The biggest hospital system in Michigan is launching what’s believed to be the nation’s largest test for novel coronavirus antibodies. The study could determine who has already been infected with the virus and may now be immune to it — information that public health officials say is vital to decisions about reopening society.

Beaumont Health will test blood samples from its 38,000 employees, as well as thousands of additional physicians and affiliates, officials said in interviews. Participation is voluntary, but officials said they expect many employees will want the test, particularly those working in emergency rooms and intensive care units who are at high risk of contracting the virus. Michigan has the third-highest number of confirmed coronavirus cases of any U.S. state.

Many public health officials have advocated such testing on a broad scale to find out how extensively the virus has circulated in the United States and who may have immunity and be able to return to work first.

The tests are used not to diagnose the illness but to find disease-fighting proteins the body creates to neutralize the virus. They can identify people who may have been infected without showing any symptoms or who had mild symptoms that may have been mistaken for something else.

“We want to reassure our health-care workers,” said Matthew Sims, Beaumont’s director of infectious-diseases research and the principal investigator for the antibody study. “And then, we really want to start to answer the question: If you have the antibodies, are you protected against getting the infection again? Nobody has the answer yet.”

Beaumont researchers hope that with such large-scale serological testing, “we could inform critical medical and public health decisions,” said Barbara Ducatman, chief medical officer at Beaumont’s Royal Oak Hospital. “That’s critical to make decisions as a society.”

By examining a single health-care system, Beaumont researchers also hope to learn more about how the coronavirus spreads and why some people develop covid-19, the disease it causes, but others don’t. Several hundred health-care workers there have tested positive for the virus, and more have shown symptoms.

“The current testing across the country . . . is very good for detecting disease in people who have symptoms,” said Nick Gilpin, Beaumont Health’s director of infection prevention. “But the test is less reliable for those who don’t — and not particularly useful for understanding the scope of the pandemic.”

The Centers for Disease Control and Prevention estimates that 1 in 4 people who carry the virus don’t show any symptoms.

Researchers around the world are racing to develop an antibody test to help guide decisions on when to ease social distancing rules and let some people return to work. But Beaumont officials said they were unaware of a test group as large as theirs.

The National Institutes of Health is conducting a study to determine how many adults in the United States without a confirmed coronavirus infection carry antibodies to it. The NIH is seeking up to 10,000 volunteers.

At the Mount Sinai Health System in New York City, researchers began working on a serological test before the first coronavirus case was confirmed in the United States. That test has been licensed to a biomedical research company that intends to test large numbers of people.

Beaumont has validated the antibody test on a group of 1,000 people, said Hans Keil, senior vice president and chief information officer. Researchers will publish the results so they can be examined by others.

The next phase is to expand the capacity to test samples from nearly 40,000 people. Beaumont is signing up volunteers, including from medical schools, to draw blood and has purchased new machines to process the samples. Researchers hope to complete their work by the end of May, Keil said.

Under privacy law, test subjects will learn their antibody status, but Beaumont researchers will use only aggregate data and not see individual results.

People with high levels of antibodies also may be able to donate their blood plasma to help others who are suffering with the infection, hospital officials said. The federal Food and Drug Administration has approved “convalescent plasma transfusion” as a therapy for coronavirus patients. The technique has also been used to treat measles, polio, mumps and influenza.

Sims said it’s still too soon to know when antibody testing will be broadly available. That may depend on how closely someone lives to a research center or health-care system that offers the test.

“At this point, they’re all just ramping up,” Sims said.

After Beaumont has finished collecting blood samples from its employees, Sims said, he expects the hospital system may offer the test to others. “But we’ll have limits on how many we can do in a day,” he added.

Intense public demand for the test, coupled with the rapid spread of the virus, has led the researchers to speed up their process.

“In normal times, with something of this size, I would normally spend six months to a year getting it ready, getting all the pieces in place and getting it started,” Sims said. “We managed to do this in three weeks.”