It will be up to vaccine manufacturers to decide whether to request E.U.A.s for their Covid-19 vaccine candidates. Many public health experts say that there are conditions under which an E.U.A. for a coronavirus vaccine would be appropriate. An application for formal approval of a vaccine can require tens of thousands of pages of documentation, and even a priority review of a new vaccine typically takes eight months after clinical trials are completed.

“There’s a lot of I’s you have to dot and a lot of T’s you have to cross” to get a vaccine approved, said Susan Ellenberg, a former F.D.A. official and a professor of biostatistics at the University of Pennsylvania. After a clinical trial was completed, she said, she could imagine that, absent any safety issues, agency officials “might issue an E.U.A. before they tied up everything for the formal approval.”

Still, Dr. Ellenberg noted, “We’ve seen an unprecedented involvement of the high-level administration in some of these issues.”

Because it is unclear how much political pressure is being put on the scientists at the F.D.A., she said, she would want to see the data from clinical trials before deciding whether to trust a vaccine authorized under an E.U.A.