FILE - In this Friday, June 12, 2020 file photo, a doctor holds a bag of blood plasma donated by a COVID-19 survivor at at blood bank in La Paz, Bolivia. Mayo Clinic researchers are reporting a strong hint that blood plasma from COVID-19 survivors helps other patients recover. But it's not proof, and some experts worry if, amid clamor for the treatment, they'll ever get a clear answer. (AP Photo/Juan Karita)