The Biden administration is buying hundreds of millions more doses of the Pfizer-BioNTech coronavirus vaccine to donate to the world, according to two people familiar with the deal, as the United States looks to increase efforts to share vaccine with the global population.

The administration is expected to purchase 500 million doses, but the terms are not finalized, said the people with knowledge of the deal, who spoke on the condition of anonymity because they were not authorized to discuss the donation.

The announcement of the purchase is slated for early next week and timed to coincide with the U.N. General Assembly meeting.

Jeff Zients, the White House coronavirus coordinator, declined to comment on the deal but said vaccine access “will be a big topic of conversation” at the U.N. gathering next week.

Pfizer spokeswoman Amy Rose said in a statement: “Pfizer is firmly committed to doing all we can to ensure equitable and affordable access to our COVID-19 vaccines for people around the world. While we don’t have any specific news to share today, we are actively working with governments around the world as well as global health partners towards that goal.”

The purchase would mark the second major effort by the United States to distribute vaccine to the world. In June, the United States purchased 500 million vaccine doses to be distributed by Covax, the World Health Organization-backed initiative to share doses globally, and officials said the vaccine would be targeted at low- and middle-income countries.

The White House also formally announced Friday that it is hosting a virtual summit of world leaders and global health advocates alongside next week’s General Assembly meeting.

“This meeting is about expanding and enhancing our shared efforts to defeat COVID-19, building out from previous gatherings of world leaders and ministers in fora like the G7, G20, and Act Accelerator to rally civil society, NGOs, philanthropists, and industry along with world leaders and align on a common vision for defeating COVID-19 together,” White House press secretary Jen Psaki said in a statement.

President Biden plans to use Wednesday’s summit to call on global leaders to make new commitments to fight the coronavirus pandemic, including fully vaccinating 70 percent of the world’s population by next September and securing billions of additional doses for the developing world, among other targets, according to a list obtained by The Washington Post.

The announcement comes amid growing criticism that the United States is not doing enough to help vaccinate the world, especially as the administration moves forward with Biden’s plan to offer booster shots to Americans.

On Friday, a Food and Drug Administration advisory committee unanimously recommended a coronavirus vaccine booster be authorized for people 65 and older and those at high risk of severe illness. But the panel resoundingly voted against recommending boosters for everyone 16 and older, which Pfizer had sought.

The WHO has called on countries to stop rolling out booster shots at least through the end of the year, arguing that poorer countries should have greater access to vaccine. Experts have said 11 billion doses are needed to vaccinate 70 percent of the global population, arguing that would significantly curb the spread of the virus and tamp down the risk of new variants.

The Biden administration has been adamant that the United States has enough vaccine supply for booster shots and global donations.

“We feel it’s a false choice to suggest it’s either give to the world or not,” Psaki said Thursday. “We are continuing to increase the supply of vaccines we’re giving to the world. We will continue to have more announcements on that because we want to be the arsenal of vaccines to the world, and we are giving more than every other country in the world combined.”

Advocates said they were heartened to learn the White House was planning to donate more vaccine to nations in need.

“The news that the Biden administration will share more covid-19 vaccine doses with the world is the kind of action we need alongside the continued push to get more Americans vaccinated,” said Carolyn Reynolds, co-founder of the Pandemic Action Network, which has worked with dozens of organizations to develop a list of goals for next week’s summit.

“Frankly, America can and should commit even more, and it is in our national interest to do so,” Reynolds said, adding that much of the world “is suffering from a vaccine famine — and we will not end this pandemic until we get more jabs in arms everywhere.”

Global health experts have spent months panning wealthy nations’ efforts to tackle coronavirus outbreaks in developing countries, calling them insufficient and urging more action — a concern echoed by congressional Democrats in a letter Friday to Biden.

“So far, 5.82 billion doses have been administered globally, but less than 2% of the population living in low-income countries received even one dose,” Sens. Tina Smith (D-Minn.) and Elizabeth Warren (D-Mass.) and six colleagues wrote to Biden, calling for “additional U.S. leadership” on the world stage. “Clearly, there is an inequitable distribution of COVID-19 vaccine doses, and it is getting worse.”