Millions of people have been tested for antibodies to the novel coronavirus, but researchers say receiving a positive result doesn't mean you should stop taking steps to protect yourself against the virus.

The 3 biggest questions about Covid-19 testing, answered

Are antibody tests reliable?

To address increasing demand and expand antibody testing capacity in the United States, FDA in March relaxed regulations that ordinarily would require manufacturers to receive the agency's approval before distributing the tests. The move allowed labs to start using the antibody tests without the products first undergoing a formal review to determine their efficacy.

This meant that some antibody tests on the market were not high quality, even though manufactures claimed otherwise—and rapid antibody tests, which are the most commonly used antibody tests in the United States, were proving to be the most unreliable, according to the World Health Organization (WHO). Some tests were even found to have reliability rates as low as 30% and false-positive rates as high as 5%.

Amid these concerns, FDA in May released stricter guidelines requiring companies that make antibody tests for the new coronavirus to submit emergency use authorization applications that prove their products meet new standards for accuracy. FDA said the tests will need to be able to detect antibodies to the new coronavirus at least 90% of the time or be able to avoid false-positive results at least 95% of the time.

So far, 18 tests have received a formal emergency use authorization from FDA. However, almost 200 antibody tests that have yet to be reviewed remain on the market, according to Bloomberg. And when Patrick Hsu, an assistant professor of bioengineering at the University of California-Berkeley, and colleagues analyzed 14 antibody tests for accuracy, they found that just three produced accurate results.

Do antibodies provide immunity?

Even if a person receives an accurate antibody test result, researchers still don't know whether having antibodies to the new coronavirus provide any immunity to reinfection.

To be immune to a virus, someone would have "to have some blood measure that serves as a correlate of that protective efficacy or immunity," according Sarah Fortune, the chair of immunology and infectious diseases at Harvard University's T.H. Chan School of Public Health. "Which sounds like it's simple, but it's much more complicated than you'd think."

That's because "[k]nowing the correlates of protection is different from knowing the mechanism of protection," STAT News reports—and antibodies are just one piece of the puzzle when it comes to immunity.   

That said, researchers studying people who have recovered from Covid-19, the disease caused by the virus, have found that the majority of patients who had the disease, including patients with mild cases, generated antibodies, STAT News reports. And, according to Marc Lipsitch, an epidemiologist and infectious disease specialist at the T.H. Chan School of Public Health, research on similar viruses indicates that "[a]fter being infected with [the new coronavirus], most individuals will have an immune response, some better than others. That response, it may be assumed, will offer some protection over the medium term—at least a year—and then its effectiveness might decline."

However, immune responses tend to vary from person to person, and it's possible that the amount of antibodies a person has could determine whether he or she is protected against the new coronavirus, according to STAT News. Researchers who tested the progression of Covid-19 in monkeys found that the higher the level of antibodies the animals had, the more protected they were against the virus. Those results reinforced the idea "that neutralizing antibodies to the virus can protect" against reinfection, but also brought up new concerns regarding the amount of antibodies a person would need to be immune, according to Dan Barouch, director of Beth Israel Deaconess Medical Center's Center for Virology and Vaccine Research.

"Some people … had high antibody titers or have had low antibody titers," said Anna Durbin, a vaccine researcher at Johns Hopkins University. "We still don't know what's going to happen to them if they're re-exposed."

In addition, researchers have explained that other factors that aren't measured by antibody tests—including types of antibodies, cytokines, and other immune cells such as T cells and B cells—could contribute to patients' future protection against the novel coronavirus. "There are a whole bunch of other things that people look at for correlates of protection," said Angela Rasmussen, a virologist at Columbia University.

"People want answers: Am I immune? Can I go back to work? But the story's not quite so simple," Hsu said.

So what should you do if you test positive?

According to public health experts, due to the current lack of understanding on coronavirus immunity and the potential for a false-positive test result, a positive antibody test result provides patients with "little if any actionable information," and should not lead to a change in behavior, Bloomberg reports. Health professionals say people who test positive for the antibodies should continue to wear face masks or coverings and practice social distancing. CDC also has said people should not rely on antibody tests to determine if they should go back to work.

People "mistakenly view antibodies as a get-out-of-jail-free card to return to normal life, a dangerous misconception," said Rep. Raja Krishnamoorthi (D-Ill.).

Caesar Djavaherian, the co-founder and medical director of Carbon Health Medical Group, said the lack of knowledge surrounding coronavirus immunity has caused some providers to consider "not offering them at all." Djavaherian said, "We're worried people will get a positive test and go about their business thinking they have immunity to the virus…The only thing worse than no data is bad data."

However, Andrew Diamond, CMO of One Medical Group, said the majority of people "end up with a negative test," which he says tends to push people to take more precautions against the novel coronavirus. "The number of people who think they've had it is far greater than the number who actually had it. Those people do change their behavior. They say, 'You know what, I should take this more seriously. I was getting cavalier out there, but now I'm going to wash my darn hands and wear this mask'" (Brown, Bloomberg, 6/16; Joseph, STAT News, 6/11, Abbott et al., Wall Street Journal, 6/10).