President Joe Biden will announce on Wednesday that the United States is doubling its commitment to buy hundreds of millions of Pfizer’s Covid-19 vaccine and ship them to other nations that are struggling to vaccinate their people, White House officials said.

Officials said the U.S. government will buy 500 million additional doses, increasing the overall commitment to more than 1.1 billion.

Biden is expected to make the announcement at a virtual United Nations Covid-19 summit in an event affiliated with the U.N. General Assembly in New York City this week.

The White House said the additional vaccine doses will begin shipping in January, and that between January and September 2022 a total of 800 million doses will be sent internationally.

“President Biden has been clear that the only way to defeat Covid-19 and protect the American people and grow the American economy is to defeat the virus both here at home and around the world,” a senior administration official told reporters on Tuesday.

“That means vaccinating our own people and vaccinating people around the world is not a choice, it is an imperative. That’s why we have mobilized a whole-of-government effort to do both.”

The White House said the government has already shipped nearly 160 million vaccine doses to 100 struggling and developing countries, including Peru, Pakistan, Sri Lanka, Sudan, El Salvador and Ethiopia.

“To put this into perspective, the United States has now delivered more free doses than every other country in the world combined,” the official added. “Millions more get shipped each day, including today. And importantly, our donations come with zero strings attached.”

The White House emphasized that the doses are manufactured by American workers and aid the domestic economy.

“For every one shot we have administered in this country to date, we are now donating three shots to other countries,” the White House official said. “No other country, or group of countries, have come close to that.”