbot test:

Multidisciplinary assessment of the Abbott BinaxNOW SARS-CoV-2 point-of-care antigen test in the context of emerging viral variants and self-administration | Scientific Reports

So the antigen test is detecting a part of the virus that is just not present in the mRNA vaccine.

There is certainly some non-zero false positive rate, as with any test. But there is no causal connection between false positives and mRNA vaccination.

Mrs CC should provisionally assume that she has COVID, take precautions and seek whatever medical care is appropriate given her condition, and get a PCR test to confirm.

From the CDC site [what to do if test positive]:… If you receive a positive test result, you should

Since she is older and immunocompromised it is really important to get in touch with her primary care provider right away so that she can be tested to confirm the result and be started on treatments to lessen the disease. These proven treatments need to started in a certain number of days so don’t wait ‘to see if anything bad is going to happen’, contact her doctor today.

You should also start techniques like in home masking and isolating to reduce the chances of you catching it while you are caring for her. The CDC site has details about home care when a family member is positive.

Call her doctor. Pursue confirmation then supportive treatment. Study home care and prevent you catching it. Take action, time and accurate information matters.

Has anyone even had a fourth booster? Aren’t we just now starting the second boosters?

Yeah - I meant 4th shot, 2nd booster. We’re a bit frazzled right now.

I should think so! Good luck with it.

Which reminds me - I’m planning to get my first booster tomorrow. I’ll need to get my vax card and bring it with me.

Huh. My doctor told me that someone who had gotten vaxxed would always test positive.

I trust y’all more.

ETA, maybe a different test?

What??? Are you sure your doctor said this nonsense?

An antibody test, maybe? That would make more sense.

Yes, because I just had my first incident of deep vein thrombosis. We were discussing what could have caused it, because no one in my family has ever had it, and my sitting habits haven’t changed. But I may have had a very mild case recently.