At the start of the pandemic, researchers began testing existing antivirals in people hospitalized with severe Covid-19. But many of those trials failed to show any benefit from the antivirals. In hindsight, the choice to work in hospitals was a mistake. Scientists now know that the best time to try to block the coronavirus is in the first few days of the disease, when the virus is replicating rapidly and the immune system has not yet mounted a defense. In a trial of hospitalized patients, however, molnupiravir seemed to have no effect on the disease. In April, the companies announced...