WASHINGTON - The White House on Tuesday told states that coronavirus vaccine doses they choose not to order will become available to other states - the most significant shift in domestic vaccine distribution since President Joe Biden took office, and part of an effort to account for flagging demand in parts of the country.

The changes were unveiled to governors as Biden set a goal of providing at least one shot to 70 percent of adults by July 4, an increase that would account for about 40 million more people in the next two months. That level of coverage could drive down cases sharply, as it did in Britain and Israel. But achieving it, experts said, depends on efficiently delivering shots to places where people are still rolling up their sleeves - or can be persuaded to do so.

"The sooner we get the most people vaccinated not only in our local regions, but around the country, the sooner we will have fewer variants developing and less spread in general," said David Kimberlin, a pediatric infectious-disease specialist at the University of Alabama at Birmingham. "Now that there are places saying, 'Our freezers are full, so please don't send any more,' there needs to be an ability to reallocate."

Each state's share of the total U.S. adult population will still determine weekly allocations. But instead of carrying over unordered doses week to week, the White House will steer untapped vaccine into a federal bank available to states seeking additional supply. Those states will be able to order up to 50 percent above their weekly allocation, while states declining their complete allotments one week will still have access to their entire share the following week.

The strategy, designed to maximize flexibility for states, could transform how vaccine flows across the country. In recent weeks, numerous states have begun leaving significant quantities of doses unordered. Last week, officials in Arkansas declined the state's entire share. The state's Republican governor, Asa Hutchinson, said he favored the changes because doses that may be unneeded in Arkansas "can be used for the urgent needs across the country, where there's a higher acceptance rate, where there's a higher demand."

"Maybe that's a motivator - that if we don't use the vaccines that are available to us here in Arkansas, then those vaccines might go to Massachusetts, because there's a higher acceptance rate there," he said during a briefing, noting in an appeal to state residents, "We have to increase our demand for it."

The scramble to use all available shots is intensifying as the pace of daily vaccinations decreases significantly - and as health authorities pivot to inoculating hesitant and hard-to-reach populations. The seven-day average of daily shots administered dropped by 17 percent during the past week, according to data analyzed by The Washington Post, and by 33 percent since April 13.

Jeff Zients, coordinator of the White House coronavirus task force, said the changes reflect how quickly the government has been able to make shots available to everyone who wants them.

People willing to drive long distances have been covered, he said. More than 80 percent of people older than 65 have received at least one dose. In the next phase of the immunization campaign, defined by efforts to improve access and build confidence, "there is a need to add more flexibility to the current system," Zients said in an interview.

The White House was at pains to avoid the perception of penalizing states - or of picking winners and losers. Those deciding not to order their full allocation in a given week will not lose out permanently, instead ceding supply on a one-time basis. The changes are sensitive because governors have grown protective of vaccine supply, eager to tout expanding availability after a period of intense scarcity in the winter.

Freeing up unordered doses "accomplishes what we all want to accomplish," New York Gov. Andrew Cuomo, a Democrat who is chairman of the National Governors Association, said on Tuesday's White House call with governors, according to a recording of the conversation obtained by The Post. "You have state control of your allocation. If a state isn't using it, then a state that can use it has access to it, which makes a lot of sense to all of us, I think."

Governors requested additional flexibility, including the option to take control of doses currently being directed to pharmacies in their states.

"I just think we can better use those doses," Delaware Gov. John Carney, a Democrat, said on the call, pointing to the state's efforts to achieve equity in access to immunization by "intentionally doing that kind of outreach." Zients said he would examine the issue.

Federal officials had weighed other changes, including metrics that would account not only for unordered inventory but the pace of administering shots already distributed to states, according to people familiar with the discussions. The White House, however, has been intent on adhering closely to the population-based formula, notably rejecting an appeal from Michigan Gov. Gretchen Whitmer, a Democrat, to expedite vaccine shipments to her state as it faced a crushing caseload in March and April.

Days before leaving office in January, Trump administration officials had announced an incentive system that would have rewarded states with additional shots if they quickly used their supplies. Biden's team scrapped that idea and instead retained a population approach in most instances, even as it gradually directed more vaccine doses to retail pharmacies, community health centers and federally run mass-vaccination sites.

As part of the changes announced Tuesday, pharmacies will have greater flexibility to redistribute doses to places where demand is greatest. Guidelines for the federal program made 80 percent of the pharmacy supply tethered to a state's population and the remaining 20 percent available for stores to reallocate. Going forward, only a majority of the supply will be dictated by a state's population, and pharmacies will have discretion over as much as 49 percent of the deliveries.

Nearly 30 million doses were allocated this week, Zients told governors, with about 18.5 million doses made available to states and other jurisdictions. He said the Food and Drug Administration could authorize use of the Baltimore plant producing Johnson & Johnson's vaccine in a "matter of a week or two," making significantly more of the single-shot vaccine available in the United States.

That vaccine, because of its easier handling and storage requirements, is key to expanding access in rural areas and among other hard-to-reach groups, Massachusetts Gov. Charlie Baker, a Republican, said. It could also be a major asset for primary care physicians poised to build confidence in the shots, he said.

The Washington Post's Dan Keating contributed to this report.