The Biden administration is buying 500 million doses of Pfizer’s coronavirus vaccine to donate to the world as the United States dramatically increases its efforts to help vaccinate more of the global population, three people familiar with the plans said Wednesday.

The first 200 million doses will be distributed this year, with the subsequent 300 million shared in the first half of next year. The doses will be distributed by Covax, the World Health Organization-backed initiative to share doses around the globe, and they will be targeted at low- and middle-income countries. Pfizer is selling the doses to the United States at a “not-for-profit” price, according to the people familiar with the deal, who spoke on the condition of anonymity to share details that were not yet public.

President Biden is slated to announce the plan this week at the Group of Seven meeting in Britain, where he is expected to be joined by Pfizer CEO Albert Bourla. The deal comes amid growing calls for the United States and other rich countries to play a more substantial role in boosting the global supply of coronavirus vaccines.

The White House and Pfizer declined to comment, but the president hinted he would be announcing his global plan as he boarded Air Force One to Britain on Wednesday.

“I have one, and I’ll be announcing it,” he told reporters.

Many public health experts and advocacy groups cheered the news of the White House’s deal with Pfizer, saying U.S. leadership on the issue will be critical to vaccinating the world — even if much remains to be done.

“It’s an extraordinary development,” said Jennifer Nuzzo, an epidemiologist and senior scholar at the Johns Hopkins Center for Health Security, adding that the announcement “sends a profound signal in terms of U.S. commitment to global health security and willingness to help end this pandemic for the world and the United States.”

The question of how to close the vaccine gap and end the pandemic is expected to be front and center at the G-7 summit this week. In the lead-up to the meeting of wealthy democracies, Biden’s vaccine-sharing strategy has been under intense scrutiny — both at home and abroad.

“The president is focused on helping to vaccinate the world because he believes it is the right thing to do; it’s what Americans do in times of need,” Jake Sullivan, Biden’s national security adviser, told reporters aboard Air Force One, while declining to discuss specifics of the president’s announcement. “When we have the capacity, then we have the will, and we step up and we deliver. And he said in his joint session that we were the arsenal of democracy in World War II, and we’re going to be the arsenal of vaccines over the course of the next period to end this pandemic.”

The Biden administration previously announced it would share at least 80 million vaccine doses with the world by the end of June. Last week, the White House detailed plans for how it would allocate 25 million doses, with about 19 million of them being shared with Covax. Roughly 6 million shots would be shared directly with countries experiencing severe coronavirus outbreaks, including India.

But congressional Democrats and some health advocates have been calling for the administration to do more. At the same time, Biden’s surprise decision to support a proposal to waive patent protections for coronavirus vaccines has faced strong pushback from the European Union. Experts have said the patent waiver, which would probably not even be approved for months, will do little to boost vaccine supply in the near term, as it could take years before countries build factories and amass the materials and expertise to produce the vaccines.

Questions about how to proceed have intensified in recent weeks as cases in the United States have receded and infections have surged in some developing countries without adequate vaccine supply, leading to fresh charges of “vaccine apartheid.”

The gap between vaccines haves and have-nots is vast. More than half the populations in the United States and Britain have had at least one dose of coronavirus vaccine, compared with fewer than 2 percent of people in Africa.

So far, the global effort to close that gap has been piecemeal. Some wealthy nations have announced plans to donate surplus doses and have expressed support for the idea of boosting global supply — but specifics on when and how to proceed are scarce.

Covax aims to deliver 2 billion doses by the end of the year, with an eye toward vaccinating 20 percent of the populations of countries in need, but it may not meet that relatively modest target. The initiative has been plagued by funding shortfalls and a severe supply crunch exacerbated by the crisis in India, leading to potentially deadly delays. To date, Covax has delivered just under 82 million doses to 129 countries.

After a successful vaccination effort in the United States, Biden tapped Jeff Zients, the covid-19 coordinator, to oversee the country’s global vaccination strategy. Zients has been working on the deal with Pfizer for more than a month, an official familiar with the deal said, and the White House wanted the announcement to be a signature part of Biden’s trip to the G-7.

“It is meaningful,” said Thomas J. Bollyky, a senior fellow at the Council on Foreign Relations and director of its global health program, “but not sufficient on its own.”

To assess the scale, he said, consider that 500 million doses is about six times the doses Covax has distributed so far. At the same time, it amounts to just a quarter of the 2 billion doses the initiative aimed to distribute this year.

“These Pfizer doses will go to many countries,” Bollyky said. “The big question is, in what order and in what amount? That will have significant bearing on what the public health impact of the commitment will be.”

With the United States’ forthcoming announcement, international health organizations are now calling on other wealthy countries to increase their global commitments, as well, warning about the need to curtail the virus around the world to prevent more dangerous variants from spreading.

“We won’t end this global pandemic anywhere unless we beat it everywhere,” Tom Hart, acting CEO of the One Campaign, an organization focused on fighting global poverty and preventable disease, said in a statement. “Donating doses to COVAX will save lives, reduce the spread of variants, and help reopen the global economy. We urge other G-7 countries to follow the US’ example and donate more doses to COVAX. If there was ever a time for global ambition and action to end the pandemic, it’s now.”

As the president travels overseas to tout the Pfizer deal, his administration still faces hurdles in the domestic vaccination effort. Biden, who has grown accustomed to touting victories over the virus, now faces two disquieting trends. The slowdown of the vaccination campaign in the United States — only about 50 percent of the population having received at least one dose — has coincided with the growing prevalence in the country of a highly transmissible variant already imperiling Britain’s path back to normal and forcing Prime Minister Boris Johnson to decide whether to fully reopen his country as planned.

“It’s highly likely we will see a very similar trend as in Britain,” said William Lee, vice president of science at Helix, a population genomics company.

The variant, first identified in India and known as delta, accounts for 6 percent of new infections in the United States, Anthony S. Fauci, the nation’s leading infectious-disease expert, said Tuesday. And while preliminary evidence suggests the two-dose vaccines are effective against the variant, there is significantly less protection after just one dose, Fauci said.

Meanwhile, the seven-day average of daily vaccinations is hovering at around 1 million, well short of the rate necessary to meet Biden’s goal of delivering at least one dose to 70 percent of adults by July 4.

The two dynamics are intertwined. And because of the stark regional variations in immunization levels, they threaten to drive summer outbreaks of the virus in areas where vast swaths of the population have yet to get the shots, especially in the southeast, experts said.

But public health officials said the global vaccination effort needs to remain a top priority given how cases are still surging in low-income countries around the world. Biden had promised the United States would lead the effort, and the Pfizer announcement will be a centerpiece of it.

The news of the deal even received bipartisan praise.

“This is exactly what the federal government should be doing: working with the companies who developed lifesaving covid-19 vaccines to make them available to the rest of the world,” Sen. Richard Burr (R-N.C.) said in a statement. “This news is a reminder of what’s possible when we partner private sector innovation with the public sector’s resources and reach.”