By Andrew Hay

(Reuters) – Arizona on Tuesday said it would provide coronavirus antibody tests for 250,000 health-care workers and first responders in the largest such testing in the United States to date.

The blood tests show who has been exposed to the novel coronavirus and successfully built immunity, the University of Arizona, which will produce and carry out the tests, said in a statement.

With experts saying that up to 50% of people exposed to COVID-19 experience few to no symptoms, determining who has developed virus-fighting antibodies is critical to restarting social interaction, said Dr. Michael Dake, senior vice president for the University of Arizona Health Sciences.

“We’re going to be the largest and certainly the first to undertake a statewide testing of all healthcare workers and first responders,” Dake, who is overseeing production of the tests, said on a conference call.

The state is providing the university $3.5 million in funding to produce the tests and provide results after clinics and hospitals send blood samples of the workers.

Dake expected the university to be able to process 5,000 tests per day once the program ramps up to full speed in early May, when some models show Arizona’s outbreak peaking.

First responders and healthcare workers are the first wave of testing, which may move onto other areas of the population, Dake said.

The University of Arizona has pledged to give antibody tests to all 45,000 of its students and 15,000 faculty.

The antibody tests, using decades-old ELISA technology, do not always pick up early-stage infections but show whether a person had the virus in the past, even if the person was asymptomatic.

    In comparison, the so called RT-PCR-technology swab tests used at drive-through stations and clinics across the country determine whether a person has the virus at that moment by looking for it in nose or throat secretions.

Both tests are critical in the coronavirus fight, but antibody tests are seen as a relatively cheap, fast means to sort populations into risk groups and measure virus spread.

Questions remain about how long coronavirus immunity levels last and whether people who have antibodies could still be contagious, according to some infectious disease specialists.

“Antibody testing is not a cure-all,” Arizona Governor Doug Ducey said in a statement. “Learning more about it is an important step to identifying community exposure, helping us make decisions about how we protect our citizens, and getting us to the other side of this pandemic.”

(Reporting By Andrew Hay in Taos, New Mexico; additional reporting by David Schwartz in Phoenix; Editing by Leslie Adler)