When news broke in April that the drug remdesivir had been shown to speed recovery in patients hospitalized with COVID-19, Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases, hailed the finding as “an important proof of concept” in the race to bring the pandemic to heel.

Unlike a vaccine, which prompts the body to mount a defense against invading viruses, remdesivir is an antiviral drug, which hampers the ability of a virus to replicate and spread. For now, results related to remdesivir are mixed, although some studies continue to suggest the drug can improve outcomes for patients with severe forms of COVID-19. Still, only a few decades ago, most scientists doubted such a thing was