The cost could be around £1billion depending on which vaccines are donated.
But Mr Johnson has rejected suggestions that supplying a billion doses of vaccines is a “moral failure” and not enough to cover the needs of poorer countries.
The Prime Minister told a news conference at the end of the G7 summit in Cornwall: “This is another billion made up of a massive contribution by the United States and other friends.
“Already of the 1.5 billion vaccines that have been distributed around the world, I think that people in this country should be very proud that half a billion of them are as a result of the actions taken by the UK Government in doing that deal with the Oxford scientists and AstraZeneca to distribute it at cost.”
He added that “we are going flat out and we are producing vaccines as fast as we can, and distributing them as fast as we can”.
The target to vaccinate the world by the end of next year will be done “very largely thanks to the efforts of the countries who have come here today”, according to Mr Johnson.