You had a weird cold a few weeks ago. You were wracked with fatigue and had trouble breathing. Oddly, you suddenly couldn't taste your favorite foods either. You're recovered now, but could you have had COVID-19?
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One way to find out is to get a COVID-19 antibody test.
But if you test positive, does it really mean that you're protected against the virus? We talked to experts to find out what a positive COVID-19 antibody test really means, and some of its limitations.
Get tips on how to stay healthy, safe and sane during the novel coronavirus pandemic.
What Are COVID-19 Antibody Tests?
When you get sick, your immune system mounts an attack against the foreign invader. Antibodies are key players in your body's defense. These proteins not only help fight infection, but they often provide protection against becoming sick from that disease again.
Most COVID-19 antibody tests look for IgM and IgG antibodies that developed in response to SARS-CoV-2, the virus that causes COVID-19. IgM antibodies are usually the first antibodies to arrive on the scene. Then, they switch to become IgG antibodies, which are longer-lasting and more specific to the virus.
"IgG do the heavy lifting for the most part for this virus. Diagnostically, they are the ones really worth talking about," says Alex Greninger, MD, PhD, assistant director of the clinical virology laboratories at the University of Washington Medical Center.
While antibody tests can identify people who have likely been infected with COVID-19, timing of the tests matters.
Researchers reviewed COVID-19 studies published through the end of April 2020 that reported on antibody tests, for a June 2020 study in Cochrane Systematic Review. What they found was that the tests were better at detecting antibodies two or more weeks after symptoms first appeared. In the first week after the onset of symptoms, tests only detected 30 percent of people who had COVID-19. The accuracy increased to 70 percent in the second week and more than 90 percent in the third week.
"When you look at the data, the ability of the test to detect the disease increases with time," says Jon Deeks, PhD, professor at University of Birmingham in the United Kingdom and lead author of the study. "It's not surprising. You don't start with antibodies. You have to build them up."
In other words, don't get an antibody test right after you start experiencing symptoms.
What if You Have a Positive COVID-19 Antibody Test?
The short story: You likely were previously infected with SARS-CoV-2, the virus that causes COVID-19. But it's also possible being vaccinated will lead to a positive result on an antibody test, per the Centers for Disease Control and Prevention (CDC).
As you can see the meaning of a positive test isn't exactly black and white. So what should you do — and not do?
Do Understand That Antibody Tests Aren’t Used to Diagnose COVID-19
That's what viral detection tests are for. Tests like reverse transcription-polymerase chain reaction (PCR) tests look for the virus in samples taken from a swab of your nose. Doctors can use this information to determine if you are currently infected with COVID-19 and potentially contagious. PCR tests are generally more sensitive if performed closer to the time when symptoms first appear, according to the University of California, San Francisco.
Antibody tests, on the other hand, can help identify if you were infected by the SARS-CoV-2 virus by looking for antibodies, even if you never developed symptoms. These tests can compliment PCR test results. For example, if you had COVID-like symptoms and received a negative PCR test, there's a chance it may be a false negative. Your doctor may suggest an antibody test to confirm whether or not you've had a COVID-19 infection.
Don’t Use an Antibody Test to Check if Your Vaccine Was Effective
As mentioned above, there's a chance that getting a COVID-19 vaccine will result in a positive antibody test, according to the CDC. But that doesn't mean you should use an antibody test to confirm if your vaccine worked, per CDC guidance.
More research is needed to understand how to interpret antibody test results for people who have been vaccinated, per the U.S. Food & Drug Administration (FDA).
Don't Assume You Won't Get Reinfected
"Just because you're antibody positive does not mean that you're protected from reinfection," says Elitza Theel, PhD, director of the infectious diseases serological lab at Mayo Clinic.
That said, research is promising.
One observational study looked at more than 3 million patients who had an antibody test. Compared to people with a positive antibody test result, people who did not have antibodies were 10 times more likely to get COVID-19, according to the May 2021 study in JAMA Internal Medicine.
And, a previous COVID-19 infection is associated with a 84 percent lower risk of reinfection, per an April 2021 study in the Lancet. The study followed participants for 7 months after their initial infection.
There's still a lot scientists don't know or understand about COVID-19 and the antibodies your body produces to fight off infection.
"The questions people are asking are how long are these antibodies going to last and what's the critical threshold that you need to block the virus," Dr. Greninger says.
And people seem to respond to SARS-CoV-2 differently. People who were asymptomatic (they didn't exhibit clinical symptoms of COVID-19) launched a weaker immune response — including lower levels of antibodies — compared to those who developed symptoms of the disease, in a June 2020 study in Nature Medicine. In contrast, people who had severe and critical cases of COVID-19 had higher levels of certain antibodies compared to those with mild cases, per a May 2020 study in Clinical and Translational Immunology.
Do Continue to Follow Public Health Guidelines
Because you could get reinfected, the CDC recommends that you continue to take precautions to prevent catching or spreading the virus.
With a positive antibody test, "I don't think you can go very much beyond saying you've got antibodies at the moment. We don't really know what that means, so please don't put yourself at risk," Deeks says.
If you are fully vaccinated, however, whether or not you have a positive antibody test, you can largely return to living the way you did prior to the pandemic, according to the CDC. You will still need to abide by local regulations and wear a mask on public transportation.
Don't Forget About False Positives
Even the best tests will produce some false positives, meaning the test will return a positive result even though you don't actually have antibodies. And thinking you have developed antibodies may give you a faulty sense of security.
False positives are particularly a problem in communities with few confirmed COVID-19 cases. Your chances of getting a false positive is higher, even if the test has a high specificity. It has to do with what's called the positive predictive value or the probability that if you test positive, you truly have antibodies to the virus.
The positive predictive value depends on the prevalence of the virus in the community. Where the prevalence rate is low there will be a higher false positive rate, Dr. Greninger explains. However, in locations where the prevalence is high, the tests are more likely to predict an accurate result.
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- Cochrane Database of Systematic Reviews: “Antibody tests for identification of current and past infection with SARS‐CoV‐2”
- University of California, San Francisco: "Testing the Tests: COVID-19 Antibody Assays Scrutinized for Accuracy by UCSF, UC Berkeley Researchers”
- Nature Medicine: “Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections”
- Clinical and Translational Immunology: “Detection of IgM and IgG antibodies in patients with coronavirus disease 2019”
- CDC: “Test for Past Infection (Antibody Test)”
- FDA: “Recommendations for Investigational COVID-19 Convalescent Plasma”
- CDC: “Interim Guidelines for COVID-19 Antibody Testing”
- Centers for Disease Control and Prevention: "Myths and Facts about COVID-19 Vaccines"
- CDC: "Interim Clinical Considerations for Use of COVID-19 Vaccines Currently Authorized in the United States"
- U.S. Food & Drug Administration: "Antibody Testing Is Not Currently Recommended to Assess Immunity After COVID-19 Vaccination: FDA Safety Communication"
- JAMA Internal Medicine: "Association of SARS-CoV-2 Seropositive Antibody Test With Risk of Future Infection"
- Lancet: "SARS-CoV-2 infection rates of antibody-positive compared with antibody-negative health-care workers in England: a large, multicentre, prospective cohort study (SIREN)"
- CDC: "When You’ve Been Fully Vaccinated"