COVID-19 convalescent plasma therapy through the lens of the third year of the pandemic

Serum obtained from recovered persons (convalescent serum) was the inaugural form of antimicrobial therapy for infectious diseases. Its efficacy required the administration of a sufficient amount of microbe-specific serum to mediate a biological effect early in the course of disease. Although serum therapy was discontinued when antibiotics were introduced early in the 20th century, the principles that governed its efficacy were prescient as they apply to the use of COVID-19 convalescent plasma (CCP) to treat COVID-19. Lessons learned from studies of CCP therapy for COVID-19 dating to the onset of the pandemic provide further proof of the principles of serum therapy. Analysis of these studies shows that like serum therapy, the efficacy of CCP in patients with COVID-19 depends on the administration of a high titer of SARSCoV-2 antibodies early in the course of disease.