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March 25, 2022

Will an Antibody Test Reveal If You’re Immune to COVID-19?

Will an Antibody Test Reveal If You’re Immune to COVID-19?

Nearly two years into the coronavirus pandemic, millions of Americans got their COVID shots months ago. If you were vaccinated early on, you might be wondering if the vaccine really worked—and if it did, whether you are still protected months later.

Rest assured, based on efficacy data that continues to be collected, all three of the available vaccines protect the vast majority of people very well. Research shows the Pfizer-BioNTech, Moderna and Johnson & Johnson vaccines continue to prevent severe illness, hospitalization, and death from COVID-19.

There is some evidence, however, that the effectiveness of the Pfizer or Moderna shots may start to wane over time. And although the J&J shot seems to be as effective as it was months ago, it was never quite as effective as the Pfizer or Moderna vaccines. For these reasons, experts say additional doses, including booster shots, can help bolster protection against the disease.

Wondering how you fit into all of this—or if you have antibodies? You may be tempted to schedule an antibody test to help set your mind at ease. Unfortunately, it won’t give you the answers you’re seeking.

An antibody test can’t tell you with certainty whether you’re immune or if you could still get COVID and potentially spread the coronavirus to others. The results of an antibody test also won’t provide you with a green light to finally throw COVID-19 caution to the wind.

Here’s why.

Antibody tests can be useful in research studies when they’re compared across large groups of people. And if you get an antibody test, it may tell you if you have either been exposed to the coronavirus or received a COVID vaccine.

But the U.S. Food and Drug Administration (FDA) warned in May 2021 that antibody tests should not be used to evaluate a person’s level of immunity or protection from COVID-19. That’s because antibody test results can’t tell you if you’re immune to the disease.

This is true for five key reasons:

Scientists still don’t know exactly what level of antibodies is needed to be immune to COVID-19. Some antibody tests tell you only whether antibodies are present or absent, but the cutoff for “absent” can vary from one test to another. Others measure antibody quantity, but scientists aren’t sure what level of antibodies translates into immunity.

A low antibody level doesn’t mean you’re unprotected. It’s not just normal for antibody levels to fall—it’s expected. Antibody levels fall as part of the normal immune response after COVID infection or vaccination.

This happens for a good reason.

When the body first encounters an invader—or a piece of one in a vaccine—stem cells known as plasmablasts start pumping out all sorts of antibodies to attack it. This blanket approach helps ensure that at least a few of them are able to latch on the invading pathogen. These early-response antibodies degrade within weeks.

But the immune system’s job isn’t over. Another group of immune cells, called memory B cells, jump into action, learning from the initial attack. B cells spends months developing antibodies that are better able to target that invader. For example, second-generation antibodies might bind more tightly to some part of a virus. 

After maturing in lymph nodes, these memory B cells can be found in multiple parts of the body, including the bone marrow, blood and lymph tissues. If re-infection occurs, they are ready with antibodies that are tailored and more effective against the repeat invader.

An antibody test won’t detect these key immune cells.

It’s unclear how long immunity persists. Researchers are also still working to understand how long protection against COVID may last over time.

Although the presence of antibodies has been linked to a lower risk of infection, this is no guarantee. You could be reinfected. And depending on the circumstances, a test that shows you have antibodies might lead you to think you’re more protected from COVID-19 than you actually are.

As a result, you may be more likely to overlook or ease up on COVID precautions, which could put you at higher risk of being infected and potentially spreading the virus. For this reason, antibody test results shouldn’t be used to group people together in places like schools or dorms. They also shouldn’t be used to exempt someone from getting screened for COVID.

Not all antibody tests work as promised. There are many tests being manufactured by a variety of companies. Some are more reliable than others. Some tests, which have been carefully vetted, are used in large-scale research studies that track many people over time. But not all antibody tests have been authorized by the U.S. Food and Drug Administration (FDA).

False negatives are possible. So, you could test negative even if you have antibodies from a previous infection or vaccination. The false-negative rate for COVID antibody tests has been reported to be as high as 30 percent.

