Updated July 31, 2020.
Since the beginning of the COVID-19 pandemic, antibody tests have been touted as a way to potentially identify people who are immune to SARS-CoV-2 (the virus that causes COVID-19), and even as a way to reopen world economies by issuing digital “immunity passports” to people with detectable antibodies. However, experts strongly caution against using antibody tests in this manner, since we still don’t know whether these antibodies lead to actual immunity to COVID-19, and if so, for how long. We’re here to help you understand what antibody testing is, how it may be beneficial, and what its limitations are.
What are antibodies?
Antibodies, also known as immunoglobulins (abbreviated Ig), are infection-fighting proteins created by the immune system in response to the unique characteristics of an infectious agent. Over the course of an infection, the immune system produces different types of antibodies including IgM, which develops within a few days of the onset of an infection, and IgG, which develops several days or weeks later. IgG antibodies may confer immunity, or resistance to reinfection with the same virus, as is the case for diseases like measles, hepatitis A, and polio.
Because antibodies to a virus can only be produced if someone has been exposed to the virus or to a vaccine (which is still in development for COVID-19), the presence of antibodies to SARS-CoV-2 is a good indicator that the person has been infected with that virus at some point in the past. We do not yet know what level of IgG antibodies, if any, are indicative of immunity to COVID-19, but research is underway and we expect to have an answer to this question in the future.
Antibodies to some diseases last a lifetime, while antibodies to other diseases may disappear over time. At this point, it appears that levels of some COVID-19 antibodies decline dramatically after several weeks, but likely persist at low levels and could be quickly reproduced by the immune system if it encounters the virus again. We don’t know for sure whether antibodies to SARS-CoV-2 are long-lasting, or whether the severity of illness affects the durability or effectiveness of the antibodies.
What are COVID-19 antibody tests?
COVID-19 antibody tests, also known as serologic tests, detect antibodies specific to SARS-CoV-2 in a blood specimen. Most COVID-19 antibody tests used today are designed to detect IgG antibodies to a part of the virus called the nucleocapsid, also known as the N protein.
The most accurate and widely-available antibody tests use one of two technologies, ELISA (Enzyme-Linked Immunosorbent Assay) or ChLIA (Chemiluminescent Immunoassay), and require a sample of blood to be sent to a diagnostic lab with specialized equipment.
Alternatively, “point-of-care” test kits known as lateral flow immunoassays (LFIAs) use a drop of blood from a pricked finger and can give results within minutes. The accuracy and availability of these kits is variable.
How is antibody testing different from PCR tests?
While antibody tests measure the host’s immune response to an infection, PCR (polymerase chain reaction) tests detect the presence of genetic material from the SARS-CoV-2 virus itself, using swabs of the nasal passages or throat. PCR tests can identify active infections in their earliest stages, before symptoms begin and before the host produces any antibodies. PCR remains the standard tool for diagnosing active infections, including in asymptomatic persons, and isolating them to prevent them from spreading the virus to others.
What are the potential benefits of antibody testing?
Antibody tests have the potential to identify people who were previously infected with COVID-19 and have recovered. This can help resolve diagnostic uncertainty about a prior illness, and provides important information to epidemiologists trying to understand the extent of viral spread in a community.
Conversely, antibody tests can identify people who don’t have antibodies and who are therefore susceptible to COVID-19. Individuals who suspect they previously had COVID-19 but test negative for antibodies might be more likely to exercise caution in their daily activities, which can help slow the spread of the virus.
What remains to be seen is whether antibody tests can determine immunity to COVID-19, either due to having recovered from the illness or due to vaccination, once a vaccine becomes available. Most infectious disease experts agree COVID-19 probably does induce some degree of immunity in those who recover from it, but this has yet to be proven. Whether immunity to COVID-19 might last a few weeks, a few years, or for life is also an open question.
What are the risks of antibody testing?
Like all diagnostic tests, COVID-19 antibody tests have important limitations. An antibody test might miss a case of infection, especially if the test is performed within two weeks of developing symptoms, before any antibodies have formed, or after several months, after antibody levels have waned.
Additionally, an antibody test might mistakenly identify antibodies to a different coronavirus, such as one of the strains that causes the common cold, leading to a “false-positive” result. Such results could cause susceptible people to believe they’re immune to COVID-19 and they might stop physical distancing measures and put themselves into risky situations where they could become infected. The likelihood of false positive results can be reduced by using antibody tests that are highly specific (nearly 100% specificity) for SARS-CoV-2.
Even extremely accurate antibody tests could still be problematic, since we don’t yet know whether the presence of antibodies is indicative of true immunity and if it is, how long immunity lasts.
How will I know whether I'm immune to COVID-19
At this point, we lack definite proof that immunity to COVID-19 is possible, though many experts believe this to be the case based on indirect evidence. Hopefully, a simple antibody test will be able to identify people who are immune, but the test will need to be highly accurate and ideally will measure the actual concentration of antibodies in the bloodstream. Such tests are in development, and we will let our members know if they become available.
What’s One Medical's approach to antibody testing?
One Medical currently offers COVID-19 IgG antibody testing. If you feel you need an antibody test, please speak with one of our providers.
We’re constantly monitoring the scientific research related to COVID-19, including antibody testing. In late July, we learned that the antibodies to the coronavirus nucleocapsid (N protein) identified by the commercially available tests commonly decline rapidly after infection. This means many people who did in fact have COVID-19 may see negative test results if the test is performed more than a few weeks after symptoms or exposure.
If you have questions about your COVID-19 antibody test result, please read our guide to lab tests, and reach out to your provider if you have additional concerns.
What are the sensitivity and specificity of the tests being used?
The antibody tests used by Labcorp and Quest have been shown to have 99.9% specificity and 100% sensitivity across the brands they’re currently using. The “sensitivity” of a test refers to its ability to correctly identify those with the condition, while the “specificity” measures its ability to correctly identify those who don’t have the condition. A test that’s highly sensitive will flag almost everyone who has the condition and not generate many false-negative results. With a high-specificity test, you can be more confident that a positive result is accurate, and that false-positives are rare.
The One Medical blog is published by One Medical, an innovative primary care practice with offices in Boston, Chicago, Los Angeles, New York, Phoenix, Portland, the San Francisco Bay Area, Seattle, and Washington, DC.
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