During the rapid spread of COVID-19 across the globe, scientists are, with the help of antibody tests, in a race against the clock to find a way to identify those who have already been infected. People who have already recovered from COVID-19 are believed to be immune to the virus and could help restart the economy without risking reinfection.

Key to finding antibodies in a person’s blood, are serological tests. So far, those tests have been used to estimate how much of the population has been exposed in different areas.

Keep reading this article, and you will find out everything you need to know about coronavirus antibody testing.

What are antibody tests?

Within hours of an ‘invader’ such as SARS-CoV-2 infiltrating the body, the immune system starts mounting a nonspecific attack. This means that the body begins to send out Y-shaped molecules called antibodies. Those antibodies target the virus and bind to specific parts of it. Antibody tests are designed to specifically detect those molecules.

Usually, antibody tests are designed to detect one of two types of molecules, immunoglobulin M and immunoglobulin G. Within a few days to a week prior to a pathogen infecting the body, the immune system starts to produce a small amount of immunoglobulin M, then several days to a week later, the body starts sending out large quantities of immunoglobulin G. Because of the timeframe the immune system responds in, newly infected patients will get negative antibody test results.

How do antibody tests work?

Generally, there are two types of antibody tests used to test SARS-CoV-2: lateral flow immuno-assays and enzyme-linked immunosorbent assay (ELISA). Both basically work the same way: a person’s blood sample or serum gets washed over a surface holding the molecules that antibodies bind to. The moment when antibodies start binding to the target molecules, the test will show some sort of chemical reaction, like a change of color.

Lateral flow assays are easy and quick to use. ELISA tests, on the other hand, must be run in a lab. ELISA tests may be ran by using pipetting and plates and they require technicians to run. Also, results take about 2 to 3 hours to get.

Each different antibody test picks a specific part of the virus as their target molecule. SARS-CoV-2 antibody tests focus on the virus’ spike protein that it uses to invade cells. The spike protein has two subunits, S1 and S2, some tests bind to the S1 region. Others bind to a small part of S1, called the receptor-binding domain (RBD in short), RBD is the specific protein that latches onto the human ACE2 receptor to enter cells. The RBD is believed to be the most specific part of the antibody for SARS-CoV-2, other parts of this virus look comparable to other coronaviruses.

What makes an antibody test good?

Summarized, you want a test that is very sensitive and very specific at the same time. Which means you want to minimize the cases of false positives and catch as many truly infected cases as possible.

To make sure that the tests have good specificity and sensitivity, it is essential that manufacturers calibrate tests. To do so, manufacturers should use blood or serum samples from people who have been confirmed to have COVID-19 and make sure the test reads their values as highly positive. On the other side blood from non-infected people should be collected and tested, there should be made sure the test does not show positive on those results.

To keep antibody tests as accurate as possible, the tests should be calibrated again for every specific region, as the virus mutates depending on the region.

Does having antibodies mean I am immune?

Antibody testing has not been confirmed as an indicator for long-term or even short-term immunity. One person who has beaten COVID-19 may not generate any antibodies at all, but that may not mean they are not immune. On the other side, some people may develop antibodies, but those antibodies are not proven to neutralize the virus.

Reliable antibody tests can give an estimate on how many people have been infected, but they can surely not tell a person whether they are immune or not.