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October 6, 2021

What Covid-19 antibody tests can (and can't) tell you about immunity

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    Concerns about waning immunity against the coronavirus have led many people to seek antibody tests to determine whether they are still protected—but several health experts say it's not currently clear what antibody levels are needed for immunity. 

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    More patients seek out antibody tests

    As some groups become eligible for booster doses of the Covid-19 vaccine and breakthrough infections become more common, more people are turning to antibody tests to check if their immunity against the coronavirus is waning, NPR reports.

    According to NPR, antibody tests—unlike antigen or PCR tests, which detect active infection—are used to identify specific antibodies generated in response to a prior Covid-19 infection or vaccination.

    Currently, NPR reports, dozens of antibody tests, which use samples from a blood draw or finger prick, are available for purchase. Some of these tests report only whether antibodies are present, while others provide scores that reflect a person's current antibody levels.

    The problems with using antibody tests to determine immunity

    Although many people are seeking antibody tests, FDA, CDC, and the Infectious Disease Society of America do not recommend using these tests to assess immunity against the coronavirus, according to a report from Pew Charitable Trusts. According to health experts, there's still considerable uncertainty about what antibody levels represent sufficient protection against the coronavirus.

    "There is no test that will give you that (degree of certainty) at this point," Gigi Gronvall, a senior scholar at the Johns Hopkins Center for Health Security, said. "There are tests that would show that you've been vaccinated or not, but is that going to be the kind of information you need?"

    Separately, Ali Ellebedy, an immunologist at the Washington University School of Medicine in St. Louis, said currently available antibody tests are not designed to identify whether a person has enough protective antibodies against the virus, especially as variants continue to evolve. According to Ellebedy, being able to detect antibodies in a blood test six months after vaccination "only means your immune system mounted a successful response then and that you have immune memory."

    Another problem with antibody tests, NPR reports, is a lack of standardization. Different tests produce different results based on varying degrees of sensitivity and different outcomes. For example, some commercial antibody tests detect only antibodies that are generated from exposure rather than vaccination—which could lead a vaccinated individual to get a negative result and falsely believe they are not protected.

    In addition, health experts said antibody levels are only one part of the body's immune response against the coronavirus. This means that even with low detectable antibody levels, a person can still be protected against the virus.

    "Antibody tests—it's really probing just one part of your immune system," Elitza Theel, director of the Infectious Diseases Serology Laboratory at the Mayo Clinic, said. "Memory immunity remains strong," she added. "That's what we need to underscore to the public."

    When antibody testing should be used

    There are some instances, however, when an antibody test can be helpful. For instance, according to Mary Hopkins, associate program director of the infectious disease fellowship program at Tufts Medical Center, antibody tests are useful for testing patients for long Covid, in which symptoms of the disease linger months after infection.

    Hopkins explained that because PCR tests can only detect active infections, the best way to determine infection months after exposure is with an antibody test. "When I see a patient in my clinic who's short of breath or has mysterious aches, I give them an antibody test to see if it's long Covid," she said.

    In addition, physicians can use antibody tests to determine the level of infection in certain populations, such as nursing home residents and immunocompromised patients, or the degree to which antibodies have decreased over time.

    "We can test those individuals and show their levels are much lower than the general population and give them a boost and test them again and see if their numbers go up," said Thomas Denny, COO of the Human Vaccine Institute at Duke University School of Medicine.

    Similarly, several states have performed large-scale antibody testing on a targeted basis to collect data about antibody levels in a specific population, Pew Charitable Trusts reports.

    For example, Derrek Asberry, a spokesperson for the South Carolina Department of Health and Environmental Control, said the agency has conducted antibody testing for vaccinated people to correlate antibody levels with reinfections or breakthrough cases.

    And in Maryland, officials conducted antibody testing on more than 500 nursing home residents and found that half had declining antibodies over time. Based on these results, Gov. Larry Hogan (R) ordered booster shots for all older Maryland residents living in congregate housing.

    Looking ahead, Mary Hopkins said large-scale antibody testing could, down the line, help determine the specific antibody levels that provide immunity against Covid-19. With this information, antibody levels could be used as correlates of protection for Covid-19, just as they are used for measles and hepatitis A and B.

    "It would be wonderful and important if you could test every month 5,000 patients 60 years and older and find out who gets reinfections or infections for the first time and see if there is a correlation," Hopkins said. "If you found infections and could say those people had antibodies below 200, that would be helpful."(Stone, NPR, 8/28; Ollove, Pew Charitable Trusts, 10/1; Bean, Becker's Hospital Review, 10/4)

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