The White House has reached an agreement with Pfizer and BioNTech to provide 500 million doses of coronavirus vaccine to about 100 countries over the next year, a pact that President Biden plans to announce as early as Thursday, according to multiple people familiar with the plan.

Under intense pressure to do more to address the global vaccine shortage and the disparities in vaccination between rich and poor nations, the president hinted at the plan Wednesday morning, when he was asked if he had a vaccination strategy for the world.

“I have one, and I’ll be announcing it,” Mr. Biden said, shortly before he boarded Air Force One for his first trip abroad as president. He was headed first to Cornwall, England, to meet with leaders of the Group of 7 nations.

People familiar with the deal said the United States will pay for the doses at a “not-for-profit” price. The first 200 million doses would be distributed this year, and 300 million would be distributed by the middle of next year, they said. Albert Bourla, chief executive of Pfizer, is expected to appear with the president when Mr. Biden makes his announcement.

The United States has already contracted to buy 300 million doses of the Pfizer-BioNTech vaccine, which requires two shots. The new agreement is separate from those contracts, according to one person familiar with it, bringing to 800 million the total number of doses the United States has agreed to purchase from the companies thus far.

The White House coronavirus response coordinator, Jeffrey D. Zients, whom Mr. Biden has put in charge of global vaccination, said in a statement on Wednesday that the president would use the “momentum” of the U.S. inoculation campaign “to rally the world’s democracies around solving this crisis globally, with America leading the way to create the arsenal of vaccines that will be critical in our global fight against Covid-19.”

The 500 million doses, all of which will be produced in the United States, still fall far short of the 11 billion doses the World Health Organization estimates are needed to vaccinate the world, but significantly exceed what the United States has committed to share so far. Other nations have been pleading with the United States to give up some of its abundant vaccine supplies.

Last week, Mr. Biden said that the United States would distribute 25 million doses this month to countries in the Caribbean and Latin America; South and Southeast Asia; Africa; and the Palestinian territories, Gaza and the West Bank.