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If you’ve taken a COVID-19 antibody test, you may be confused about what your results mean. Do high antibody levels mean you’re immune from getting COVID-19? Do you have an active infection? Do you still need to receive a vaccine?
Here’s what you should know about your test results and what they may indicate about your susceptibility to COVID-19.
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What are antibodies?
Antibodies, also called immunoglobulins (Ig), are Y-shaped proteins that the immune system uses to identify and destroy foreign invaders, such as bacteria, viruses, or allergy-causing substances (Justiz Vaillant, 2021).
When a virus (or another invader) enters the body, part of the virus binds to B-cells. B cells (also called B-lymphocytes) are a type of white blood cell that helps your body fight infections. When the virus binds to B-cells, the cells divide and change into plasma cells, which make antibodies. The antibodies travel through the body to find more of the same virus and target them for destruction (Justiz Vaillant, 2021).
After an infection (or receiving a vaccine), your body has circulating antibodies against that particular virus or bacteria. These antibodies can help protect your body against future infections from that particular virus or bacteria (Justiz Vaillant, 2021).
Types of antibodies
There are five main types of antibodies. Each works differently to either directly eliminate or produce an immune response against the invader (Justiz Vaillant, 2021; Schroeder, 2010).
- Immunoglobulin M (IgM): IgM antibodies are the first antibodies produced after the body recognizes an invader. These antibodies help fight off an infection before other antibodies are made.
- Immunoglobulin G (IgG): IgG is the most common antibody and lasts the longest. It is also the only antibody that crosses the placenta, which allows a mother’s protective antibodies to be delivered to the baby.
- Immunoglobulin A (IgA): IgA antibodies are found in body fluids, such as saliva, tears, and breastmilk, and in the gastrointestinal, genital, and respiratory tracts. These antibodies help eliminate foreign substances before they can travel further into the body.
- Immunoglobulin E (IgE): IgE antibodies are involved in producing an allergic reaction (you can thank them for your runny nose and itchy eyes during the springtime). They also help fight against parasitic infections.
- Immunoglobulin D (IgD): The role of IgD antibodies is not as well understood, but they may help B-cells differentiate into antibody-producing plasma cells.
What is a COVID-19 antibody test?
A COVID-19 antibody test (serology test) is a blood test that looks to see if you have antibodies specific for SARS-CoV-2 (the virus that causes COVID-19). You can develop antibodies against SARS-CoV-2 if you were previously infected with COVID-19 or have received a COVID-19 vaccine (CDC-a, 2021).
Antibody tests typically detect either IgM or IgG (or both). IgM antibodies form 5–10 days after the initial infection, while IgG antibodies can take at least 7–10 days before they are produced (West-a, 2021). For this reason, it is best to wait at least 14 days after any symptoms begin before taking an antibody test to ensure you receive the most accurate results (West-b, 2021).
COVID-19 antibody testing is different from diagnostic testing (viral testing). Antibody testing can’t determine if you have an active infection. On the other hand, viral testing looks for evidence that the virus is currently in your body (CDC-a, 2021).
Understanding antibody test results
After taking a COVID-19 antibody test, it is essential to understand what your results mean (and what they don’t).
Positive test result
A positive test result means antibodies against SARS-CoV-2 were detected in your blood sample. You may have SARS-CoV-2 antibodies from past infection with COVID-19 or if you have received a COVID-19 vaccine. Not all tests identify the antibodies produced from vaccination, so be sure to speak with your healthcare professional to ensure the correct test is ordered (CDC-a, 2021).
It’s also possible that a positive antibody test could indicate a current infection—especially if the test is looking for IgM antibodies (the first antibodies produced during an infection). However, it’s best to receive a diagnostic COVID-19 test, not an antibody test, if you feel sick (West-a, 2021).
Sometimes a test may produce a false-positive result. This means you received a positive test result without actually having antibodies against COVID-19. A false-positive could happen if your body has produced antibodies against another coronavirus, such as one that causes a common cold (Liu, 2021).
