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April 27, 2020

Are COVID-19 antibody tests accurate? Questions abound as scores hit market

Here's what scientists know – and don't know – about immunity to the coronavirus

Health News COVID-19
Serology testing coronavirus Anthony Behar/Sipa USA

Serology testing determines whether people have been infected by the coronavirus – even unknowingly – and developed immunity by detecting antibodies in patient blood samples. Above, a health care worker administers an antibody test at a drive-in test site in Franklin Square, New York.

Antibody testing has emerged as an essential tool as public officials across the United States search for ways to reopen the economy safely during the COVID-19 pandemic.

In response, scores of serology tests have hit the marketplace. But questions abound about these tests and the level of immunity that COVID-19 antibodies provide. 

How accurate are the tests? Can they determine whether people are safe from reinfection? And, if so, how long might that immunity last? 

Here's what is known – and isn't known about antibody tests:

WHAT ARE ANTIBODY TESTS?

Antibody tests measure the amount of antibodies a person has developed against a specific infection. The immune system is a complex fighting machine, with different levels of defense against intruders like SARS-CoV2, the scientific name given to the novel coronavirus. 

Antibodies bind to the virus along with T cells, forming a second level of defense after other cells, like macrophages, are sent to slow the virus.  

When someone successfully recovers from COVID-19, those antibodies remain present in the blood. Antibody testing therefore gives a more accurate picture of the number of people who have been exposed to the coronavirus, including those who never developed symptoms.  

If those antibodies provide some level of immunity, people who test positive could move about without fear of reinfection. 

HOW RELIABLE ARE THEY?

While serology testing has ramped up in the United States and other parts of the world, some scientists and governments officials are concerned about the tests' accuracy, particularly the propensity to deliver false-positive and false-negative results. 

A group of researchers recently evaluated 14 antibody tests currently on the market, looking to determine which were accurate. Only three showed reliable results and only one never developed a false positive. The other two tests deemed reliable didn't produce a false-positive result 99% of the time. 

But even those three tests occasionally failed to pick up the presence of antibodies. The research has not been peer-reviewed yet or published in a medical journal. 

"The proportion of people in the United States who have been exposed to the coronavirus is likely to be 5% or less," Scott Hensley, a microbiologist at the University of Pennsylvania told The New York Times. "If your kit has a 3% false positive, how do you interpret that? It's basically impossible. 

"If your kit has 14% false positive, it's useless," he said.

It also isn't clear whether all antibody tests are able to distinguish the SARS-COV-2 antibodies from those developed against the other six coronaviruses known to infect humans, including two that cause the common cold. 

HOW ARE THEY REGULATED?

While antibody tests show promise, more regulatory oversight is needed, according to the chair of the House Subcommittee in Economic and Consumer Policy, Raja Krishnamoorthi, of Illinois. 

Most of the companies selling antibody tests are doing so without approval from the U.S. Food Drug Administration, he warned. More than 100 tests have the the market and only four of the companies behind them sought federal authorization. The rest relied on self-validation. 

"Because of this kind of voluntary compliance type of regiment, there is absolutely no incentive for a junk test-maker to actually produce their results to the FDA," he said.

So how can people know which antibody tests they can trust? Krishnamoorthi suggests only using one of the tests authorized by the FDA, which are listed on its website.

WILL 'IMMUNITY PASSPORTS' WORK?

Among the other major unknowns related to serology testing: Whether COVID-19 antibodies offer protection against future reinfection and the length of time someone maintains their immunity. 

Antibodies against some viruses, like measles, last a lifetime. Others, like various cold and influenza strains, do not.  Scientists are still trying to determine the effects of COVID-19 antibodies.

Some countries are considering issuing so-called "Immunity Passports" as a way to allow healthy, seemingly "risk-free" people to re-enter society. 

Still, the World Health Organization has cautioned against this, saying there is not enough evidence to support the use of a such a strategy right now. While most studies have shown that people who recovered from COVID-19 have developed antibodies against the infection, no study has investigated the possibility of reinfection.

"People who assume that they are immune to a second infection because they have received a positive test result may ignore public health advice," the WHO stated last week. "The use of such certificates may therefore increase the risks of continued transmission."

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