Testing at a hospital in Chicago found that 30-50% of patients who have been tested for COVID-19 have antibodies in their system, which means they likely already had the virus and are immune, at least for an unknown period of time.

“A phlebotomist working at Roseland Community Hospital said Thursday that 30% to 50% of patients tested for the coronavirus have antibodies while only around 10% to 20% of those tested have the active virus,” Chicago City Wire reported on Thursday.

Sumaya Owaynat, a phlebotomy technician, said she tests between 400 and 600 patients on an average day in the parking lot at Roseland Community Hospital. Drive-thru testing is from 9 a.m. to noon and 1 to 4 p.m. each day. However, the hospital has a limited number of tests they can give per day. Owaynat said the number of patients coming through the testing center who appear to have already had coronavirus and gotten over it is far greater than those who currently have the disease. TRENDING: Obama Statement on Ginsburg Demands GOP Senate Honors Her Dying 'Instructions' and Put Off Vote on Supreme Court Nominee Until New President Sworn In “A lot of people have high antibodies, which means they had the coronavirus but they don’t have it anymore and their bodies built the antibodies,” Owaynat told Chicago City Wire.

More than 500,000 Americans have tested positive for COVID-19, and more than 20,000 have died. According to one model, the number of deaths per day peaked on Friday and will begin to fall.

A key to re-opening the U.S. economy will be testing to find out who already has had the virus (some reports say up to 50% of those who have contracted SARS-CoV-2 show no symptoms). Those with antibodies can return to work as they’ll be immune to the virus.

The Centers for Disease Control and Prevention (CDC) has already begun preliminary studies to try to find out who is carrying antibodies.

“The first, which has already begun, will be looking at blood samples from people never diagnosed as a case in some of the nation’s Covid-19 hot spots, to see how widely the virus circulated,” STAT reported last week. “Later, a national survey, using samples from different parts of the country, will be conducted. A third will look at special populations — health care workers are a top priority — to see how widely the virus has spread within them. Bresee said the CDC hopes to start the national survey in the summer; he gave no timeline for the health workers study.”

The Food and Drug Administration (FDA) has also authorized the first blood test, known as a serology test, to look for antibodies in the blood. Cellex Inc., a medical device company based in North Carolina, says the test could help physicians determine how widespread the virus is and the duration of immunity for people after they recover.

“Based on the totality of scientific evidence available to FDA, it is reasonable to believe that your product may be effective in diagnosing COVID-19,” FDA chief scientist Denise Hinton wrote in an April 1 letter to James Li, CEO of Cellex. “The known and potential benefits of your product when used for diagnosing COVID-19 outweigh the known and potential risks of your product.”

“Your body likely starts making antibodies against the coronavirus between seven and 21 days after you’re infected,” Vincent Racaniello, a professor of microbiology and immunology at the College of Physicians and Surgeons at Columbia University, told Mother Jones. “Those initial antibodies typically stick around in your system for about three months, while other antibodies take a little longer to develop and will likely be found in your blood years after you’ve had COVID-19.