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COVID-19 Antibody Tests: What You Need to Know

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COVID-19 Antibody Tests: What You Need to Know

Research is ongoing, but here are experts' thoughts — and warnings — on the current diagnostics.

One of the big failings of the United States' response to the novel coronavirus pandemic was a massive shortfall in the availability of tests to diagnose when someone is sick with COVID-19, the illness caused by the virus. And though we continue to need (and use) those tests to identify new infections, talk has turned to a different kind of test — one that measures antibodies, a biological marker that someone has contracted COVID-19 and recovered from it.

Before we dive in, it’s important to emphasize that research on the novel coronavirus is evolving at a rapid rate, says Jordan Crofton, RN, medical administrator at THE WELL. The following information is true to the best of our knowledge today, but it may be different tomorrow or the next day as experts continue to make discoveries.

With that in mind, here’s what we know about COVID-19 antibody tests — so far.

What is an antibody test?

An antibody test is a blood (serology) test that checks for the presence of antibodies, a type of protein that the immune system produces when it encounters a new disease-causing virus, explains Sheldon Campbell, MD, PhD, a microbiologist and associate director of Yale Medicine’s Clinical Microbiology Lab. “If the test is accurate and done right, it tells us that the patient has been exposed to and contracted SARS-CoV-2, the COVID-19 virus.”

The body produces five types of antibodies (or immunoglobulins), and there are three types relevant to COVID-19 — IgG, IgM, and IgA. Some antibody tests on the market only indicate one or two of those antibodies, Crofton says, which doesn't give the whole, complicated picture, especially in a situation like COVID-19.

Plus, many COVID-19 tests aren't able to accurately provide insight into the type, amount and quality of the antibodies, which is what helps us understand whether the antibody will be able to fight off a future exposure to the virus.

Here’s the basic breakdown:

  • IgM antibodies are the first on the scene to fight a new infection. Only having IgM antibodies means that the person is likely still in the acute or early stage of an infection — at which point they may still be able to transmit the disease.
  • IgG antibodies typically take several weeks (at least) to show up and usually indicate that a person has had a fairly recent infection.
  • IgA antibodies serve as the principal antibody in mucosal secretions, which line your respiratory passageways and gut, for example, to protect them from a threat such as a virus or bacterium. These antibodies linger for a longer period of time than the IgM or IgG antibodies.

Because it can take days to weeks for your body to make antibodies after contracting a virus (and we're not quite sure how long viral shedding can last in the case of COVID-19), it’s possible to test negative for antibodies while still in the acute phase of an infection, says Crofton. This means you might still be at risk of spreading the disease — even if you’re asymptomatic.


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How do I know which tests can be trusted?

Simply put, not all antibody tests on the market are created equal and the results aren’t as accurate as we’d like them to be. In fact, the majority of the COVID-19 antibody tests being sold have not been vetted by the FDA or CDC, and, adds Crofton, “have poor specificity and sensitivity.” One report by more than 50 scientists found that only three out of 14 tests had accurate results.

Translation: There is potential for significant false positive rates (signaling you have antibodies but don’t) as well as false negative rates (saying you don’t have antibodies, but do).

Even more troubling: Because there are many types of human coronaviruses (like ones that cause a common cold), it’s possible for an antibody test to “cross-react” to a different, past virus that a person has had. “While this cross-reaction may show up as a positive antibody test, it does not mean that the person has definitively had COVID-19," warns Crofton.

On Monday, May 4, the FDA took a stronger stance and released a statement on Monday saying that any companies producing antibody tests must submit data proving accuracy within the next 10 days — or the tests will be removed from the market. "This means that we're likely about to see big changes in the market as tests are more properly vetted," Crofton says. Plus, the CDC is currently evaluating commercial antibody tests for COVID-19 to measure efficacy, so it's a safer bet to wait to purchase a test until it gets approval from either the FDA or the CDC.

Does a positive COVID-19 antibody test signal immunity?

The short answer? No one knows for sure, and it’s dangerous to assume you are, even if the test you got accurately detected COVID-19 antibodies. “Antibody tests in general are good, but not perfect,” Campbell says. There are several viruses for which having the antibody doesn’t mean you are immune, such as HIV. While it’s “likely” that most people with antibodies to COVID-19 will be immune, not all will be, he explains.

“Some people may get an infection, recover from it and have antibodies present, but they may not be sufficient enough to protect them from the infection again in the future,” explains William Schaffner, MD, a professor of medicine in the Division of Infectious Diseases at the Vanderbilt University School of Medicine.

This is often the case for immunocompromised people, including the elderly, Schaffner notes. Why are some people’s immune systems capable of producing a response significant enough for future protection, but others’ aren’t? Unfortunately, we don’t know the answer to the question yet, he says.

Antibody tests in general are good, but not perfect.

Does a positive COVID-19 antibody test mean you can no longer spread the virus to others?

For most viruses, it is usually true that a positive antibody test means you won’t transmit the virus, Campbell says. But in the case of COVID-19, it’s not clear yet. (Beginning to see a pattern here?)

Though rare, “we do see some people with a positive antibody test and simultaneously, a positive test for the [active] virus," he says — which would mean they are still contagious. Bottom line: The more we learn about the novel coronavirus, the more we see what a tricky microorganism we're dealing with.

If you had COVID-19 this year, can you catch the "second wave" six months from now?

For many respiratory viruses, you’re immune for a while — several months to a few years — and then immunity wears off, allowing you to potentially contract that same virus again. This is called waning immunity, and it’s the reason that adults need booster vaccines in addition to the vaccines you receive as a child.

It’s somewhat unlikely (but not impossible) to catch the same cold or flu twice, because those viruses are constantly evolving (and there are an estimated 200 cold viruses). In that sense, you are also not immune to a virus that has mutated significantly. It’s too soon to tell if the new-to-humans nemesis that is COVID-19 will be highly mutable.

Will THE WELL offer COVID-19 antibody tests?

THE WELL has every intention of offering antibody testing — if and when the medical team gains confidence in the efficacy of the technology being developed. “Our top priority is the safety and health of our patrons and patients,” says Crofton. "As the collective research of top scientists fuels the development of these tests, we are cautiously optimistic that we'll soon be able to offer a reliable test that will provide the information and reassurance everyone is seeking right now."


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