It sounded like a simple solution to the shortage of monkeypox vaccine: Merely by changing the way doses are injected, the federal government could vaccinate five times as many people with the supply it has in hand.

But the approach — injecting one-fifth of the current dose into the skin instead of a full dose into underlying fat — is not actually all that simple, experts say. And some federal officials are concerned about changing the method without more research, even though Dr. Robert M. Califf, the head of the Food and Drug Administration, described the proposal on Thursday as promising.

Some outside experts, too, are urging caution. “From a basic science perspective, this should work,” said Dr. Jay K. Varma, the director of the Cornell Center for Pandemic Prevention and Response. “But, of course, there are lots of things in life, in science, that we think should work, and then when we actually do them, they don’t.”