The University of Minnesota and Mayo Clinic are close to unveiling antibody tests that can determine if people have already been infected by the coronavirus that causes COVID-19 and are no longer threats to get or spread the infection.

State health officials see these tests as a key part of Minnesota's pandemic response, because they could define the breadth of the outbreak and maybe identify previously infected individuals who could move about in public freely and volunteer in response efforts.

Identifying people with immunity to the virus "gives us the ability [to know] who wouldn't need to be quarantined, who could be out and not infecting others," Gov. Tim Walz said on Monday.

Antibodies are proteins produced by the immune system in response to infections and can be found through tests of blood serum — the clear liquid that separates out when blood clots. In the case of SARS-CoV-2, the virus causing the current pandemic, antibodies don't show up in sufficient quantities for eight to 11 days.

That makes antibody testing ineffective for the purpose of early diagnosis, but a potentially powerful tool for identifying anyone who was infected, whether they had symptoms or not. "In theory, you probably want to test everybody in the country," said Marc Jenkins, director of the University of Minnesota's Center for Immunology.

Testing could be particularly important for health care workers, whose infection risks could be heightened if they run out of protective masks and face shields when working with any surge of COVID-19 patients. If they've already been infected, those workers could have immunity — although there is only limited evidence so far that people cannot be reinfected by this coronavirus.

Jenkins' team used a common chemical assay test, known as ELISA, and found that it correctly distinguished samples from two people with COVID-19 from two reference samples taken from people before the pandemic.

Researchers nationwide are racing to develop antibody tests, which under current emergency pandemic conditions would not need the usual level of review and approval from the U.S. Food and Drug Administration. Exactly how much verification of tests is needed is unclear, but Jenkins said he wants to at least get successful results for 10 positive and 10 negative specimens before even considering the test for public use. He expected to have that many results in the next couple of weeks.

"It's going to take a lot more samples … to validate the test," he said, "but we have a pipeline to do it and the results from these early tests were so clean that I'm pretty hopeful."

Mayo has used more than 300 samples to evaluate the accuracy of three different assay tests, and has found one ELISA-based test to be most accurate, said Elitza Theel, director of Mayo's Infectious Diseases Serology lab.

Theel said it will be critical for doctors to understand that the current molecular tests are still the best for diagnosing symptomatic patients — and that new antibody tests won't solve any national shortage of those. However, from a disease-detective standpoint, health officials will be able to use antibody tests to determine when a community has achieved "herd immunity" — which means that enough people have already been infected that the risk of new infections starts to diminish.

COVID-19 is a respiratory illness caused by the novel coronavirus that emerged earlier this winter in Wuhan, China, and spread globally. Initial reports indicated a death rate of anywhere from 1% to 3.4%, but an abundance of antibody testing would provide a more accurate picture because it would allow for testing of both sick and healthy people to find out how many of them have been infected.

"When you have an actual true denominator, you're able to determine the true case fatality rate," Theel said.

Local studies on antibody testing didn't occur until recently, because researchers needed substantial samples from COVID-19 patients to do it, and because there was a more immediate need to develop diagnostic molecular tests for the illness.

Quality problems with a molecular test created by the U.S. Centers for Disease Control and Prevention initially delayed states such as Minnesota from using it. Then, supply shortages prevented its broader use, even as the coronavirus was spreading nationwide.

Mayo and other private labs created their own versions of the molecular tests, and now have conducted more such tests than the state's public health lab, which has prioritized its limited supplies for hospitalized patients, long-term care facility residents and health care workers.

Jenkins said he first started looking at antibody tests when news of testing supply shortages emerged. Knowing that antibody testing could provide invaluable public information, he said he wanted to create a pipeline for these tests on which Minnesota could rely.

Jenkins acquired a COVID-19 protein that is critical to the test from Fang Li, the U researcher whose recent Nature journal article explored the genetic origins of this coronavirus and its evolved mechanism for infecting cells.

Other coronaviruses cause the common cold and also were responsible for global SARS and MERS outbreaks. Jenkins said he will be checking to make sure the newly developed test identifies only cases of COVID-19 infections.

Whether antibody testing could be used as an all-clear system for people is based on a key assumption: that people develop at least short-term immunity to the coronavirus.

One small study in China found no reinfections in monkeys when they were exposed to the virus a second time. Researchers also believe that people develop immunity to other coronaviruses that lasts months or maybe years. Antibodies were found in patients six years after the 2002-2003 SARS epidemic.

Jenkins said it appears likely there is at least temporary immunity, but it's unclear whether people will get lifetime immunity after only one infection from this virus — as is the case with polio and chickenpox. On the other hand, people only develop immunity one at a time to the many different types of rhinoviruses, which cause common colds, he said.

As many as 80% of people infected by this coronavirus suffer mild or no symptoms. A reliable test could be a relief to many people if they knew they now had immunity, Jenkins said. "Just for the psychological well-being of a lot of people, this could have real value."