As of Sunday, the University Hospital in Madison, Wis., part of UW Health, had transfused 11 Covid patients with convalescent plasma under the expanded-access protocol, said William Hartman, an anesthesiologist and one of the investigators on the study. Eight of the patients were in life-threatening situations and now are in various stages of recovery, he said. The other three received plasma before or just after admission to the intensive-care unit and have shown improvement: One was discharged from the hospital; one was taken off a ventilator within a day and symptoms have improved. The third hasn’t worsened and hasn’t required ICU admission, he said.

“There is no lab test that proves convalescent plasma caused these results,” Dr. Hartman said. “Based on when we gave them the transfusion and the outcomes, we are encouraged.”

Between 5,000 and 10,000 people may ultimately be eligible to enroll in the convalescent plasma program, the Mayo Clinic’s Dr. Joyner said. Investigators will compare patients who get the plasma with similar patients who didn’t receive it, such as very ill patients at a hospital where the therapy wasn’t available. Researchers hope the knowledge they gather can inform future trials and aid doctors and researchers in another outbreak.