WASHINGTON — In a sharp break with the Trump administration, President-elect Joe Biden intends to release nearly all available doses of the coronavirus vaccine soon after he is inaugurated, rather than hold back millions of vials to guarantee second doses will be available.

The decision is part of an aggressive effort to “to ensure the Americans who need it most get it as soon as possible,” the Biden transition team said Friday. The vaccination plan, to be formally unveiled next week, also will include federally run vaccination sites in places like high school gyms and sports stadiums, and mobile units to reach high-risk populations.

The president-elect has vowed to get “at least 100 million COVID vaccine shots into the arms of the American people” during his first 100 days in office.

The decision to release the vast majority of vaccine doses set off a sharp debate among public health experts. The two vaccines that have received emergency approval each require two doses, and the Trump administration has so far been holding back about half of its supply to ensure that booster doses will be available for those already inoculated.

Officials at Operation Warp Speed, the Trump administration’s vaccine program, had noted that doses would stop being sequestered after the first few weeks of rollout. But the announcement by the Biden administration sets the clearest bench mark yet for front-loading shots, then distributing them as they become available. And Warp Speed officials Friday were critical of the president-elect’s decision.

The Food and Drug Administration — whose advice Biden has pledged to follow — has spoken out strongly against changing the dosing schedule, as some other countries have opted to do, calling such a move “premature and not rooted solidly in the available evidence.” Some public health experts fear that second doses would be delayed by the decision.

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But others called it a smart measure and said it was imperative to get as many people vaccinated as quickly as possible — so long as the second doses are not delayed. The Biden team said it was confident that the supply would be enough, and that Biden would invoke the Defense Production Act if necessary to bolster the supply of second doses.

“The president-elect believes we must accelerate distribution of the vaccine while continuing to ensure the Americans who need it most get it as soon as possible,” said T.J. Ducklo, a spokesperson for the Biden transition team.

The announcement that Biden intends to free up extra doses coincided with a letter from eight Democratic governors — including Andrew Cuomo of New York and Gretchen Whitmer of Michigan, both of whom have clashed with President Donald Trump — imploring the current administration to release all available doses to the states as soon as possible.

“The failure to distribute these doses to states who request them is unconscionable and unacceptable,” the governors wrote in the letter, which was obtained by The New York Times and sent Friday to the secretary of health, Alex Azar, and Gen. Gustave F. Perna, who is in charge of vaccine distribution.

“We demand that the federal government begin distributing these reserved doses to states immediately,” the letter said.

Biden’s promise of 100 million shots in arms is an ambitious one, and the Trump administration’s rocky rollout — which Biden called “a travesty” Friday — has not made his task any easier. As of Thursday, the Trump administration had shipped more than 21 million vaccine doses, and millions more were already in the federal government’s hands.

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Yet only 5.9 million people had received a dose. State and local public health officials, already overwhelmed with rising infections, have been struggling to administer the vaccine to hospital workers and at-risk older Americans while most people remain in the dark about when they might be protected.

The biggest problem so far has not been a lack of vaccine, but the difficulties that state and local governments face in distributing the doses they have. Capacity and logistics, not shortages, are keeping vaccines from being administered.

Dr. Leana S. Wen, an emergency physician and public health expert at the George Washington University School of Public Health, said she was surprised and concerned about Biden’s new strategy.

“This is not the problem we’re trying to solve right now,” Wen said.

Officials at Operation Warp Speed were also critical.

Speaking at a news briefing Friday, Dr. Stephen M. Hahn, the FDA commissioner, urged states that have used only a small part of their supply to begin vaccinating lower-priority groups, while still observing government guidelines. Most states are still prioritizing front-line health care workers and older Americans in group residential settings.

Expanding the targeted groups “will go a long way toward using these vaccines appropriately and getting them into the arms of individuals,” Hahn said.

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Biden advisers have not discussed the rest of their plan to overhaul vaccine distribution; they say more details will be released next week. Biden has always promised a far more muscular federal response than Trump’s leave-it-to-the-states approach, and he has been outlining his vision in public appearances and interviews with local radio stations while campaigning early this week for Georgia’s Democratic Senate candidates.

“Our plan is going to focus on getting shots into arms, including by launching a fundamentally new approach, establishing thousands of federally run or federally supported community vaccination centers of various size located in places like high school gymnasiums and NFL stadiums,” Biden said during an interview with WFXE-FM in Columbus, Georgia.

“And,” he went on, “they can be run by federal staff, contractors, volunteers, including FEMA, you know, the emergency management group, the Centers for Disease Control, the U.S. military, the National Guard.”

A person familiar with Biden’s vaccination plan said it would take time to establish mass vaccination sites. Biden himself said Friday that the vaccine effort would be the “greatest operational challenge we will ever face as a nation” — one that would cost “billions of dollars.”

Joshua M. Sharfstein, a former top FDA official who is now a professor at the Johns Hopkins School of Public Health, said mobile units would be especially important in reaching people in rural areas, as well as underserved populations, where transportation or lack of interest was a barrier to vaccination.

“We need to make the vaccine easily available to the people who are excited to get it now, and we need to go out and reach people who are at special risk but still have questions or are unable to make it to vaccine sites,” he said. “If we just vaccinate people who are excited to be vaccinated, we will miss people who really need to be vaccinated and deserve to have special access.”

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The dosing schedule, though, is a major complication. Booster shots for Moderna’s vaccine must be given four weeks after it is administered; for the vaccine made by Pfizer-BioNTech, the interval is three weeks. On Friday, officials at the World Health Organization issued recommendations saying that the interval between the administration of two doses of the Pfizer vaccine could be doubled to six weeks. But the FDA has not approved such a shift.

Biden, who was given the vaccine nearly three weeks ago, will have his second dose Monday, his spokesperson, Jennifer Psaki, told reporters Friday.

With the virus raging across the country, and more than 367,000 Americans already dead from COVID-19, some experts have expressed hope that hastening the vaccine rollout would help bring the pandemic under control.

But there is not enough data yet to know what effect the vaccine has on transmission of the virus. And without knowing the effect that the vaccine has on transmission, it is impossible to predict whether vaccinating more people will drive down caseloads.

Models outlining the impact of front-loading first doses are intriguing, said Olivia Prosper, a mathematical modeler at the University of Tennessee.

But while “they bring up a lot of food for thought and some hypotheses to test,” it is still premature to use them to inform public policy, she said. Models also have their limits, she noted, because most do not take into account the nation’s tiered vaccination strategy, which prioritizes high exposure and high-risk people for early shots.