A global trial of the pill enrolled 775 people with mild or moderate covid-19. Participants had at least one risk factor for severe covid-19, such as obesity or advanced age. They had to start the drug regimen within five days of symptom onset and be unvaccinated.

Half of study participants received the drug and half received a placebo. No deaths were reported among people receiving the drug, but eight deaths were recorded among those who received a placebo. The rate of hospitalization and death in people who received the drug was 7.3 percent — about half the level for those who received a placebo.

Those results, showing the drug afforded significant protection, led an independent safety board to suggest halting the trial.

Laboratory and animal experiments suggest the pill may be effective against known variants, including delta. Unlike vaccines or antibodies that target specific proteins on the surface of the virus, molnupiravir works by introducing genetic errors that garble the coronavirus’s genetic code. That means it may be more resistant to mutation, and may even work on other coronaviruses or RNA viruses.