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?
As cases spike across the United States, you might be thinking it would be a good thing if you already battled the infection and "got it over with." 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.
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 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 Assume You Won’t Get Re-Infected
"Just because you're antibody positive does not mean that you're protected from re-infection. We just don't know yet," says Elitza Theel, PhD, director of the infectious diseases serological lab at Mayo Clinic.
Initial research seems to suggests that antibodies are present for two to three months, she says. Preliminary studies in monkeys suggest that antibodies provide partial immunity for a short period of time. "We really need longer studies to decipher how long that immunity lasts and if these findings are transferable to humans," Theel says.
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.
Plus, most tests on the market right now generally detect the overall presence of antibodies against COVID-19. "We're just telling you that an antibody is binding to an antigen. We're not doing a functional test," Dr. Greninger explains.
That means we don't know if and how much of the virus-fighting proteins produced by your body are so-called neutralizing antibodies, a subset of antibodies that protect against re-infection. Some laboratories, such as Mayo Clinic Laboratories and Labcorp, recently introduced new neutralizing antibody tests.
Do Continue to Follow Public Health Guidelines
Since you could get re-infected, the Centers for Disease Control and Prevention (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.
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. In a city like Seattle, where the prevalence rate is currently between 2 and 3 percent, there will be a higher false positive rate, Dr. Greninger explains. However, in a city like New York, where the prevalence is between 20 and 25 percent, the tests are more likely to predict an accurate result.
With infections rising in previously low prevalence areas, the implications of antibody tests are in flux right now in many places across the United States, Dr. Greninger says.
Do Consider Donating Your Plasma
You've likely heard about convalescent plasma programs. Plasma, the liquid portion of the blood, is collected from people who have recovered from COVID-19 and developed antibodies against the virus.
Currently, there are a number of clinical trials that are evaluating administering convalescent plasma to patients with serious cases of COVID-19 as an experimental treatment. Convalescent plasma has been studied in other outbreaks like the 2009 to 2010 H1N1 influenza virus pandemic and the 2012 MERS epidemic, according to the U.S. Food and Drug Administration (FDA).
One of the main uses for COVID-19 antibody tests is to screen for potential convalescent plasma donors, Theel says.
If you've tested positive for antibodies, you may be eligible to donate, but you have to meet certain requirements. According to the FDA, you need evidence of a confirmed COVID-19 diagnosis and must be symptom-free for 14 days. Find out more information about donating plasma.
Don’t Count on an Immunity Passport
"Early on there was a lot of discussion about using antibody testing to restart the economy and to provide immunity passports, which would then allow people to walk about freely," Theel says.
However, antibody tests should not be used to determine if you're immune to COVID-19, per the CDC. There are still many unknowns, which makes interpreting the tests results tricky. "At this point, it's premature to rely on antibody testing to restart economies. More data is needed before we can say that," Theel says.
While the meaning of an antibody test result for an individual is pretty limited right now, these tests can provide important epidemiological information.
"We can use antibody testing to look at the seroprevalence rates in communities," Theel says. "This can help on a regional, state and national level to better understand how this virus is spreading. There is such a high number of asymptomatic individuals who aren't presenting clinically and aren't getting tested, but they will still develop antibodies."
And remember, antibodies are just one part of your body's defense system. "The immune system is complex. It's not just antibodies that play a role in our immunity," Theel says.
Concerned About COVID-19?
Read more stories to help you navigate the novel coronavirus pandemic:
Is This an Emergency?
- 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”