Unfortunately, the presence of antibodies does not necessarily mean you are immune to COVID-19. Researchers don’t know how long the antibodies last or if you can become infected even if you have antibodies (West-a, 2021). The Centers for Disease Control and Prevention (CDC) recommends all eligible people receive a COVID-19 vaccine and continue to follow guidelines for masking and social distancing, regardless of antibody status (CDC-b, 2021).
Allergies vs. COVID-19: how to tell the difference
Negative test result
If you receive a negative result, this means the test did not detect any antibodies against the COVID-19 virus. This could be due to one of several reasons (CDC-a):
- You were never infected with COVID-19 and have not gotten a COVID-19 vaccine.
- You tested too soon after the infection, and your body has not had enough time to make antibodies.
- You were infected, but your body did not produce detectable levels of antibodies.
- You were infected, but your antibody levels have declined.
- You received a false-negative result, meaning you have antibodies, but the test is inaccurate.
Values and ranges
Some COVID-19 antibody test results provide a numerical value or range indicating the number of antibodies in your blood. Because we are still learning about this disease, it is unknown what antibody level is needed for immunity. These values are primarily used for research purposes but may become helpful in the future to determine your immune status after receiving a vaccine (West-b, 2021).
How long do antibodies last?
Researchers don’t know for sure how long antibodies against COVID-19 last. Studies have shown that people who developed more severe symptoms tended to have higher antibody levels that stayed in the body longer compared to people who developed mild symptoms. Some evidence suggests that antibodies may protect you against reinfection for at least 3–6 months, but how new strains of the COVID-19 virus will impact this is unknown (CDC-a, 2021).
How to get a COVID-19 antibody test
A COVID-19 antibody test must be given by a licensed healthcare provider. Talk with your healthcare provider to determine if an antibody test is appropriate for you. Tests are available at local clinics, lab sites, and drug stores.
Protecting yourself against COVID-19
At the start of the pandemic, many people hoped antibody testing could provide information about who had developed immunity against COVID-19. Unfortunately, we still don’t know enough to determine what level of protection antibodies offer. The best way to protect yourself and others is to receive your COVID-19 vaccine and continue following CDC guidelines for masking and social distancing. If you’re interested in receiving an antibody test, talk with your healthcare provider to determine if the test is appropriate for you and what your results could mean.
- Centers for Disease Control and Prevention (CDC)-a. (2021). Interim guidelines for COVID-19 antibody testing. Retrieved Aug. 31, 2021 from https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html
- Centers for Disease Control and Prevention (CDC)-b. (2021). Test for past infection. Retrieved Sept. 1, 2021 from https://www.cdc.gov/coronavirus/2019-ncov/testing/serology-overview.html
- Justiz Vaillant, A. A., Jamal, Z., & Ramphul, K. (2021). Immunoglobulin. [Updated Jul 28, 2021]. In: StatPearls [Internet]. Retrieved on Sep. 3, 2021 from https://www.ncbi.nlm.nih.gov/books/NBK513460/
- Liu, G. & Rusling, J. F. (2021). COVID-19 antibody tests and their limitations. ACS Sensors, 6(3), 593–612. doi: 10.1021/acssensors.0c02621. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33544999/
- Schroeder, H. W., Jr. & Cavacini, L. (2010). Structure and function of immunoglobulins. The Journal of Allergy and Clinical Immunology, 125(2 Suppl 2), S41–S52. doi: 10.1016/j.jaci.2009.09.046. Retrieved from https://pubmed.ncbi.nlm.nih.gov/20176268/
- West-a, R. M., Kobokovich, A., Connell, N., & Gronvall, G. K. (2021). Antibody (serology) tests for COVID-19: a case study. mSphere, 6(3), e00201-21. doi: 10.1128/mSphere.00201-21. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33980676/
- West-b, R., Kobokovich, A., Connell, N., & Gronvall, G. K. (2021). COVID-19 antibody tests: a valuable public health tool with limited relevance to individuals. Trends in Microbiology, 29(3), 214–223. doi: 10.1016/j.tim.2020.11.002. Retrieved from https://pubmed.ncbi.nlm.nih.gov/33234439/
Dr. Steve Silvestro is a board-certified pediatrician and Manager, Medical Content & Education at Ro.