FDA Authorizes First Test that Detects Neutralizing Antibodies

As the global tally for confirmed coronavirus cases passes 50 million and COVID-19 hospitalizations in the United States hit an all-time high, scientists are racing to develop both vaccines and tests that can tell us more about the deadly virus. A recent example includes an antibody (serology) test that earned emergency use authorization (EUA) from the Food and Drug Administration (FDA) on Friday, November 6.

The blood test was developed by researchers at the Duke-NUS Medical School in Singapore and issued to GenScript USA Inc. It is the first to detect neutralizing antibodies from recent or prior infection with SARS-CoV-2—the virus that causes COVID-19.

There are currently over 50 other serology tests that have been granted EUAs by the FDA too, but those tests look for binding antibodies. This new test, known as cPass, looks for neutralizing antibodies. What’s the difference? Let’s break it down.

Binding antibodies bind to the virus and do not necessarily decrease infection.

Neutralizing antibodies block the virus from entering and infecting human cells. 

“Neutralizing doesn’t mean killing [SARS-CoV-2]; it means preventing infection," James Crawford, MD, PhD, professor at the Feinstein Institutes for Medical Research and senior vice president of Northwell Health’s laboratory services, tells Verywell.

Regardless of whether they look for neutralizing or binding antibodies, antibody tests detect if someone has had previous exposure to COVID-19. Some experts say discovering neutralizing antibodies in someone’s blood may be a sign of potential immunity against the disease.

“Neutralizing antibodies do block viral entry, so they're indicators [of immunity]," Gigi Gronvall, MD, senior scholar and associate professor at the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, tells Verywell. "Are they only indicators and is there a particular level at which we know that there’s protection? We don’t know that yet, but this type of test will help to get at those questions."

The cPass test will make it easier to answer some of those types of questions partly because of its simplified process. According to a preprint of a study focusing on cPass, the new test, which is a surrogate virus neutralization test, can detect neutralizing antibodies without the “need to use any live virus or cells and can be completed in one to two hours” in most research or clinical labs. Previously, it would take researchers multiple days in a specialized laboratory using a live virus sample—meaning testing called for a much higher level of biocontainment to prevent the potential spread of deadly pathogens. 

“As a scientist, I think this will be a valuable assay to have on hand and to map out what we can learn about the diverse populations to then get a vaccine," Crawford says. "The expectation is that we’ll be able to measure neutralizing antibodies and this is a much simpler assay than doing a living viral culture assay."

How Long Can Neutralizing Antibodies Last Within the Body?

Even though studies are constantly offering conflicting information about the length of time antibodies can stay in our system, Gronvall says people shouldn’t get too hung up on a number because information about the virus is going to change over time. For a given virus, she says antibody levels typically peak at two- or three-months post-infection and then gradually wane as time goes on.

“[Antibody levels] decline but that doesn’t necessarily mean that’s the end of immunity,” Gronvall says. “Your immune system is very complicated. There are these things called memory cells, so even though the levels of the antibodies wane, your immune system has some memory and can be spurred into action upon contact with the virus again."

A Neutralizing Antibody Test May Help Assess Vaccine Performance

Measuring neutralizing antibodies in people after vaccination may give scientists a better look at how vaccine candidates hold up over time. Studies have already shown the potential value of developing vaccines that induce high levels of neutralizing antibodies, which could make cPass an important tool for evaluating their effectiveness.  

“Once vaccines start getting administered to millions of people, the question is going to be are we going to do testing on people to find out whether they are truly protected? Crawford says. "To be protected you need to measure neutralizing antibodies, not just any old antibody."

Gronvall adds that she is interested in seeing more studies that evaluate neutralizing antibody levels over time. That way, when levels inevitably dip, researchers will have a sense of when they're unacceptably low and a revaccination is needed. 

Although it’s unclear whether cPass will be used in a physician’s office any time soon, the FDA was clear in saying that just because someone has neutralizing antibodies, it doesn’t mean they are immune to COVID-19.

“Patients should not interpret results as telling them they are immune, or have any level of immunity, from the virus,” Tim Stenzel, MD, PhD, director of the Office of In Vitro Diagnostics and Radiological Health in the FDA’s Center for Devices and Radiological Health, says in an FDA press release.

The information in this article is current as of the date listed, which means newer information may be available when you read this. For the most recent updates on COVID-19, visit our coronavirus news page.