What is a COVID-19 Antibody Test and What Does it Mean?

Coronavirus Disease 2019 (COVID-19), spread by SARS-CoV-2, can be detected in people if they actively have the virus and if they have an immune response to the virus. The immune response is measured by assessing the number of antibodies in the blood of the person who may have had COVID-19. The antibodies measured in the blood are called IgM and IgG. Symptoms of COVID-19 usually begin after being infected for about five (5) days. IgM is present in the blood by about day 8 and then decreases shortly after. After about day 19 (around 14 days since symptom onset),  IgG is detectable in the blood.

 

At this point, it is unknown how long IgG will last in the blood. The IgG of other coronaviruses lasts about six (6) months. It is also unclear if the presence of detectable IgG will be protective against becoming reinfected with COVID-19. There may need to be a certain amount of IgG antibodies present to have a protective effect. A recent study demonstrated that monkeys with high levels of SARS-CoV-2 that cleared the virus and were reinfected had a significant decrease in SARS-CoV-2 levels during the second infection. This suggests there is a protective effect of IgG, however, more research is needed.

 

Getting an antibody test can tell you if you had COVID-19, whether or not you had symptoms. A positive test will tell you that you may have some protection against COVID-19, but should still take precautions to prevent getting it again. We do not know how long this can be protective for at this point, but many researchers are currently studying this. Having a positive antibody test will also allow that person to donate plasma to be used in the experimental treatment of COVID-19.

Figure 1. Antibody levels relative to SARS-CoV-2 infection onset. Source: https://www.diazyme.com/covid-19-antibody-tests

Chandrashekar, A., Liu, J., Martinot, A. J., McMahan, K., Mercado, N. B., Peter, L.,...Barouch, D. H. (2020). SARS-CoV-2 infection protects against rechallenge in rhesus macaques. Science, 1-11. doi: 10.1126/science.abc4776

Edridge, A. W. D., Kaczorowska, J., Hoste, A. C. R., Bakker, M., Klein, M., Jebbink, M. F., … van der Hoek, L. (2020). Human coronavirus reinfection dynamics: Lessons for SARS-CoV-2. doi: 10.1101/2020.05.11.20086439

Diazyme. (2020).  Why do we need antibody tests for COVID-19 and how to interpret test results. Retrieved from https://www.diazyme.com/covid-19-antibody-tests
U.S. Food and Drug Administration. (2020). Donate COVID-19 plasma. Retrieved from https://www.fda.gov/emergency-preparedness-and-response/coronavirus-disease-2019-covid-19/donate-covid-19-plasma

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