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Six sites across Los Angeles County will be used to test for coronavirus antibodies in 1,000 people to find out how many have already fought off the virus, USC Price School of Public Policy officials said Thursday.

The pilot serology tests will start on Friday and Saturday and take place at drive-thru sites throughout the county every two weeks to track the trajectory of the pandemic, according to a news release from the school.

Working with the L.A. County Department of Public Health, the school said it hopes to find if people have the antibodies or immune cells that potentially give them immunity against COVID-19, which had sickened nearly 8,000 and killed 223 patients countywide as of Thursday.

“Testing for antibodies to the virus that causes COVID-19, in addition to testing for the absence of the virus itself using traditional COVID-19 tests, could potentially identify those who can enter the workforce with minimal risk of infecting others or getting infected themselves,” the school said.

Antibodies are specific proteins made in response to an infection and are found in the blood and in other tissues of those who are tested after infection.

Finding antibodies during a serology test would mean that a person had an immune response to the coronavirus, even if they hadn’t developed any symptoms, according to the Centers of Disease Control and Prevention.

Using a representative sample of 1,000 randomly pre-selected people, the study also hopes to find out how many people in the county have ever contracted the coronavirus, painting a more accurate picture of the spread of the virus.

The school said it will not disclose the locations of the testing sites to protect the privacy of participants and said that people can’t just walk up and get tested.

The CDC is also working to develop a new serology laboratory test to find out how much of the U.S. population has been exposed to the coronavirus.

The U.S. Food and Drug Administration had issued its first emergency use authorization for a coronavirus serology test last week.

The USC Price School said Friday and Saturday’s tests will be among the first conducted in the U.S.

“We continue to be very encouraged about serologic testing for the state of California. We have some of the best laboratories, academicians and scientists around serology testing,” Secretary of the California Health and Human Services Mark Ghaly said at a news conference Thursday.

Ghaly said the tests could help with understanding how much the virus is prevalent in a community and possibly immunity to the virus, so the state wants to use the tests “responsibly.”

“We in California are looking at serologic testing with a great deal of hope and anticipation, but also being very methodical on how we use these to guide our decisions,” he said.

Those interested in information about taking part in future antibody studies over the next three to four months can sign up online.

Besides looking for past infections, L.A. County has been focused on expanding testing for current coronavirus infections by setting up more drive-thru testing sites throughout the county and removing some restrictions on who can be tested.

Whereas previously testing was limited to people over 65, those with underlying health conditions, or people who have been exposed to a confirmed case, any resident with COVID-19 symptoms can now apply for a test online.

As of Thursday, over 38,300 people in L.A. County had been tested, and 15% were found to be positive, according to the county’s health department.