If you’ve been vaccinated against COVID-19, you may be thinking about getting an antibody test to see if the vaccine “worked.” Or, if you donate blood at MD Anderson Blood Bank or elsewhere, you may get back your antibody test results after you donate blood.

Antibody testing identifies individuals who may have developed an immune response after infection with SARS-CoV-2, the coronavirus responsible for COVID-19 disease.

But can these antibody test results really tell you if the COVID-19 vaccine was effective? Here’s why you shouldn’t be surprised to get a negative result even if you’ve received a COVID-19 vaccine.

What your COVID-19 antibody test results mean

A positive result suggests that you had COVID-19 at some time in the recent past, even if you didn’t show any symptoms. You’ll need to continue to practice caution in the community and protect yourself and others from potential exposure to COVID-19 by following safety precautions, including masking, social distancing and handwashing.

A negative result suggests that you haven’t had a recent COVID-19 infection. However, it doesn’t prove that there has been no prior or current infection. Antibodies could be present at levels below the test’s threshold for detection. It takes one to three weeks after an infection for antibodies to be detectable. In the months after an infection, antibody levels may decrease below the detectable level.

For either result, if you’re experiencing cough, fever, difficulty breathing, sore throat, loss of taste/smell, chills/muscle pain and/or gastrointestinal symptoms, self-isolate and contact your doctor.

Antibody testing isn’t conclusive for predicting COVID-19 immunity

While serology testing for COVID-19 is increasing in the United States, experts still have a lot of questions about what can be determined from the results.

Despite several versions of the antibody tests on the market from multiple manufacturers, current evidence suggests antibody testing alone isn’t conclusive for diagnosing a person with current or prior COVID-19 or for predicting a patient’s sustained immunity. It remains particularly unclear how long antibodies exist in a person’s body following a COVID-19 infection and how many antibodies are needed to neutralize SARS-CoV-2 following another exposure.

Can antibody tests determine if the COVID-19 vaccine was effective?

The Centers for Disease Control and Prevention (CDC) discourages antibody testing for assessing immunity after getting the vaccine.

A vaccinated person is very likely to get a negative result from a serology test, even if the vaccine was successful and protective. That’s because different serology tests detect antibodies to different parts of the virus.

Some tests detect antibodies to the spike protein of the virus, which are produced in response to viral infection or the vaccine. Others detect antibodies to a different part of the virus called the nucleocapsid protein, which are produced in response to infection, but not by the current vaccines.

MD Anderson’s Blood Bank uses an antibody test designed to detect antibodies to the nucleocapsid protein, which means donors who have received the COVID-19 vaccine will likely receive a negative antibody test result.

“A vaccinated person should not be alarmed or worried if they receive a negative antibody test result because this test does not detect antibodies from the Pfizer, Moderna and Johnson & Johnson’s Janssen COVID-19 vaccines, which were developed against the spike protein of the virus,” says Fernando Martinez, M.D., medical director of Laboratory Medicine. “This reinforces the guidance from the CDC that serology tests should not be used to test for immunity.”

Request an appointment at MD Anderson online or by calling 1-877-632-6789.

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