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Experts say COVID-19 antibody tests only confirm exposure to the coronavirus, not immunity. CErgin Yalcin/Getty Images
  • Food and Drug Administration officials say antibody tests should not be used to determine whether someone has immunity to COVID-19.
  • They say the tests only determine whether someone has been exposed to the coronavirus and not if they’ve built sufficient immunity.
  • Officials add the tests also have a higher false-positive rate than other tests.
  • They also note that people who do show antibodies on these tests should still get a COVID-19 vaccination.

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Officials at the Food and Drug Administration (FDA) say using COVID-19 antibody test results to evaluate a person’s immunity or protection from the disease is a bad idea.

The federal agency released a statement last week saying the tests weren’t a reliable indicator of a person’s immunity from COVID-19, even after the person has been vaccinated.

“Antibody tests can play an important role in identifying individuals who may have been exposed to the SARS-CoV-2 virus and may have developed an adaptive immune response,” said Dr. Tim Stenzel, the director of the Office of In Vitro Diagnostics and Radiological Health in the FDA’s Center for Devices and Radiological Health, in the statement.

“However, antibody tests should not be used at this time to determine immunity or protection against COVID-19 at any time, and especially after a person has received a COVID-19 vaccination,” he added. “The FDA will continue to monitor the use of authorized SARS-CoV-2 antibody tests for purposes other than identifying people with an adaptive immune response to SARS-CoV-2 from a recent prior infection.”

The agency pointed out that currently authorized COVID-19 antibody tests haven’t been validated to assess immunity or protection from the disease.

It also said the tests should only be ordered by healthcare professionals who are familiar with the use and the limitations of the test.

Antibodies are a blood protein the body produces in response to the presence of a specific antigen.

Antibodies combine chemically to fight off invaders, which include bacteria, viruses, and other foreign substances, that show up in blood.

Experts say antibody tests have been used during the pandemic to determine whether someone has been exposed to the virus, which is helpful when it comes to deciding who should quarantine.

“However, the mere presence of antibodies to a virus does not necessarily mean an individual has protective immunity to reinfection,” Ian Chan, CEO of Boston-based biotech developer Abpro, told Healthline.

“A correlation between positive antibody tests and protective immunity needs to be demonstrated for each individual test,” he said. “This has been demonstrated for a variety of viral infections such as the hepatitis B virus but needs to be determined for COVID-19 by clinical trials and epidemiological studies.”

Chan said there also tends to be a higher false-positive rate with antibody tests.

“Due to the current lack of current understanding if positive antibody tests correlate with protective immunity and the risk of antibody false-positive, it’s not advised to avoid a vaccine simply due to a positive antibody test,” he said.

COVID-19 is still relatively new enough that the necessary data needed to test for antibodies that actually block the virus is still being developed.

“The challenge with the current antibody tests is that very few of them actually test for neutralizing (receptor binding domain antigen) IgG antibodies, which are the only antibodies that actually block SARS-CoV-2 infection,” Gerald Commissiong, CEO of research company Todos Medical, told Healthline. “Most of the antibodies tests test for nucleocapsid antigen IgG antibodies, which may not neutralize the virus, and so it doesn’t make sense to rely on them to determine immunity status.”

“The FDA never said to rely on antibody tests to determine immunity, only to determine prior exposure. With the significant confusing and conflicting information, it is understandable the market doesn’t understand this,” he added.

Commissiong said antibody tests shouldn’t be used for vaccine avoidance, but they could be useful in considering whether someone should get a future booster shot.

“[It’s] effectively the same concept, because if you’ve been infected, the question is when does immunity wane?” he said. “Given how different everyone’s immune system is, time may not be the best factor when determining if a booster is needed.”

Even with vaccines being rolled out, researchers will still have their hands full with figuring out the ins and outs of COVID-19 for some time.

“A lot of work is being done to understand what threshold of neutralizing antibody levels are needed to establish immunity,” Commissiong said. “But we don’t have that data yet.”