Antibody test won’t reveal if you gained immunity from your COVID vaccine, experts say

For the cautious and vulnerable, such as those with weakened immune systems, taking a coronavirus antibody test after vaccination might offer some assurance that their bodies built immunity against COVID-19.

But the tests are unnecessary and unreliable, and should not be used to determine how much protection someone gains from the vaccines, the U.S. Food and Drug Administration says.

That’s because the tests weren’t designed to reveal that kind of information; they should only be used to determine if a person had a coronavirus infection in the past, not a current one. What’s more, the test may provide false confidence that could put people in harm’s way.

“If antibody test results are interpreted incorrectly, there is a potential risk that people may take fewer precautions against SARS-CoV-2 exposure,” the FDA says. “Taking fewer steps to protect against SARS-CoV-2 can increase their risk of SARS-CoV-2 infection and may result in the increased spread of [the virus.]”

The Centers for Disease Control and Prevention also recommends against taking the test after coronavirus vaccination.

The vaccines produce specific antibodies that the antibody tests, ironically, cannot detect, according to the CDC. This means an antibody test result could come back negative after COVID-19 vaccination, which is known to spur the production of antibodies, in people without a history of previous natural infection.

“When most people sign up for [an antibody] test, most laboratories and providers are typically testing for anti-nucleocapsid antibodies,” Dr. Luis Ostrosky, an infectious disease specialist with UT Physicians in Texas, wrote in a blog post. “The problem with that is those are not antibodies that would be created by the vaccine, but only through natural infection.”

And if your test shows you have low antibody levels or none at all, “that does not mean that your vaccination didn’t work,” Ostrosky added.

Scientists are still trying to learn how long antibodies last after natural infection, but early studies show they linger for at least several months and offer some level of protection, although how much is still unclear. If the novel coronavirus is anything like others in the coronavirus family, such as the Middle East Respiratory Syndrome, then antibodies can last up to two years after infection, the CDC notes.

Similar questions remain about antibodies from vaccination.

Immunity against COVID-19 doesn’t only involve antibodies

There are several elements of the immune system that play a role in protection from viruses aside from antibodies, which naturally decline over time no matter the illness.

B cells and T cells, for example, are immune cells that communicate with each other to kick start a chain reaction of responses when exposed to a virus or bacterium. There are also “memory B cells” that can remember if you’ve been infected with or vaccinated against a pathogen.

A non-peer reviewed study of 185 people who recovered from COVID-19 found that antibodies started to decline about six to eight months after infection, while T cells only slightly diminished and B cells grew in number over time. In some other infections, B cell memory is “long-lived,” the study said, “including 60+ years after smallpox vaccination or 90+ years after infection with influenza.”

These other players may carry more important roles in immunocompromised people who face greater risks of coronavirus infection after complete vaccination than those without weakened immune systems.

Studies have found this population, including people with AIDS, cancer and organ transplants, may not respond to vaccines as healthy adults do, McClatchy News reported. Experts call the phenomenon “vaccine failure” or those who experience it, “vaccine non-responders.”

Ostrosky told NPR he thinks antibody tests among the immunocompromised might be helpful, giving them “some sense they may be protected,” given they are at risk of severe disease and death.

“If an immunocompromised patient does not have antibodies, doctors may consider bolstering their protection with another dose of the vaccine, continued masking and other precautions,” Ostrosky told the outlet.

But Dr. Ghady Haidar, an infectious diseases physician specializing in immunosuppressed populations at the University of Pittsburgh Medical Center, said people with weakened immune systems should avoid getting antibody tests after COVID-19 vaccination, which is a common approach for this group and other vaccines such as for hepatitis B.

“Please, please try not to get your antibodies tested if you are immunocompromised,” Haidar told McClatchy News in April. “Whether or not you make antibodies, please consider yourselves still somewhat at risk for COVID-19 and continue precautions.”

If you want to get an antibody test, Ostrosky recommends waiting two weeks after your final dose of a COVID-19 vaccine and finding a test that can detect at least some of the antibodies the vaccines produce.