President Trump’s announcement two weeks ago that he was taking the antimalarial drug hydroxychloroquine as a measure against Covid-19, if true, is puzzling, because  scientific evidence is growing that the drug offers no benefit to Covid-19 patients and could be dangerous. Richard Malley and Marc Lipsitch write in the New York Times that Trump’s decision does highlight the urgent need to devote more attention and resources to drugs that can prevent COVID-19 or stop mild cases from getting worse. There is an urgent need to help the people most severely affected, but we think a parallel effort is equally urgent: conducting clinical trials to identify drugs that could prevent people from becoming infected with SARS-CoV-2 in the first place or prevent mild cases of Covid-19 from becoming severe.