President Biden is set to announce on Wednesday that the United States is buying 500 million more doses of the Pfizer-BioNTech COVID-19 vaccine to donate to countries around the world, a pledge that will bring the total promised U.S. vaccine donations to more than 1.1 billion.Biden will make the announcement during remarks at the start of a virtual summit aimed at boosting commitments from other nations and the private sector. The effort's goal: vaccinate 70% of the world's population for COVID-19 within a year's time, senior administration officials told reporters.That goal would require about 5 billion more doses for low- and middle-income countries, meaning the U.S. pledges will account for about one-fifth of the global need, said Carolyn Reynolds, co-founder of the Pandemic Action Network. "The challenge now is timing and delivery," Reynolds said. While there should be enough supply to meet the 70% global target by next year, more work is needed to get shots in arms much faster, Reynolds said.The donation comes as the United States and other wealthy countries face withering criticism from the likes of the World Health Organization for pouring resources into plans to offer booster shots to fully vaccinated people, even as vast swaths of the world have yet to receive a single shot.On Tuesday, United Nations Secretary-General António Guterres excoriated world leaders, opening the U.N. General Assembly with withering criticism of the "lack of political will, selfishness and mistrust" in the response to the pandemic. "This is a moral indictment of the state of our world. It is an obscenity," said Guterres, who will participate in the Wednesday summit. Dr. Tedros Adhanom Ghebreyesus, the director-general of the WHO, will also speak.The White House has argued that it can both prepare for offering booster shots while also helping other parts of the world. The U.S. has so far shipped almost 160 million doses to 100 countries around the world, a number that exceeds all other donations combined, a U.S. official told reporters ahead of the summit. The new tranche of Pfizer doses will be manufactured in U.S. plants and will be bought at a "not-for-profit" price, the officials said. Most U.S. vaccine shipments — 800 million doses — will ship between January and September 2022.Biden will speak at the beginning of the summit, and Vice President Harris will lead another session. In total, it's expected to last more than three hours and will include about 30 speakers, including six heads of state. Officials from more than 100 countries and more than 100 organizations, private sector companies and philanthropists are expected to participate.The White House wants to set specific targets for vaccines as well as donations of oxygen, tests, therapeutics and protective gear to parts of the world that need it — and work on a health security financing mechanism so that the world is more prepared for future pandemics. Leading up to the summit, the White House made a series of specific requests for commitments from participants — a list that officials said would be unveiled at the end of the summit. Copyright 2021 NPR. To see more, visit https://www.npr.org.