The D.C. government will conduct a citywide survey to study the spread of coronavirus antibodies throughout the District.

The city’s Health Department will partner with the Centers for Disease Control and Prevention (CDC) to conduct the serology survey, which identifies antibodies in blood that show if someone has fought off COVID-19.

Officials say 850 randomly selected households will receive letters inviting all residents of the household to get a free antibody test before Aug. 15. Those who agree will get free transportation to the testing site, as well as a $25 Visa gift card.

Some doctors say the presence of antibodies might protect people from catching the virus again, but health experts warn that a positive test does not guarantee immunity.

D.C. Mayor Muriel Bowser announced that the city will open a third free antibody testing site Tuesday at the Hillcrest Recreation Center in Southeast. It first began offering free antibody testing in mid-June at two sites, one in Southeast and the other in Northwest.

Washingtonians who are interested in getting a free test have about a month left to get one from the city: Bowser announced that the free testing program will end on Aug. 15, the same day the survey will conclude.

Residents can still get antibody testing through their health care providers after that date, but those tests aren’t guaranteed to be free. The federal government’s CARES Act requires insurance companies to cover the cost, but many labs do not accept insurance.

On Monday the city also reported its fourth-straight day without COVID-19 related deaths. It’s the longest stretch of days without deaths from the virus since the pandemic began in March.