What to know about antibody tests in a post-vaccine world

A Centers for Disease Control and Prevention panel once again reminded consumers this week not to use commercial antibody tests to check whether or not they are immune to COVID-19. These tests, the panel said Thursday, are not consistent and can't be used to measure protection against the virus.

Antibody testing is primarily used to determine if a person has previously been infected with the COVID-19 virus. Early reports suggested that people who tested positive for antibodies after either a known or asymptomatic infection had some level of immunity. Now, most health officials agree that having a positive antibody test does not mean a person is immune to contracting the virus.

"Antibodies after a period of time start to come down again, that's not a bad thing," said Dr. Todd Ellerin, director of infectious diseases at South Shore Health. But the presence of antibodies is only part of the story when it comes to immunity.

"Your antibodies are only one part of the immune system. You have other parts of the immune system as well, such as T-cells," said Ellerin. "T-cell response is probably very important to protecting against severe disease because they can attack the virus directly."

With high-profile breakthrough infections among sports teams and Olympians, many are wondering if they still have the same level of protection against the delta variant as when they first got their vaccines.

But scientists are still searching for a so-called "correlate of protection" -- a marker in the blood that would reliably indicate protection. And they don't necessarily think it's antibodies. That means the presence of antibodies -- or even more specific antibody levels -- does not necessarily correlate with a person's level of protection.

"The correlates of protection are not known, and so it is not known whether a positive antibody test after COVID-19 infection represents protection against any variant, including the delta variant," said Dr. Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center.

The Food and Drug Administration strongly warns against antibody tests as a way to check for immunity among people who are fully vaccinated, writing in May that "currently authorized SARS-CoV-2 antibody tests should not be used to evaluate a person's level of immunity or protection from COVID-19 at any time, and especially after the person has received a COVID-19 vaccination."

Experts agreed that this warning might be confusing for some consumers. After all, antibody testing is routinely done to test for immunity against other viruses such as hepatitis B, measles, mumps, rubella and varicella.

But COVID-19 is different, experts said.

"It is generally not recommended to use commercial antibody tests to monitor responses to vaccines, because the meaning of these assays has not been well defined in terms of whether they can predict vaccine protection or not," said Barouch.

We know the COVID-19 vaccines work not because of a blood test, but from large clinical trials that compared a group of unvaccinated people against a group of vaccinated people, finding that unvaccinated people were overwhelmingly more likely to become infected and fall seriously ill with COVID-19.

"What we generally think is that all of the vaccines get you to high-enough levels where even if they wane, you're still protected against hospitalization," Ellerin said. "Because the vaccines have done such a great job, all of them, at preventing hospitalizations, we think, based on some of the modeling studies, that you only need 3% of your average neutralizing antibody response to keep you out of the hospital."

Unfortunately, there is no commercially available antibody test that can tell us if we are truly immune to COVID-19 or how long any immunity gained through natural infection or vaccine-mediated will last.

According to the CDC and FDA, the only acceptable time to use an antibody test to check for immunity is if you have volunteered for a carefully monitored clinical study. Some people with immune compromised conditions -- like cancer or autoimmune diseases -- don't mount a good response to vaccines.

People with those medical conditions are being monitored for antibody levels using very specific antibody tests, and under the close supervision of doctors. And early data suggests that for some, a third shot might help.

No vaccine is 100% effective is preventing infections, but health officials continue to recommend vaccination as the best way to prevent COVID infections.

"Even if we find a correlate of immune protection, we want it to have clinical relevance, not just in vitro or test tube relevance," said Ellerin. "The national guidelines and I do not recommend antibody testing for the general public, but for immunocompromised patients that's a different story."

Odelia N. Lewis, M.D. is a recent graduate of SUNY Downstate's Family Medicine Residency Program and a contributor to the ABC News Medical Unit.