Antibody Tests for COVID-19: What Can They Tell Us?

On July 31st, the FDA authorized the first two antibody tests designed to give more than a yes-or-no response. Instead of merely indicating the presence or absence of COVID-19 antibodies, these tests display an estimated quantity of such antibodies present in an individual’s blood. 

Since the beginning of April, the FDA has approved 33 antibody tests, all of which provide a positive or negative result. The two semi-quantitative tests are altered versions of previously approved assays developed by Siemens under their Atellica and ADVIA Centaur labels. 



The new assays are designed to detect IgG antibodies. These typically appear in the blood weeks after symptoms begin to show. Compared to other types of antibodies, IgG antibodies remain in circulation for much longer after an infection has subsided. 

So, why has the FDA introduced a semi-quantitative approach? What can antibody tests tell us that we don’t already know? The answers may not be as clear-cut as we’d like. 

Antibody tests reveal if someone has already been exposed to an infection. Lab-based assays detect the presence of Y-shaped proteins in an individual’s blood or serum. 

Antibody tests aren’t usually used to diagnose an ongoing infection. That’s because it takes some time for the body to generate antibodies. 

So, what can the number of antibodies in an individual’s blood tell us about their response to a previous SARS-CoV-2 infection?

In short, we don’t know yet. We’re still figuring out what the presence of antibodies may mean. Semi-quantitative antibody tests are simply another set of tools for scientists to study the effects of COVID-19 on the immune system. 

The FDA has warned the public that antibody test results do not indicate any level of immunity from the virus. Moreover, antibody tests do not detect active or asymptomatic infections—social distancing procedures and mask-wearing mandates should still be followed wherever possible. 



The body’s immune response to an infection is incredibly complex. New evidence shows that focusing on antibodies may not actually give us the full picture we’re hoping for. 

The human body’s first line of defense against a potential intruder is non-specific—the body simply makes itself as inhospitable as possible (e.g., raising body temperature). 

Once the innate immune system has been overwhelmed by a pathogen, the adaptive immune system kicks in. That’s when markers like protective antibodies and killer T cells emerge. Antibodies keep the virus from invading healthy cells, while killer T cells destroy cells that have already been invaded and infected. 

The goal is to stop the virus from spreading within our bodies. It makes sense, then, that antibodies and T cells would appear in tandem. That’s the way our immune systems typically respond—except, it seems, in the case of a COVID-19 infection. 

Until now, we’ve relied on antibody tests over T cell tests because the former are cheaper, faster, and easier to administer while the latter must be performed in a specialized lab. If antibody and T cell responses occur in tandem, relying on antibody tests only makes sense. But that’s no longer the case. 

Scientists around the world have uncovered a perplexing detail about the immune response to COVID-19. In a subset of people who tested positive for the virus, no antibodies were detected—only T cells. 

What does this mean about the body’s immune response to severe cases of COVID-19? Will this affect how a vaccine may or may not be effective? Much of the work that has been done is only preliminary.