As the number of COVID-19 cases in the US surpasses 800,000, many Americans want to know if recovered patients have immunity to the novel coronavirus.

The answer could come from an antibody test.

Antibody tests measure the number of antibodies—proteins made by plasma cells—in the blood. The body's immune system uses antibodies to neutralize pathogens such as viruses and bacteria.

While these tests are commonly used to tell if someone is immune to diseases such as measles or chickenpox, they're not yet widely available for COVID-19. But it's not clear what the results would mean, said infectious disease specialist Rekha Murthy, MD, vice president of Medical Affairs and associate chief medical officer at Cedars-Sinai. 

"We have the expectation that a positive antibody test can be associated with protection against future infections. But since this pandemic has evolved so quickly, the data isn't yet available to back up this interpretation," Murthy said.

Cedars-Sinai Newsroom recently spoke with Murthy to learn more about antibody testing, whether it's ready for use in the battle against COVID-19 and why the public should be wary of unproven tests.

How is an antibody test different from a COVID-19 diagnostic test?

Antibody tests are not used to diagnose current infections with COVID-19. Diagnostic tests rely on recovering evidence of viral particles through collection methods such as the nasal swabs being used in the community or lung tissue samples collected from patients on ventilators. We run those samples through a process in the lab that allows us to detect bits of the novel coronavirus's genetic code. Antibody testing looks for antibodies in your blood that your body produced in response to an infection, which remain there for a certain period of time after you've recovered. Antibodies may not be detenced until one to two weeks after becoming ill with COVID-19, so an antibody test isn't a good diagnostic.

What needs to happen to confirm that an antibody test for COVID-19 works?

These tests need to be validated, meaning they have to be vetted by the US Food and Drug Administration (FDA) and the laboratories that perform the tests to ensure that they meet certain quality standards. Cedars-Sinai and other healthcare institutions have been working to validate tests. One of the ways we do this is by collecting antibodies from people in the hospital who tested positive for COVID-19 and from people who tested negative. Then we run the tests on the antibodies from both groups to see if they work consistently.

But what about reports of a variety of tests that supposedly identify COVID-19 antibodies?

As of today, very few tests have been reviewed by the FDA, and yet there's been an onslaught of tests being offered by many companies. Some of these non-validated tests are being promoted as being able to capture COVID-19 antibodies when they may actually be picking up other standard coronaviruses that cause the common cold. Always consult your medical provider before trusting a test you see on the market. Do not assume that any test being offered by someone who is not a licensed healthcare professional is validated and reliable. I would urge the public to get updates on testing from credible sources such as the Centers for Disease Control and Prevention. 

Is Cedars-Sinai currently offering COVID-19 antibody tests?

We've done some preliminary testing to validate new tests, and we've only offered these tests to selected healthcare workers so far. We're working to find a test that we can offer to patients more broadly.

Will these antibody tests eventually give us a simple "yes" or "no" answer as to whether someone has already had COVID-19?

It won't be a clear-cut "yes" or "no." Results from these tests have to be interpreted by the physicians who ordered them, and we need to develop guidance for physicians. We need studies and data to help us determine the level of antibody response that's reflective of a recent or past infection.

If someone tests positive for COVID-19 antibodies, will they be immune to the disease for life?

Again, we need more research to determine if an antibody response protects against future infections either by preventing infection, or whether it means that if you do get another infection it might be milder. The reality is that this virus may be more like influenza than the measles in terms of the way in which immunity is conferred. You get lifelong immunity after a case of the measles or getting a measles vaccine, but influenza is on the other end of the spectrum. The virus that causes the seasonal flu keeps changing every year. So even if you caught the flu last year or got the vaccine, you still need a new shot the following year. We don’t know yet where COVID-19 may be in that spectrum of short-term or long-term immunity.

What will it mean for society if many people test positive for COVID-19 antibodies?

We need more research to help us figure out if the presence of the right level of antibodies in enough people allows us to relax some social distancing and use of personal protective equipment such as masks. But first we need solid data. We may have a false sense of security without the science to back this up. Everyone, including physicians, wants to be able to offer these tests broadly to the public. We just have to do it the right way.