Vaccines have been allocated state by state based on population - a formula the Biden administration held to even as some states such as Michigan saw increases in coronavirus infections.
Under the new policy, unordered doses would be made available to states with a more immediate need, White House spokeswoman Jen Psaki told reporters.
"They can still order ... up to their full allocation based on population," Psaki said. "They can also order less than their full allocation and ... the doses that are not being given to that state will then go into kind of an overarching supply that could then be distributed to other states by population."
The shift, first reported by the Washington Post, was conveyed to governors by the Biden administration on Tuesday.