Science’ s COVID-19 reporting is supported by the Pulitzer Center and the Heising-Simons Foundation. A study in U.S. nursing homes has shown for the first time that monoclonal antibodies, mass-produced in a laboratory, can protect people from developing symptomatic COVID-19. Their manufacturer, Eli Lilly, hopes these antibodies will provide an additional way to protect people at risk of serious disease from the pandemic coronavirus. But given the success of COVID-19 vaccines and their increasing availability, it’s not clear that the expensive and somewhat cumbersome intervention will be widely used. Both Eli Lilly’s monoclonal antibody and a similar two-antibody cocktail from Regeneron Pharmaceuticals—famously used to treat former U.S. President Donald Trump in October 2020—have already received emergency use authorization (EUA) as a therapeutic for those who have become infected and are at high risk of developing severe COVID-19. So far, they are not widely used because they must be given early in infection and infused in a hospital or clinic. But now that they appear effective at preventing even mild disease, Eli Lilly plans to ask the U.S. Food and Drug Administration to expand the EUA to include use as a preventive.