Meanwhile, some people may get a false positive result and test positive for COVID antibodies, but the test could actually have picked up antibodies against a different virus. For example, it might detect any of the four coronaviruses that cause the common cold. Fortunately, false positives are less common than false negatives.

Complicating matters, different tests could rely on different testing methods and report results in a variety of ways, making it hard to draw meaningful conclusions.

An antibody test may not measure the “right” antibodies. Neutralizing antibodies are the type of antibodies considered truly protective against COVID. But an antibody test may look for different antibodies. For example, the Pfizer and Moderna COVID vaccines teach the immune system to recognize the spike protein of the coronavirus. Meanwhile, a COVID infection may result in antibodies against other parts of the coronavirus, like nucleocapsid proteins. If an antibody test only detects nucleocapsid antibodies, a vaccinated person could test negative—even if this person has other protective antibodies.

Bottom line:
If you decide to get an antibody test, talk to a health care provider (HCP) and learn as much as you can about that individual test and how to interpret your results.

And regardless of what your test results show, it’s important to follow the CDC’s recommendations for protection from coronavirus, which includes wearing a mask in public or poorly ventilated or crowded indoors spaces, practicing good hand hygiene, and avoiding touching your face with unclean hands.

As researchers learn more about how the immune system responds to COVID-19 and as authorized antibody tests become widely available, they may eventually give people more useful information about their susceptibility to COVID-19. But we’re not there yet.

Sources:
U.S. Food and Drug Administration. “Antibody Testing Is Not Currently Recommended to Assess Immunity After COVID-19 Vaccination: FDA Safety Communication.” May 19, 2021.
U.S. Food and Drug Administration. “Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers.” May 19, 2021.
Callaway E. “COVID super-immunity: one of the pandemic’s great puzzles.” Nature. Oct. 14, 2021.
Centers for Disease Control and Prevention. “Using Antibody Tests for COVID-19.” Sept. 10, 2021.
Centers for Disease Control and Prevention. “Interim Guidelines for COVID-19 Antibody Testing.” Sept. 21, 2021.
Centers for Disease Control and Prevention. “Immunization Schedules: Table 1. Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger, United States, 2021.” Feb. 12, 2021.
Centers for Disease Control and Prevention. “Vaccines and Preventable Diseases. Diphtheria, Tetanus, and Whooping Cough Vaccination: What Everyone Should Know.” Jan. 22, 2020.
National Cancer Institute. “COVID-19, Vaccines, and the Immune System: Emerging Research from NCI’s SeroNet.” Aug. 30, 2021.
West R, Kobokovich A, Connell N, Gronvall GK. COVID-19 Antibody Tests: A Valuable Public Health Tool with Limited Relevance to Individuals. Trends Microbiol. 2021;29(3):214-223.
Shmerling RH. “Which test is best for COVID-19?” Harvard Health Blog. Jan. 5, 2021.
Centers for Disease Control and Prevention. “How to Protect Yourself & Others.” Aug. 13, 2021.
U.S. Food and Drug Administration. “Serology/Antibody Tests: FAQs on Testing for SARS-CoV-2.” May 19, 2021.
U.S. Food and Drug Administration. “Removal Lists of Tests That Should No Longer Be Used and/or Distributed for COVID-19: FAQs on Testing for SARS-CoV-2.” Oct. 22, 2021.
Clifford K. “Can antibody tests tell you if a COVID-19 vaccine worked?” MD Anderson Cancer Center. Mar. 29, 2021.
Abbasi J. The Flawed Science of Antibody Testing for SARS-CoV-2 Immunity. JAMA. Published online October 21, 2021. doi:10.1001/jama.2021.18919
Zhao E, Xu H, Wang L, et al. Bone marrow and the control of immunity. Cell Mol Immunol 9, 11–19 (2012).
Healy B, Khan A, Metezai H, Blyth I, Asad H. The impact of false positive COVID-19 results in an area of low prevalence. Clin Med (Lond). 2021;21(1):e54-e56.

 

 

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