Plasma from people recovering from infection, particularly after severe illness, may contain high levels of polyclonal, pathogen specific antibodies. These antibodies may confer passive immunity to recipients, and in viral diseases are thought to act mainly by neutralising viral particles. 1 Convalescent plasma, or purified antibodies from the plasma (hyperimmune globulin), was often used in clinical practice before the advent of vaccines, including during the influenza pandemic of 1918. 2 Hyperimmune globulin is still used for post-exposure prophylaxis against various viral infections, including hepatitis B, varicella zoster, and rabies.

The use of convalescent plasma to treat patients with covid-19 has understandably attracted a lot of attention, but definitive evidence of efficacy has been elusive. Nevertheless, on 23 August the US Food and Drugs Administration authorised its emergency use for hospital patients with covid-19. At the time, only two small underpowered trials had been published. 3 4