Antibody Test for the Coronavirus Could Reveal Who Is Immune

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As the coronavirus pandemic drags on, governments and people around the globe are trying to stay safe while also figuring out what work, school, and socializing will look like with COVID-19 still spreading.

For public health experts to begin to understand whether regular interactions between people are safe enough to resume, they will need to know if people who were infected with SARS-CoV-2, the coronavirus that causes COVID-19, have developed some immunity to the virus.

Such immunity can theoretically be detected using what’s known as a serological or antibody test, which looks for proteins in your blood that your body produces after it has been infected with the virus. (This is different from the diagnostic tests that identify an active infection by looking for samples of viral genetic material, usually by sticking a swab far up the nose.)

Antibody tests may be especially important with this pandemic because it’s likely that there have been many undiagnosed cases in the U.S., according to W. David Hardy, an adjunct professor of medicine in the Division of Infectious Diseases at Johns Hopkins University School of Medicine in Baltimore. That’s largely because many people infected with SARS-CoV-2 don’t show clear symptoms of being ill and because diagnostic testing has not been widely and consistently available in the U.S.

But for now antibody tests aren’t accurate enough and don’t tell us enough about immunity to allow for people to change any of their behaviors. People who test positive still must abide by social distancing guidelines and use protective gear, such as face coverings, according to guidelines for antibody testing released Aug. 18 by the Infectious Diseases Society of America (IDSA).

Here’s what you should know about antibody tests for the novel coronavirus.

If You've Had COVID-19, Are You Now Immune?

One of the reasons antibody tests may be useful is to see who has developed immunity against SARS-CoV-2. But scientists don’t know yet whether those who have had COVID-19 develop long-term immunity against the virus, according to Maria L. Alcaide, an associate professor of clinical medicine and director of the Infectious Diseases Research unit at the University of Miami, and a fellow with the IDSA.

After you recover from a viral infection, your body’s immune system has several ways of preventing reinfection, including by developing antibodies.

Sometimes that protection lasts from months to a few years, as is the case with many of the viruses that can cause the common cold. People infected with the deadlier SARS and MERS coronaviruses tend to have some protection for two to three years after infection. With some other viruses, such as measles, people infected once are generally protected against reinfection for the rest of their lives.

It’s reasonable to think that people who were infected with SARS-CoV-2 will have some protection from reinfection for a period of time ranging from several months to several years, according to Alcaide and other experts. But she cautions that it’s still too soon to say for sure.

There have been several anecdotal reports of reinfection, though most probably stemmed from a false negative test that mistakenly indicated a full recovery, or from an infection that went dormant and then reemerged, experts say. There has been at least one documented case of reinfection, however. Months after recovering from being ill with COVID-19, a Hong Kong resident tested positive for a different strain of the virus, though he did not have symptoms the second time.

Still, most evidence continues to support the idea that people have some protection after they’ve developed antibodies to the virus. In one small study published Aug. 21 in the Journal of Clinical Microbiology, researchers documented how three crew members on a fishing vessel who already had antibodies didn’t get sick when an outbreak sickened 104 of the other 122 people on the ship.

While immunity is still “somewhat of an open question,” according to Eric Blank, chief program officer for the Association of Public Health Laboratories (APHL), initial research indicates that this virus may behave like other coronaviruses, where people have at least some short-term protection. “You might have milder symptoms than you may have experienced before,” he says. (The Hong Kong patient who caught the virus for a second time didn’t develop the disease, something that could be related to a protective antibody-related immune response.)

How Do Antibody Tests Work?

Though there are a wide variety of antibody tests available for the coronavirus, they’re all looking for the same thing: antibodies created by your immune system in response to the coronavirus infection, Hardy says.

The first of those antibodies don’t tend to show up until five to seven days after an initial infection, Alcaide says. That’s too late to make antibody tests an ideal tool for diagnosing disease, she says. Because antibodies persist after an infection is cleared, they’re better tools for widespread monitoring, to gauge how many people in a population have been exposed to a disease.

But scientists are still learning how long antibodies last and what that means.

So far, research confirms that most people develop antibodies after being sick with the coronavirus. The “sweet spot” for antibody detection—when levels are highest—appears to be three to four weeks after a patient develops symptoms, says Angela Caliendo, a professor of medicine at the Warren Alpert Medical School of Brown University in Providence, R.I. But even at that point, the best tests still are only sensitive enough to identify about 95 percent of coronavirus infections, says Caliendo, who spoke at a briefing explaining the IDSA’s guidelines for using antibody tests.

Within a couple of months, antibody levels may fall below the level at which they can be detected, and we have almost no data on whether antibodies last longer than three months, said Kimberly Hanson, an associate professor of internal medicine at the University of Utah School of Medicine in Salt Lake City.

If a Test Says You Have These Antibodies, What Does That Mean?

Everyone’s hope is that a positive antibody test means they can resume life in public without fear. But even if an accurate test definitively identified antibodies against SARS-CoV-2, that wouldn’t necessarily mean you are immune.

Even if tests can confirm the presence of neutralizing antibodies that will help fight the coronavirus, you still might need a certain level of them in your blood to have good protection against reinfection, these experts say. We don’t yet know what that level is, and only certain types of tests can measure it.

