In trials of this size, researchers will know if a vaccine is effective after as few as 150 to 175 infections, said Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, in a call with reporters Friday.

Although the safety board can recommend stopping a trial, the ultimate decision to halt a study is made by the scientists running the trial, Topol said. A vaccine manufacturer could then apply to the Food and Drug Administration for an emergency use authorization, which can be granted quickly, or continue through the regular drug approval process, which requires more time and evidence. Topol and other scientists have sharply criticized the FDA in recent weeks, accusing Commissioner Stephen Hahn of bowing to political pressure from the Trump administration, which has pushed the agency to approve COVID treatments faster. Stopping trials early poses a number of risks, such as making a vaccine look more effective than it really is, Topol said. “If you stop something early, you can get an exaggerated benefit that isn’t real,” because less positive evidence only emerges later, Topol said. Ending vaccine trials early also carries safety risks, said Dr. Paul Offit, a vaccine developer who serves on an NIH advisory panel on COVID vaccines and treatments. A smaller, shorter trial could fail to detect important vaccine side effects, which could become apparent only after millions of people have been immunized, said Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.