Coronavirus Antibody Test: What Is It And What Does It Reveal?

Experts share what you need to know about the blood tests that show a COVID-19 infection and possible immunity.

By now, you’ve probably heard about the coronavirus antibody tests, and how they may be the best shot we’ve got at opening up the economy and returning to some sort of normalcy.

Researchers and hospital systems across the country have started conducting antibody tests on thousands of people to map out just how far the novel coronavirus spread over the past few months. Initial antibody test results out of California suggest that up to 4% of residents in hard-hit counties may have already had COVID-19, whether or not they realized it.

Though the tests are giving us a stunning snapshot of just how many people may have already been infected, too many unknowns still surround COVID-19 ― mainly the ”are recovered people immune?” question ― to say that antibody tests are our ticket out. But, used in conjunction with other testing and tracking efforts, they could very well help us get control of the pandemic.

Here’s how the antibody tests work, and what they mean for our health.

First, what is an antibody?

When a virus gets into the body, it attaches to a cell and begins to replicate itself.

At the same time, our cells realize that something’s not right and send out distress signals. Those distress signals zip through the body and jump-start the immune system, our body’s defense system against any harmful germ, virus or bacteria.

“They say, ‘Virus here!’ and the immune system gets those messages and shows up and starts trying to figure out what virus it is,” said Sheldon Campbell, associate director of Yale Medicine’s Clinical Microbiology Lab and associate professor of laboratory medicine at the university.

Then, depending on the virus and how dangerous it is, the immune system mounts a response that involves a whole bunch of different cells banding together to wipe out the invader. One of the system’s tools in its fight are antibodies, these large Y-shaped proteins that fuse with a part of the virus called the antigen, and disarm it, Campbell said.

Typically, the antibodies — along with all the other ammo the immune system has in its arsenal — control the infection and protect us from harm’s way.

And those antibodies don’t just do their job and disappear. They linger in the body, for varying amounts of time depending on the illness, so that if the virus strikes again, the body can act fast and knock it out. (For reference, SARS antibodies were maintained for about two years after infection, MERS antibodies seem to last at least 34 months. We still don’t know how long COVID-19 antibodies hang around.)

The tests examine a blood sample for COVID-19 antibodies

Antibody tests start with a physician drawing blood for a test, sometimes with just a finger prick. The blood is then taken to a lab, where the serum and plasma are isolated from the sample. The patient’s plasma is then exposed to a part of the coronavirus (the antigen) to see if the antibodies remember the virus. If antibodies to the coronavirus are present thanks to a past infection, they’ll stick and bind to the virus.

“What it tells you is if the person has been exposed to the virus and had an immune response to the virus,” said James Zehnder, director of clinical pathology at the Stanford University School of Medicine.

It’s worth noting that antibody tests are not the same as diagnostic tests, like the nasal and throat swabs. Antibodies tell us a person has been infected after the fact, not during.

It takes antibodies a couple of weeks to develop after being exposed to a virus, according to Akiko Iwasaki, a professor of immunobiology at Yale’s School of Medicine. Therefore, an antibody test should not be conducted on a person until they’ve been recovered for at least two weeks.

Antibody tests are giving us a snapshot of just how many people may have already been infected by the coronavirus, which could help us -- along with other measures -- get control of the pandemic.
solarseven via Getty Images
Antibody tests are giving us a snapshot of just how many people may have already been infected by the coronavirus, which could help us -- along with other measures -- get control of the pandemic.

There’s a catch with current coronavirus antibody tests

However — and this is a big however just because someone has antibodies for COVID-19 does not mean they’re out of the woods.

The presence of antibodies shows that a person has been infected with the virus before, but it does not guarantee that person is now immune. Nor does it tell us for how long they may be immune.

As Iwasaki puts it, antibodies tell us about the history of an infection. “Since we do not know what type of immune responses are needed to protect someone from COVID-19, antibodies alone may not tell us whether someone is protected from reinfection,” Iwasaki said.

Also concerning: People who have antibodies to the coronavirus could still be infectious and actively spreading the virus, Iwasaki added. We just don’t know yet.

Zehnder, who helped coordinate Stanford’s antibody test development, said it’s reasonable to assume the antibodies provide some level of protection, since most other viruses generally confer immunity after exposure. But out of an abundance of caution, we cannot say for sure if these antibodies grant immunity to COVID-19.

“It’s important to not make assumptions of being protected before you actually know because that could lead to dangerous behavior and spreading of a disease,” Zehnder said.

The antibody tests for COVID-19 aren’t perfect

Antibody tests can also be a bit glitchy.

“The immune system can be sloppy,” Campbell said, so we can’t always discern which specific antibody a test is picking up. The common cold is also a type of coronavirus, so many of the antibodies generated to fight that illness may also react to a COVID-19 test. That can result in a mix-up.

“They’re related and they’ve got bits in them that are very similar,” Campbell said, noting that researchers do try to differentiate the two as best as possible.

Also, some people with autoimmune diseases and other health conditions generate a ton of antibodies that bind and stick to anything, commonly causing false-positive tests, according to Campbell. A false positive could cause someone to think they’re in the clear, and presume they’re protected from reinfection when they’re really not.

On a bigger scale, risk exists that if and when we start conducting antibody tests on everyone, we could end up with a bunch of false positives. That could give people a false sense of security, Campbell said.

Antibody tests do give us an idea of how many people have been infected

The main use of antibodies tests, at least now, is to help researchers understand the prevalence of COVID-19. We’re quickly learning that many people who were symptom-free, or even tested negative for COVID-19 on a diagnostic test, actually had the infection.

With that information, we can get a clearer read on how infectious the coronavirus is.

“If there are enough tests, it would be important to expand the testing to everyone so we understand the extent of viral spread in populations in the future,” Iwasaki said.

Antibody tests alone can’t open up the economy, but used alongside diagnostic tests, contact tracing, distancing and quarantining, these measures can help health officials get a better handle on the coronavirus. If we want to open up the economy, we need to know exactly what is going on with the infection, Zehnder said.

“To imagine reopening the economy without having the ability to know what the status of the population is, who’s infected, being able to take care of that quickly … you’re flying blind without having the testing ability,” he said.

Experts are still learning about the novel coronavirus. The information in this story is what was known or available as of press time, but guidance concerning COVID-19 could change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.

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