President Biden will announce Wednesday that the United States is purchasing an additional 500 million doses of Pfizer's COVID-19 vaccine to donate to the world, as he seeks to boost the global vaccination effort.

The announcement comes as part of a virtual global COVID-19 summit that the White House is hosting on Wednesday, coinciding with the United Nations General Assembly. 

The new purchase will bring the total U.S. commitment of vaccine doses for global distribution to more than 1.1 billion, a senior administration official told reporters.

The newly announced doses will ship between January and September of next year, the official said. The administration has so far shipped about 150 million doses.

The White House will also be calling on the world to meet a target of vaccinating 70 percent of the global population before the next U.N. General Assembly in September 2022, a target an official called “ambitious.”

Still, even with the new dose donation announcements, there is a long way to go to vaccinate the world. The World Health Organization estimates 11 billion doses are needed to vaccinate 70 percent of the world’s population.

The U.S will also be calling on other countries to step up their commitments at the summit on Wednesday, officials said.

The moves come as the Biden administration has been under pressure to do more to help vaccinate people in other countries.

In addition to dose donations, experts and advocates have called on the U.S. to help boost global vaccine manufacturing to make more doses, and to facilitate the transfer of manufacturing know-how to lower-income countries.

Biden will host a session of the virtual summit on Wednesday, along with other officials like Vice President Harris and U.S. Agency for International Development Administrator Samantha Power. In addition to vaccines, other sessions will focus on supplies like oxygen and protective equipment, as well as preparing the world for the next pandemic.

On Tuesday, U.N. Secretary-General Antonio Guterres called out the inequity in global vaccinations in his address to the General Assembly, calling it an “obscenity.”

“A majority of the wealthier world vaccinated. Over 90 percent of Africans still waiting for their first dose,” Guterres said. “This is a moral indictment of the state of our world.”

“We urgently need a global vaccination plan to at least double vaccine production and ensure that vaccines reach seventy percent of the world’s population in the first half of 2022,” he added.