As officials begin to discuss when they can lift stay-at-home orders, companies are rushing to develop coronavirus antibody tests that could help them make those decisions without risking a second wave of infections.

Leading the fight against COVID-19, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Friday morning on CNN that antibody testing can show who has developed immunity to the coronavirus and can safely go back to work without getting reinfected.

“As we get to the point of at least considering opening up the country, as it were, it’s very important to appreciate and understand how much this virus is penetrating this society,” he said.

How would antibody tests factor into decisions to lift social distancing orders, and how reliable are those tests? Here’s everything you need to know about antibody testing. 

What is an antibody test?

Testing to see if people have antibodies in their blood isn’t the same as testing to see if they have been infected with the coronavirus that causes COVID-19. 

The antibody test can determine if someone was previously infected and recovered, whereas the molecular test shows whether that person was infected with the virus at the time the test was taken. 

Antibodies are the body’s way of remembering how it responded to an infection so it can attack again if exposed to the same pathogen. If a person has antibodies in his blood, that means he has immune cells available to fight the virus, which lowers the risk of re-infection.  

Antibody tests look for two antibodies in the blood, immunoglobulin M (IgM) and immunoglobulin G (IgG). IgM antibodies are the first line of defense, appearing within several days of infection. IgG antibodies come later, as the body is clearing up the infection.

IgM and IgG antibodies fight all kinds of infections. The blood tests for COVID-19 look for a protein particular to this coronavirus, which shows whether the body is producing antibodies to it and not, for example, the seasonal flu.

Marc Lipsitch, a professor of epidemiology at Harvard and an expert in public health interventions, told the USA TODAY Editorial Board on Wednesday that a significant portion of the population must be immune to the coronavirus before social distancing restrictions can be lifted. Widespread antibody testing can determine how many people carry the antibodies.

Why antibody testing is important

Antibody tests can reveal who is immune to a disease, but it can also determine how widely it has spread and how deadly it is.  

Dr. Neeraj Sood, professor and vice dean of research and faculty at the University of Southern California Sol Price School of Public Policy, is leading a study in conjunction with the Los Angeles County Department of Public Health using antibody tests to answer that question.

There have been 223 deaths related to COVID-19 in the county, according to the health department. If, based on the antibody study, researchers determine that about 2,500 people had been infected, it would be considered a deadly disease. But if more than 2 million people had been infected, it wouldn’t be considered that dangerous. 

“If we find out COVID is far less deadly than the flu, we can open up the economy. You don’t need to hit herd immunity to open it up.” Sood said. “But if you find out that COVID is 10 times deadlier than the flu, then you have to keep it closed.”

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Antibody testing combined with the molecular coronavirus test can determine if a person is immune and can transmit the disease, he said. This is crucial in deciding who can go back to work.

Those who are immune and can’t transmit the disease can be on the front lines of the epidemic, keeping daily life afloat in grocery stores, hospitals and other essential businesses, Sood said.

Antibody tests can also identify people who had COVID-19 but didn’t have any symptoms — a group that may be much larger than we know.

“There’s some data out there that 15% to 16% of kids have had asymptomatic infections. They could be the secret spreaders. But without tests, we don’t know,” said Mark Slifka, a professor of viral immunology at Oregon Health and Science University in Portland, Oregon.

Doing broad testing of even a portion of the population in the coming months could also give researchers a sense of how much of the U.S. population was infected this year. They could use that to predict who might be immune if the virus comes back again this fall, said Slifka.

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Immunity to viruses can last from months to a lifetime. William Schaffner, a professor of preventive medicine and infectious disease at Vanderbilt University, expects that contracting the coronavirus once would likely give someone “pretty solid protection for a year.” But he cautioned predictions for a newly emerged virus are difficult to make.

“We don’t know that this new coronavirus has read the textbook and knows what it’s supposed to do,” he said.

When will antibody testing be available to the public?

On the TODAY show Thursday, Fauci said a large number of antibody tests should be available in a matter of days or weeks, according to the companies developing them.

Dr. Elitza Theel, director of the Infectious Diseases Serology Laboratory at the Mayo Clinic, said laboratories throughout the country have been working to validate and select antibody tests. 

She expects they will be widely available in about one to three weeks.  

Cellex Inc. of Research Triangle Park in North Carolina was the first company to receive emergency use authorization of its test by the U.S. Food and Drug Administration last week. 

How reliable are these tests?

Over 50 commercial manufacturers are seeking FDA approval for antibody tests, Theel said.

But Dr. Raed Dweik, chairman of the Respiratory Institute at the Cleveland Clinic, said some of the tests he’s seen aren’t accurate enough to determine if someone is truly immune to the coronavirus. 

This is because antibodies for the coronavirus that causes COVID-19 look very similar to antibodies that respond to coronaviruses that cause other illnesses, such as the common cold.

Tests could mistakenly identify antibodies as being for the coronavirus that causes COVID-19, producing a false positive. Dweik said it will take more time to develop a test that can accurately detect the right antibodies.

“The timing is tricky,” he said. “Developing this test has been fraught with problems.” 

In addition, scientists don’t know a lot about the protective value of coronavirus antibodies, said Dr. Carlos del Rio, an infectious disease expert and executive associate dean at Emory University. The test to identify HIV, for example, is an antibody test. “Having those antibodies doesn’t necessarily mean immunity,” he said.

Few of the coronavirus antibody tests have been vetted to determine their clinical accuracy, Theel said. No test is 100 percent accurate, she said, and it’s up to the clinical laboratories to decide which ones are best for their patients. 

“It’s one of the biggest concerns right now because there’s so many tests out there and we don’t know how the vast majority of them work,” she said.

Contributing: Letitia Stein and Elizabeth Weise, USA TODAY. Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT. 

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