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Ventura County residents can get free antibody tests to find out if they’ve already fought off the coronavirus, officials said Friday.

The first round of serology testing will begin 10 a.m. Monday in Simi Valley and will be followed by additional testing in Thousand Oaks, Moorpark, Camarillo, Santa Paula Fillmore and Ojai over the next two weeks. A full list of dates and locations can be found here.

Ventura residents can get tested at the Barranca Vista Park Wednesday, the Ventura City Fire Department said, adding that officials are expected to announce additional dates for the city in the future.

Residents can arrive to get the blood test done without an appointment, on a first come, first-served basis, county officials said.

Serology testing is different from testing that seeks to diagnose current infections. It can find people who had been infected in the past by looking for the antibodies in their blood.

Antibodies are proteins made in response to an infection and are found in the blood. Finding them 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.

The effort to detect antibodies mirrors that in Los Angeles County, where public health officials are working with the University of Southern California to test thousands of residents to better understand how widespread the virus is in the county.

In sharing the preliminary results of the tests, L.A. County officials said hundreds of thousands of adults in the county may have been infected, even without showing any symptoms.

But the CDC and multiple health experts have warned that serology testing may be wrong up to half the time and tests are not accurate enough to use to make important policy decisions.

And even if someone does have the antibodies in their blood, it doesn’t necessarily mean that the person has become immune to the coronavirus, and it’s not yet clear if they won’t get sick if they’re exposed again.

As of Friday, Ventura County had recorded a total of 1,835 coronavirus infections, with 42 people who have died of the respiratory illness it causes. There were 156 cases confirmed in the city of Ventura.

The county operates four free drive-thru coronavirus testing sites in Ventura, Oxnard, Santa Paula and Moorpark. So far, Ventura County has tested more than 44,900 people for the virus.