Some activists say that Biden’s new plan for donating vaccines is not enough.

When President Biden announced in June that the United States would buy 500 million Pfizer-BioNTech coronavirus vaccine doses for poorer nations, there was a gaping hole in his plan: To fund it, the administration quietly diverted hundreds of millions of dollars that had already been promised to countries for helping to get shots into people’s arms.

Mr. Biden did not make the same mistake a second time.

His announcement on Wednesday that the United States was donating an additional 500 million Pfizer doses came paired with a promise of an additional $750 million for vaccine distribution, roughly half of it through a nonprofit involved in global vaccinations. That reflected a growing awareness on the part of global leaders that turning vaccines into actual vaccinations represents one of the most significant challenges of this phase of the pandemic.

Even so, the Biden administration’s schedule for shipping the newly announced Pfizer doses frustrated activists: Of the 1.1 billion doses that the United States has committed to donations, only 300 million are expected to be shipped this year. The longer the virus circulates around the world, the more dangerous it can become, even for vaccinated people in wealthy countries, scientists have warned.

“Purchasing doses for donation sometime next year is helpful, but it does not meaningfully expand the global supply, and it is not justice,” said Peter Maybarduk, the director of the Access to Medicines program at Public Citizen, an advocacy organization.

Some experts pleaded for more aggressive action by Mr. Biden to pressure American vaccine makers to share their formulas with nations that desperately need more shots.

“Where is the monthly calendar of what each wealthy country is going to deliver?” said Kate Elder, senior vaccines policy adviser for Doctors Without Borders’ Access Campaign. She added, “Where are the announcements about meeting the calls of regions and middle-income countries that are begging to be self-sufficient?”

It was not immediately clear how the administration was planning to allocate the new money for vaccine distributions. Some countries have been struggling to train and pay vaccinators and to transport doses. Pfizer doses present especially steep challenges: They must be stored at ultralow temperatures, requiring the installation of freezers and, in many cases, backup generators.

Early this year, supply shortages represented the most pressing problem for global vaccinations. Rich nations had raced to secure doses while slow-walking pledges of money and supplies to Covax, the United Nations-backed program to immunize the world.

But as those manufacturing difficulties have eased, different problems emerged. One was blatant inequality: More than 5.9 billion shots have been given globally, but overwhelmingly in wealthier nations. Another was that poorer countries had been left without the money needed to move shots from airport tarmacs into people’s arms.

Although the Biden administration’s pledges have outpaced those of most Western nations, analysts said other wealthy countries were setting an extraordinarily low bar.

And even as countries have promised to donate more doses, they have been slow to follow through, with the World Health Organization saying that only 15 percent of promised donations have been delivered. The W.H.O. has set a target of vaccinating 40 percent of every country’s population this year, requiring wealthy nations to share doses significantly more quickly.

There remains considerable excess supply: Wealthy nations will have 1 billion more vaccines than they need by the end of 2021, even if they administer booster shots, according to Airfinity, a science analytics company.

Global health officials have urged wealthy nations to let go of those supplies before they expire. Some countries have donated vaccines so close to their expiration dates, and in such small quantities, that poorer countries have struggled to use them.

Some 11 billion doses are needed globally, and activists said on Wednesday that the wealthy world’s response remained far too piecemeal. The new Pfizer doses purchased by the U.S. will be shipped through Covax starting in January.