In this Tuesday, May 5, 2020 file photo, a health worker draws blood from a patient for a COVID-19 coronavirus antibody test in DeLand, Fla. Scientists are still working to figure out how well antibodies for the new coronavirus may shield someone from another infection, or how long that protection might last. (AP Photo/John Raoux)

At the height of the coronavirus lockdown, President Donald Trump and his top health advisers trumpeted a new test that would help Americans reclaim their lives—one that would tell them if they already had the virus and were protected from getting it again.

Their arrival