Home / Science / Health /  Covid-19 antibody tests in demand as people worry about immunity


Some people are taking Covid-19 antibody tests to determine whether they might be protected against the virus. Many health officials and doctors wish they wouldn’t.

Antibody tests are one tool some people are deploying to help them decide which precautions to take to protect themselves and curb the spread of Covid-19. Some vaccinated people say they want to know whether their protection has weakened to the point that they should get a booster, while some previously infected people say they want to measure the strength of the response the virus generated in their immune systems.

Katy Savage, 41 years old, doesn’t want to get a Covid-19 vaccine because she worries about side effects for her unborn child. She survived an August bout with the disease that put her on a ventilator for nine days and thinks immunity from that infection is enough to protect her. The Centers for Disease Control and Prevention has recommended since August that expecting mothers or women who plan to get pregnant take the vaccine, citing research showing the shots are safe and effective during pregnancy.

She plans to get a test later this month to see if she still has antibodies circulating in her blood.

“To me that would be, ‘OK, cool, I’m still good. I’m still safe,’ " Ms. Savage said.

But it isn’t known what level of antibodies effectively prevents infection or a severe case of Covid-19, said Emily Volk, president of the College of American Pathologists. She said getting a positive antibody test isn’t a substitute for getting vaccinated.

The CDC and the Food and Drug Administration discourage antibody testing to assess immunity after infection or vaccination in part because the tests can’t say how much protection those antibodies might provide.

Studies have found a connection between higher antibody levels, especially for so-called neutralizing antibodies that prevent the virus from entering cells, and increased protection against Covid-19. The specific threshold for what level of antibodies a person needs to be protected from infection or severe disease is still being determined in research.

“Whenever you order a laboratory test, typically you’d want to have in mind what you’re looking for," Dr. Volk said. “If you’re just sort of shooting in the dark and then getting a number that’s difficult to place into clinical context, it just adds confusion."

Antibody tests also don’t account for immune cells known as B-cells, which produce antibodies, or T-cells, which can help identify the virus or kill virus-infected cells.

“Antibody testing doesn’t give you a full picture of a person’s immune response to Covid. It just shows you a snapshot of one branch of the immune system," said Elitza Theel, head of the infectious diseases serology lab at the Mayo Clinic in Rochester, Minn.

The FDA has authorized about 90 antibody tests, also called serology tests, that search for Covid-19 fighting proteins that develop after an infection or vaccination. Most of the tests require a blood draw and are analyzed in a laboratory. Some require only a finger-prick, and one uses a saliva sample.

Patients often can access the tests through their doctors or companies including Laboratory Corp. of America, Quest Diagnostics or CVS Health Corp. They can cost anywhere from $40 to $150 and are sometimes covered by insurance. The volume of serology testing conducted by Labcorp jumped in August and September this year, according to a Wells Fargo & Co. analysis, though Quest didn’t see as much of an increase.

Antibody tests are useful in some contexts. Public-health researchers use them to estimate what proportion of a population has had a Covid-19 infection. Immunologists and other experts use them to study the immune response to Covid-19 after both infection and vaccination.

The tests can help rule out whether certain symptoms in children, such as fever with stomach pain or dizziness, are caused by a condition called multisystem inflammatory syndrome, which can develop after a Covid-19 infection. They also can help determine whether someone had Covid-19 if they didn’t get a viral test when they were sick.

Antibody tests can also benefit immunocompromised patients. Roberta Massaro, a retired pharmacy-inventory specialist in South Bend, Ind., takes anti-rejection drugs that suppress her immune system by killing the antibodies that might attack the transplanted liver she received in 2013.

After receiving her second vaccine dose in April, she took an antibody test that indicated she had Covid-19 antibodies in her system. “It was just for peace of mind," Ms. Massaro said. She still takes precautions such as masking indoors, but she said she had begun shopping again at the supermarket during more crowded times of day.

Some clinicians and pathologists said an antibody test about a month after a second or even third dose can help determine whether an immunocompromised patient has responded to the vaccine. If not, some doctors are cautioning those patients to act as if they haven’t been vaccinated.

“The antibody testing tells us who we need to look out for that will need help after vaccination," said Alan Wells, medical director of the University of Pittsburgh Medical Center clinical laboratories.

But Dr. Wells said checking antibody levels of patients outside of those specific cases has little benefit for now. Antibodies generally peak shortly after infection before dropping to a baseline level, so seeing a lower antibody response many months out from a vaccination doesn’t say much about how well that person is protected, Dr. Wells said.

For some viruses, including the hepatitis B virus, scientists have identified a level of antibody prevalence that corresponds with protective immunity. “We just don’t have that yet for Covid, so outside of public-health labs and big studies, there’s not a lot of clear use for the results right now," said Kelly Wroblewski, director of the infectious-diseases program at the Association of Public Health Laboratories.

Another challenge is that not all tests hunt for the same antibodies, and results aren’t uniformly reported across different tests. Ligia Pinto, director of the Vaccine, Immunity and Cancer Directorate at the Frederick National Laboratory for Cancer Research in Maryland is leading a government-backed effort to get laboratories and researchers to standardize antibody-test results to make it easier to compare results across different scientific studies and get a better understanding of the data.

To figure out the signals for protective immunity from infection or disease, researchers need to track certain parts of the immune response in large cohorts of patients over time and match them with cases of breakthrough infections or reinfections, she said.

“We have very strong tests," Dr. Pinto said. “We need to understand better what a certain level means."


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