The trade-offs are very different when considering emergency authorization for products intended to prevent rather than treat disease. The FDA has used this pathway for a vaccine only once, in 2005, allowing a decades-old vaccine to be used to prevent anthrax in a bioweapon attack. When the agency authorized emergency use of other preventive drugs, as it did for anthrax in 2008 and H1N1 influenza in 2009, the drugs were already FDA-approved. Emergency authorization of a new vaccine for the novel coronavirus would be unprecedented.

Emergency treatments may be given to thousands, or tens of thousands, of very sick patients. But a vaccine could reach millions — even tens of millions or hundreds of millions — of healthy people. Because vaccine recipients are not sick, their potential to benefit stands to be much less than that of seriously ill patients given a drug, especially in light of the effectiveness of alternatives such as physical distancing and good hygiene. If data ultimately shows that vaccine effectiveness is limited while the risks are substantial, harms to millions of uninfected individuals could be enormous. False reassurance from a flawed vaccine might also worsen the epidemic, setting back progress.

Public trust in vaccines is another critical consideration. Already, the anti-vaccine movement has begun to sow doubt about a coronavirus vaccine. Even if an option ultimately proves to be safe and effective, it won’t do much good if many people refuse it. And if a covid-19 vaccine causes harm, efforts to counter other vaccine-preventable diseases may be set back as well.