Medical literature published during the Spanish flu pandemic of 1918 includes case reports describing how transfusions of blood products obtained from survivors may have contributed to a 50% reduction in death among severely ill patients. In 1934, a measles outbreak at a Pennsylvania boarding school was halted when serum harvested from the first infected student was used to treat 62 fellow students. Only three of the 62 students developed measles — all mild cases.

More recently, plasma-derived therapy was used to treat patients during outbreaks of Ebola and avian flu. And on Wednesday the Japanese drugmaker Takeda Pharmaceutical Co. said it was developing a new coronavirus drug derived from the blood plasma of people who have recovered from Covid-19. Its approach is based on the idea that antibodies developed by recovered patients might strengthen the immune system of new patients.

But there are plenty of unknowns about whether such treatments could work with Covid-19. And if they are useful in treating severely ill patients, they are far from a cure-all.