The Prime Minister described the pledge as one of the "triumphs" of the G7 summit, despite the World Health Organisation calling for 11 billion doses.
Earlier the former prime minister Gordon Brown told Sky News the summit would go down as a "missed opportunity" and accused leaders of "unforgivable moral failure" over the figure.
Speaking at a press conference on Sunday, Mr Johnson said: “I must really reject that.
“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.
"This is June to June – now until next June – and don't forget this vaccine has literally only been invented very recently, these vaccines have only come on stream very recently."
Mr Johnson 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”.
Earlier Mr Brown had warned the pandemic was hitting the poorest countries hardest and would widen inequality.
He said: “Millions of people will go unvaccinated and thousands of people, I’m afraid, will die.
“I think this summit will also go down as an unforgivable moral failure, when the richest countries are sitting around the table with the power to do something about it.
“We will have a huge problem of a division between the richest countries that are safe and the poorest countries that are not safe.
“But then the problem will come back to haunt the richest countries because we will have contagion spreading that will hurt even the people who are vaccinated because of mutations and variants.”
Following the conclusion of the G7 summit, leading charities accused world leaders of having "fallen disappointingly short".
In a joint statement, UNICEF UK, Crisis Action and Action for Global Health said: "The success of this year's G7 summit should be judged by whether leaders have put their money and resources where their mouths are.
"Without 10 billion vaccines, the removal of patents and investment in healthcare systems pledges to inoculate the world by the end of next year ring hollow."
The Prime Minister also used the press conference to play down a row with the EU as UK foreign secretary Dominic Raab claimed leaders had been “offensive” by suggesting Northern Ireland was not part of the country.
Emmanuel Macron, the French president, is believed to have suggested Northern Ireland was not part of the UK during his talks with Mr Johnson during the G7 summit in Cornwall.
Mr Johnson said: “Of course we make the point continuously that we are part of one great, indivisible United Kingdom.
“What I am saying is that we will do whatever it takes to protect the territorial integrity of the UK, but actually what happened at this summit was there was a colossal amount of work on subjects that have absolutely nothing to do with Brexit.
"I can tell you that the vast, vast majority of the conversations that we have had over the last three or four days have been about other subjects and there has been a fantastic degree of harmony between the leaders of our countries.”
His claims of harmony came despite the foreign secretary earlier risking a diplomatic row by criticising the comments allegedly made by Mr Macron.
Mr Raab said: “We have serially seen senior EU figures talk about Northern Ireland as if it was some kind of different country to the UK.
"It is not only offensive, it has real world effects on the communities in Northern Ireland, creates great concern, great consternation.
“And the truth is Northern Ireland cannot be talked about as a separate country to the UK. It’s offensive. And that kind of approach speaks volumes.
“That is one of the reasons we have the problems we do with the Northern Ireland Protocol, because there isn’t a proper appreciation and there’s been a lopsided approach.”
The spat came as Mr Macron used his own G7 summit press conference to pile pressure on the UK Government by insisting the Brexit deal must be honoured.
He said: “I believe that as far as this subject matter is concerned everybody has got to come back to reason.
“My wish, my will is that we succeed – we succeed collectively – to put into operation what we decided upon a few months ago.
“We are respectful and for a number of years after Brexit we have established certain rules, a protocol agreement and a trade treaty for future relations.
"We just want them to be respected seriously, calmly and professionally – that’s all.
"Prime Minister Johnson was well aware at the time that there was a control issue and he himself signed a protocol agreement which is valid for Northern Ireland, which does envisage controls.
“Full respect of sovereignty, including with regard to Northern Ireland, must not have the consequence of non-respect of the 27 member states, which decided to create a single market.
“You mustn’t make the EU deal with certain incoherences that you were well aware of from the beginning.”