Regarding hasty approvals of treatments during a pandemic, some will earnestly ask, “What’s the harm?” Until now, the harms of convalescent plasma have been played down. That is a mistake. Physicians and scientists decried the premature use of hydroxychloroquine, promoted by the president, on grounds that it has potentially damaging cardiac side effects, leading the FDA to revoke its earlier emergency-use authorization for the drug. The FDA was right. But the cardiac risk was always relatively low. It is troubling that even some medical experts assume that plasma is safer.

Plasma transfusions of all kinds have a well-documented rate of serious adverse events. These events include lung damage and even life-threatening anaphylaxis. Transfusions can cause an array of immune reactions, some welcome and some not, depending on the context. Given that covid-19 is in part driven by an autoimmune reaction to the novel coronavirus, more study is essential before doctors proceed with the plasma treatment.

Some data are already available. In a recent study of plasma in covid-19 patients, 25 serious reactions occurred among 5,000 transfusions. That’s 1 in 200. Four people died. Though it is hard to know if these deaths were directly or indirectly related to plasma, they must be taken seriously.