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As officials begin to discusswhen 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 immunityto 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, its 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 arethose tests? Here's everything you need to know about antibody testing.

Testing to see if people haveantibodies in their blood isnt the same as testing to see if they have been infected withthe coronavirus that causes COVID-19.

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

Antibodies are the bodys 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 hasimmune cells available to fight the virus, which lowers therisk 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 itand 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.

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, isleading 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-19in 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 dont 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 testcan 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 cant transmit the disease can be on the front lines of the epidemic, keeping daily life afloatin 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.

Theres some data out there that 15% to 16% of kids have had asymptomatic infections. They could be the secret spreaders. But without tests, we dont 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. populationwas infected this year. They could use that to predict who might be immune if the virus comes back again thisfall, 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 dont know that this new coronavirus has read the textbook and knows what its supposed to do, he said.

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, directorof theInfectious 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.

Over50 commercial manufacturers are seeking FDA approval forantibody tests, Theel said.

ButDr. Raed Dweik, chairman of the Respiratory Institute at the Cleveland Clinic, saidsome of the tests he's seen aren't accurateenough to determine if someone is truly immune to the coronavirus.

This is becauseantibodies 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 determinetheir clinical accuracy, Theel said. No test is 100 percent accurate, she said, and it's up to the clinical laboratories to decidewhich 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 dont know how the vast majority of them work," she said.

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

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