The COVID-19 antibody test, also known as a serology test, is a blood test that looks for antibodies to determine whether you have had a recent or past infection of SARS-CoV-2, the virus that causes COVID-19.
What are antibodies?
- Antibodies are proteins produced by your immune system to fight an infection.
- Antibodies can be produced after infection or vaccination. It may take about two to three weeks (or longer) for the body to develop enough antibodies to be detected in your blood. Antibodies can be detected in blood for several months or more after you have recovered from COVID-19.
- Antibodies may be present after:
- Mild or severe infection
- Asymptomatic infection
What is the antibody test used for?
- Assess people at high risk, like health care workers or frontline workers, and determine whether they have developed immunity to COVID-19 or are still prone to infection.
- Contribute to public health research and epidemiology. The study of antibody seropositivity helps researchers understand the extent of infection and immunity in a population.
- Determine whether a vaccine was successful or needs to be repeated.
How does an antibody test work?
Because antibodies are abundant in the bloodstream, an antibody test is usually a blood test. Antibodies are also present in saliva, but salivary antibody tests aren’t as efficient.
During an antibody test, technician will take a blood sample and test for one of the antibodies to the coronavirus:
- IgM antibodies develop early in an infection and indicate active or recent infection.
- IgG antibodies show up later. IgG antibodies are usually present 14 days days after symptoms start and can stay in your blood long after you have recovered from an infection. IgG antibodies can indicate past infection.
- Spike protein antibodies indicate immunity buildup in someone who has been vaccinated and can indicate the effectiveness of the vaccine.
- The coronavirus has two different types of proteins: spike protein (S protein) which is on the surface of the virus and nucleocapsid protein (N protein) which is inside the virus
- Since the coronavirus enters human cells through the S protein, most COVID-19 vaccines are designed to target this spike protein.
Types of antibody tests
Binding antibodies test
This test uses purified proteins of SARS-CoV-2 (not a live virus) to detect the presence of binding antibodies that attach to a virus, such as the S protein and N protein. The test does not indicate the extent or effectiveness of the immune response.
Neutralizing antibodies test
This is a newer and more sensitive test that detects a subgroup of antibodies that may inactivate the virus. It helps indicate how effective the antibodies are in blocking the virus to protect you from another COVID-19 infection.
You may be administered a neutralizing antibodies test after testing positive for binding antibodies.
What do your antibody test results mean?
Positive antibody test
If you test positive, this could mean:
- You have antibodies from an infection with the virus that causes COVID-19. This could be an infection from a different virus from the same family of coronaviruses.
- You have antibodies to the virus that causes COVID-19 that may provide some protection from getting infected again. However, it has yet to be confirmed how long the protection may last. A positive antibody test result doesn’t mean that you have built an immunity to COVID-19; a vaccine may still be necessary.
Negative antibody test
If you test negative, this could mean:
- You have never had COVID-19.
- You could have a current or recent infection.
- You have had COVID-19 in the past but have not yet developed a detectable amount of antibodies.
- Since it takes a few weeks for your body to develop antibodies, it is possible that your result is negative if it was taken less than 21 days after infection or less than 14 days after vaccination.
- Some people who are infected may not ever develop antibodies.
Are COVID-19 antibody test results reliable?
However, it’s important to keep in mind that COVID-19 antibody testing may have misleading results:
- False-positive: The test result may be positive, but you may not have antibodies to SARS-CoV-2 and may not have been exposed to the virus in the past.
- False-negative: The test result may be negative, but you may still be infected by SARS-CoV-2. You may not have a detectable level of antibodies, or the test was done too soon after infection.
Whether your antibody test comes back positive or negative, you may still contract the virus that causes COVID-19 or spread it to others.
Can antibody testing predict COVID-19 immunity?
Antibody testing alone isn’t conclusive enough to predict sustained immunity. It remains unclear how long antibodies stay in your body after a COVID-19 infection and how many antibodies are required to neutralize the SARS-CoV-2 virus after another exposure. Though rare, there have been some confirmed and suspected cases of reinfection.
Regardless of your test results, it’s important to take the necessary precautions to help protect yourself and others, such as physical distancing, hand washing and wearing masks in public.
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World Health Organization. "Immunity passports" in the context of COVID-19. https://www.who.int/news-room/commentaries/detail/immunity-passports-in-the-context-of-covid-19
United States Food and Drug Administration. Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers. https://www.fda.gov/medical-devices/coronavirus-covid-19-and-medical-devices/antibody-serology-testing-covid-19-information-patients-and-consumers
McIntosh K. COVID-19: Epidemiology, Virology, and Prevention. UpToDate. https://www.uptodate.com/contents/coronavirus-disease-2019-covid-19-epidemiology-virology-and-prevention