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The search for IgG: What you need to know about antibody testing

Photo of physician conducting antibody research, as researchers say antibody testing will play an important role in navigating decisions on reopening the country safely. (Photo credit: Getty Images)
Researchers say antibody testing will play an important role in navigating decisions on reopening the country safely. (Photo credit: Getty Images)

The first few U.S. studies investigating antibody testing for COVID-19 revealed that there were far more community cases of the virus than previously anticipated. As officials consider how to reopen the country safely, researchers say antibody testing will play an important role in navigating those future decisions. But not all antibody tests are created equal, none are U.S. Food and Drug Administration (FDA)-approved, and the presence of antibodies doesn’t necessarily equate to COVID-19 immunity.  

Public health experts and clinicians with The University of Texas Health Science Center at Houston (UTHealth) help break down what we know so far about antibody testing, also referred to as serology, and what we can expect to see in the future.  

What are antibodies?

Simply put, antibodies are proteins in your blood that your body produces when you have an infection to help fight it off and also protect you from getting it again, said Catherine Troisi, PhD, an associate professor and epidemiologist with UTHealth School of Public Health. They are tested through a blood draw.  

Antibodies are also known as immunoglobulins (Ig) and there are two different kinds. George Delclos, MD, MPH, PhD, a respiratory disease and occupational medicine specialist explains:

  • IgM – These proteins start to show up during the acute phase of the infection. As the infection progresses, they continue to rise until the infection begins to resolve.
  • IgG – Typically, as the levels of IgM decline, IgG proteins appear. The IgG proteins may help protect you from a repeat infection.

“When IgM levels are low and IgG levels are high, that indicates a past infection and possible immunity,” said Delclos, professor at the Southwest Center for Occupational and Environmental Health and Marcus M. Key, MD, ­Shell Occupational and Environmental Health Endowed Chair at UTHealth School of Public Health. “In COVID-19, most antibody tests are focusing more on levels of IgG and their meaning.”

How quickly does your body make them?

“It takes about two weeks from the first time your body is introduced to a new pathogen to develop antibodies, and COVID-19 is new to everyone because it is a novel virus. The next time your body comes in contact with that pathogen, your antibodies will ramp up quicker, keeping you safe from the infection or ensuring that it is less severe,” Troisi said.  

Will an antibody test tell you if you’re immune to COVID-19?

An IgG antibody test will only confirm if you had the disease at least two weeks ago – it won’t tell you if you have it currently, Troisi said.

“Since between 25% and 50% of people who have the virus don’t show symptoms but can still transmit the disease, we don’t know how widespread it is. By looking at who’s immune, we can see who had it and who’s gotten over it, and get a better sense of how far it spread,” Troisi said.

“If IgG antibodies are detected in someone’s blood, there’s likely to be some level of immunity. We just don’t know yet for how long, and what level of antibodies are needed to be effective,” Delclos said.

There will be upcoming research studies following people who have antibodies over a couple of years to see if the antibodies wane, if chances for reinfection rise, and if a second-time infection would be less severe, Troisi said.  

Pros and cons of testing

Antibody testing will be critical to understand how widespread the COVID-19 infection is and who might be immune. This could be especially important as people begin to return to work, but there are also some concerns to consider, Delclos said.

“The first is the lack of accurate tests – most of the commercially available tests in the U.S. have not been validated. So far, only a handful of tests have been validated for clinical use by the FDA. There is work being done to validate all the tests that are out there, but it will take some time,” Delclos said.

If a test has been “validated for clinical use,” it does not mean it is FDA-approved. The FDA is issuing emergency use authorizations to companies who wish to bring their antibody tests to market. Emergency use authorization allows unapproved medical products to be used in an emergency to diagnose, treat, or prevent serious or life-threatening diseases or conditions when they are no other viable alternatives.  

“Under emergency use authorization, the FDA verifies the tests could potentially work and asks the companies to perform internal validations. There is no additional oversight from the agency after emergency use authorization is granted, so it’s important to note that those tests are not FDA-approved,” said Michael Chang, assistant professor of pediatrics with McGovern Medical School at UTHealth and a pediatric infectious disease specialist with UT Physicians.  

The FDA is working with the National Institutes of Health, the Centers for Disease Control and Prevention (CDC), and other partners to validate the more than 70 commercial tests that have been created in the wake of the pandemic, some of which are using misleading claims about FDA-approval and the test’s ability to  detect antibodies to COVID-19.  

“The second issue is how we interpret the test results, since we’re really not sure what level of antibody detection we’re looking for to be effective against COVID-19. Antibody testing can confirm that someone had the virus, but to assume they have immunity is a big jump. We need more data to be able to see whether or not that is the case,” Delclos said.  

It is also possible that antibody testing could pick up on other coronaviruses that aren’t COVID-19, causing false positives.

“There are four common human coronaviruses that circulate mostly in winter, but possibly year-round. It’s estimated that they may account for up to 20% of cases of the ‘common cold.’ Like with any other infection, the body will generate antibodies against these coronaviruses, which could trip up an antibody test, and/or overestimate how much COVID-19 immunity there actually is.” Chang said. “It’s possible to design antibody tests that are specific enough, but we don’t know if companies have done that yet.”  

Troisi also worries that once we start identifying who has already had COVID-19 and who hasn’t, it could put some people at a disadvantage.  

“For example, would someone with potential immunity have privilege over someone still susceptible to the virus in terms of getting a job or returning to work? Especially if that person was already in a vulnerable population, it could add another layer of difficulty to their life,” Troisi said.  

And there’s another unintended consequence that people could try to infect themselves on purpose to gain potential immunity.  

“People, especially the younger population, might think it’s okay to have COVID parties so that they can get it, get it over with, and go back to work. That is extremely dangerous because even young people can be hospitalized and die, or could transmit the virus to vulnerable populations, such as seniors or those with comorbid conditions,” Troisi said.  

Can I be tested now?

Antibody testing studies are being done in phases.  

During the week of March 30, the CDC and its public health partners started the first stage of studies of community transmission using samples collected from the state of Washington and New York City.  

Then testing will expand into areas with high numbers of confirmed cases. It will also include studies of households in some states

“This completely new virus was first reported late last year, and look how much we’ve learned in the first few months. The pace at which we’re gaining knowledge and learning has been spectacular, and I think with time we will continue to gain the knowledge we need to handle this virus,” Delclos said.

For more information on antibody testing, visit the CDC website.  

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