The Biden administration will buy 500 million doses of the Pfizer-BioNTech coronavirus vaccine to share with countries around the world, with the option to buy an additional 200 million, two sources familiar with the deal tell Axios.

Why it matters: That’s a big step toward making the U.S. a major global vaccine supplier and comes as Biden departs for his first foreign trip as president.

Details: The doses were purchased at a not-for-profit price rather than the $19.50 per dose the U.S. paid in its initial Pfizer contract, according to the sources. Most or all of the doses will be distributed through the global COVAX mechanism.

The administration has already said that Biden will press other rich democracies to share doses, including at the G7 on June 11–13. Biden laid out a plan to share an initial 25 million doses last week.Driving the news: Around 200 million available this year and another 300 million in the first half of next year, Politico reports. The Washington Post first reported the purchase of 500 million additional doses.What to watch: Biden could potentially pursue similar deals with Moderna and Johnson & Johnson.The big picture: Biden had employed an “Americans first” approach until very recently, keeping almost all doses produced in the U.S. at home. The U.S. is second in production only to China, which has been sharing and selling doses all over the world.

Now with demand slackening at home, the Biden administration has started to shift its focus to donations while also allowing companies like Pfizer and Moderna to start exporting doses.But based on existing contracts, those companies would be expected to ship a large percentage of doses to rich countries. By buying the doses, the U.S. will be in control of where they are sent.It could also give the U.S. flexibility to donate doses now with the confidence that it has a secure future supply of doses in case they are needed for boosters or to vaccinate additional populations, like children.The state of play: Rich countries like the U.S. still control the lion’s share of global doses even as vaccination rates remain below 1% in sub-Saharan Africa and supply is desperately needed in countries like Brazil or Nepal.

Meanwhile, the COVAX initiative, which was designed to supply doses to low- and middle-income countries all over the world, is months behind schedule because of export curbs from hard-hit India.That puts additional pressure on the likes of the European Union, another big vaccine producer, to expedite plans to share doses globally.