Gay rights activists, who have been sharply critical of the administration, have been demanding an emergency declaration for weeks. “This is all too late,” said James Krellenstein, a founder of PrEP4All, an advocacy group that works to expand treatment for people with H.I.V. “I don’t really understand why they didn’t do this weeks ago.”

The F.D.A.’s plan to consider fractional doses of Jynneos took some federal scientists by surprise.

There is some data to suggest that injecting one-fifth of a regular dose of Jynneos between skin layers would be just as effective as the approach being used now, administering a full dose under the skin. The skin is rich in immune cells that mediate the response to vaccines, so this approach is sometimes used, especially with vaccines in short supply, although it requires more skill.

Researchers at the National Institutes of Health had planned to test the strategy for Jynneos in a clinical trial that was set to begin in a few weeks, with results expected later in the fall.

“That was our plan, so we’ll have to see how it fits into the new landscape, which has changed,” said Dr. Emily Erbelding, who directs the N.I.H.’s division of microbiology and infectious diseases. “We thought that there was a desire to get a more robust data set, but if it’s a race against time, then this is a different situation.”

“Things are moving fast,” she added.

Declaring an emergency gives the C.D.C. more access to information from health care providers and from states.

During the outbreak, federal health officials have regularly shared information on testing capacity or on the number of vaccines shipped to states. But the C.D.C.’s data on the number of cases lags that of local public health departments, and the number of people vaccinated, or their demographic information, is mostly unavailable.