The Biden administration will buy 500 million additional doses of the Pfizer/BioNTech coronavirus vaccine to share with countries around the world, the Washington Post first reported.

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.

• The administration has already said he'll press other rich democracies to share doses, including at the G7 from June 11-13. Biden laid out a plan to share an initial 25 million doses last week.

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 those doses to rich countries. By buying the doses, the U.S. will be able to decide 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 U.S. and EU, which are producing vaccines to expedite plans to share them globally.

Worth noting: In a previous contract, Pfizer sold doses to the U.S. at a cost of $19.50 each. That would put the price of this new move at just under $10 billion, though the terms of this deal may be different.