On Aug. 18, 2021, the final results of the Clinical Trial of COVID-19 Convalescent Plasma in Outpatients (C3PO) demonstrated that COVID-19 convalescent plasma did not prevent disease progression in a high-risk group of outpatients with COVID-19, when administered within the first week of their symptoms.

The trial was stopped in February 2021 due to lack of efficacy based on a planned interim analysis. The formal conclusions from the trial appeared in The New England Journal of Medicine.