Eventually, scientists will have a better understanding of whether most people who were exposed to SARS-CoV-2 developed enough antibodies to protect against another infection. For now though, it’s likely that we’re still months away from knowing whether people are immune after infection, Florian Krammer, professor of vaccinology in the department of microbiology at the Icahn School of Medicine at Mount Sinai in New York City, said on Twitter on July 13.

Can You (And Should You) Get a Test?

First, it’s important to know that antibody tests not only provide limited information but also cannot be 100 percent accurate. The results can go wrong if the test isn’t specific enough, which means it could give you a false positive—making it look (falsely) like you’ve been infected and have some protection against reinfection. Tests may also give a false positive result if they identify antibodies against some other coronavirus, like the four that regularly cause common colds.

False negative results—where a test says you do not have antibodies, but you actually do—are also a possibility. This can happen if the test is done too early, before antibodies develop, or if it just isn’t sensitive enough to detect the antibodies in your blood.

It’s understandable, if you spent any part of 2020 coughing and feverish, that you might want to know if you had COVID-19. But right now, IDSA experts say most people shouldn’t bother getting an antibody test out of curiosity, or to try to see whether it’s okay to stop wearing a mask or social distancing. It’s not.

These tests “should not be used to determine immunity or risk of reinfection,” according to IDSA guidelines.

And remember, if it has been more than a few months since you got sick, your antibodies could be undetectable.

That doesn’t mean you won’t end up taking a test, as part of a study or if your doctor is trying to figure out whether an illness you’ve had for several weeks could be COVID-19. And hospitals and other medical centers that have developed their own tests are often offering them to their own staff as well as to certain patients, Hardy says.

If you have gotten a test, either from your doctor or from a testing provider, such as LabCorp or Quest Diagnostics, don’t expect to necessarily get clear answers. Be especially cautious of results from an older antibody test. Many of the first tests that were rolled out were fairly inaccurate, but the tests available now are better than they used to be.

If you’re unsure about an antibody test being offered at your job or at a clinic, you can check the Food and Drug Administration’s website to see if the test has an emergency use authorization (EUA) from the agency, which does not mean it is perfectly accurate but suggests that it may be of higher quality than one not on the list.

Does Insurance Cover Antibody Tests?

It’s a little murky. Under the Families First Coronavirus Response Act, private insurance, Medicare, and Medicaid should cover antibody testing (PDF) for COVID-19 as long as the tests have been authorized by the FDA or by your state, or have or are seeking an EUA.

Thus, as National Jewish Health of Denver says on its website, “If you are taking the [COVID-19 antibody] test with a physician referral, your insurance may cover it.”

But the law doesn’t require that antibody tests for uninsured people be covered. And insurers might not cover antibody tests ordered directly by a consumer, rather than by a healthcare provider. In general, tests directly ordered by a patient are that patient’s financial responsibility, according to Quest. HealthLabs, another company offering antibody tests directly to consumers, says it doesn’t accept insurance but will provide receipts that consumers can submit to their insurers.

So always check with your health insurer first.

When Will We Know More?

All the tests available right now don’t necessarily answer the questions that consumers want answers to. But as the FDA and other groups review data on the tests, they’re getting a better idea of how much we know and which tests work better than others.

These reviews have helped identify some of the key antibodies these tests should be looking for, according to Eric Blank at the APHL. “Things have changed for the better,” he says. But we still need to know how to make use of the results, since we still have more to learn about immunity.

Last, Hardy says, we will need a national strategy for who should get these tests first and how they should be administered.

What Else Will Antibody Testing Tell Us?

From a diagnostic perspective, the IDSA recommends using antibody tests to diagnose likely COVID-19 cases if someone tests negative using the molecular nasal swab test, especially if the patient has been sick for several weeks. Antibody tests can also help identify children with multisystem inflammatory syndrome, which can develop weeks after even an asymptomatic coronavirus infection.

But the main use of antibody testing is for surveillance: tracking the rate of infection in a community. Once accurate tests are in widespread use and conducted in a timely fashion, epidemiologists will be able to tell roughly how many people were infected with the novel coronavirus and how many people are still vulnerable to it, says Maria L. Alcaide at the IDSA.

If, in certain communities, the prevalence of people who have already been infected is very high, that community may be protected at least temporarily, she says. But if the prevalence is still low, “that will give an indication that the population is still at a very high risk of infection, and perhaps social distancing measures should be maintained,” she says.

This could be particularly important for figuring out how extensive the initial exposure rates were for communities of color that we already know have been hit particularly hard by the pandemic, Blank says.

In New York City, for example, some early data that the urgent care chain CityMD provided to the New York Times showed that in certain communities of color in hard-hit Queens, antibody levels among patients tested at CityMD topped 50 percent; at a CityMD location in a wealthier, predominantly white neighborhood in Brooklyn, only 13 percent of antibody tests came back positive.

When deciding when to relax stay-at-home and social distancing orders, health authorities are trying to avoid a second wave of infections that could overwhelm already stressed healthcare systems.

“That’s why doing these tests is so important before making decisions about reducing social distancing,” Hardy says. “It gives us an idea of what we’re dealing with.”

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