To receive an authorization for emergency use, a developer must demonstrate that—based on the totality of scientific evidence—the products’s known or potential benefits outweigh the known and potential risks. Some worry that this is a lower standard for access that shouldn’t be applied to a vaccine, where the bar for safety needs to be high, as vaccines are given to the healthy. But the FDA has latitude to apply the standard appropriately to different settings—in particular, requiring more-rigorous evidence for treatments used on healthier populations than for seriously ill hospitalized patients.

This authority enables the staged entry of a vaccine. It’s unlikely that a Covid-19 vaccine will receive full approval and broad distribution right away. Instead, the FDA will probably authorize vaccines for use in targeted groups of people at high risk from Covid and most likely to benefit from the vaccine. For them, it may make sense to provide access to the vaccine before long-term follow-up studies that address very remote risks.

This might include health-care providers or first responders, who face greater exposure, or older people, who are more prone to severe complications if infected. But even to receive emergency use approval, trials must show that these groups could safely benefit from the vaccine. All this means that at least initially, Covid vaccines won’t provide the sort of herd immunity that can help extinguish an epidemic.