Antibody (serology) tests: What we know and don’t know

With community spread of COVID-19, many of us may wonder whether that cough we had a few weeks back was perhaps a mild case of COVID. Maybe you are a health care worker or other essential worker who wonders if you are immune and could care for someone who has COVID-19 or continue to work without the risk of getting re-infected.

There is clearly important value in understanding true individual immunity as well as the level of immunity to COVID-19 across the population.  But with the science on COVID-19 still emerging, there is still a lot we need to learn about how well antibody tests can help answer these questions.

About antibody tests and immunity
An antibody test, also known as a serological test, looks for antibodies in the blood that indicate whether a person has been exposed to a virus or bacteria. Antibodies are generated by a person’s immune system when you are fighting off viruses and bacteria.

A critical question that scientists are trying to answer is whether a person who has been infected with the virus that causes COVID-19 is immune from further infection or not and whether antibody tests accurately predict immunity.

Scientists don’t currently know how much immunity or protection results after a person is infected with SARS-CoV2, the virus that causes COVID-19, or how long that immunity lasts. However, based on other coronavirus infections, most feel that infection will produce some protection.

Enter the antibody test.
A new type of test that measures antibodies to the SARS-CoV2 virus has been getting a lot of attention lately and is becoming available, but there are important limitations.

Antibody tests may be useful in understanding if someone has been infected with COVID-19 in the past, because the tests indicate that a person’s immune system fought the virus. Because it takes time for antibodies to develop, it is important to remember that these tests are not reliable for diagnosing if a person who is currently ill has COVID-19, and these tests should not be used for that purpose. 

Reliable, validated antibody tests can provide important information about how many people in the population have been exposed to COVID-19 and may have some protection to future infections. However, we do not know how much, if any, protection against future COVID-19 infection may be provided by antibodies detected by different antibody tests.

“Serology (antibody) tests are useful in understanding how many people in a population may have been infected previously and who may have some immunity protecting against future infection. However, at this point we do not know how much, if any, protection against future COVID-19 infection is provided by antibodies detected by various serological tests,” said Dr. Jeff Duchin, Health Officer for Public Health – Seattle & King County.

“It’s important to understand that there is no reliable way at this time to know if someone is protected based on results of an antibody test, and any product that provides an “immunity certificate” or other statement indicating protection based on the test result is unlawful and should be reported to the FDA and the Washington State Office of the Attorney General.”

Limitations of the tests
There are many types of antibody (serology) tests and depending on the reliability of the specific test used, they may produce a falsely positive or falsely negative result. In other words, a negative test does not rule out that someone indeed had COVID-19, and a test may be positive even in someone who has not had COVID-19.

Most antibody tests have not been sufficiently evaluated to know how reliable they are. The Infectious Disease Society of America advises that antibody test results should not be used to make staffing decisions or decisions regarding the need for personal protective equipment.

Because of the COVID-19 emergency, the US Food and Drug Administration (FDA) has allowed serology tests to be marketed based on the manufacturer’s evaluation only and without the usual FDA evaluation – few serology tests have been evaluated by the FDA at this time.

FDA is currently working with the National Institutes of Health (NIH) and the Centers for Disease Control and Prevention (CDC) to evaluate serological tests.

The bottom line: Does Public Health recommend getting an antibody test?
Antibody tests can help us understand how widely SARS-CoV-2 has spread in the population.  If reliable tests were widely available, they could have a useful role in identifying people who may be at lower risk for infection and can return to work and other activities, although we don’t know enough about whether a positive antibody test indicates protection to make recommendations at this time. No one should draw definite conclusions about their protection from COVID-19 based on currently available tests.

Original posted April, 21, 2020