US President Joe Biden has pledged that America will be the "arsenal of vaccines, as it was the arsenal of democracy in World War II" by donating 500 million Covid vaccine doses to poorer countries.
He confirmed that the United States will donate 200 million Pfizer/BioNtech vaccine doses this year and 300 million next year for 92 poorer countries.
President Biden called the donation a "historic step" in the fight against the global pandemic.
"This is about our responsibility, our humanitarian obligation to save as many lives as we can," he told reporters on the eve of the G7 summit in Cornwall, southwest England.
He added that it was also in the US interest because of the risk of variants.
"Half a billion vaccines will start to be shipped in August,as quickly as they roll off the manufacturing line," Mr Biden said at a news conference.
"The United States is providing these half billion doses,with no strings attached. Our vaccine donations don't include pressure for favours, or potential concessions. We're doing this to save lives."
Earlier, the White House said this "largest ever purchase and donation of vaccines by a single country" will "help supercharge the global fight against the pandemic".
Mr Biden "will also call on the world's global democracies to do their parts in contributing to the global supply of safe and effective vaccines" the spokesperson said.
The US has come under criticism for sitting on huge stocks of unused vaccines, something the government says was initially necessary as a precautionary measure in its own race to get shots into the arms of Americans.
However, with domestic vaccination rates now having reached 64% of adults receiving at least one dose, Washington is moving quickly to reclaim global leadership on the issue.
Dismissing suggestions that the United States is in a so-called vaccine diplomacy contest with Russia and China, the White House describes its initiatives as a return to multilateral action after the presidency of Donald Trump.
This new, huge surge in donations aims "to save lives and end the pandemic," the White House said, adding, "the United States is using the power of our democracy, the ingenuity of American scientists, and the strength of American manufacturing to beat the pandemic globally by helping to vaccinate the world.
"The vaccines will be distributed through the international COVAX programme to "low and lower-middle income nations."
The announcement came on top of an earlier pledge to donate some 80 million vaccines by the end of June, as well as a $2 billion contribution to COVAX funding.
"President Biden has been clear that borders cannot keep this pandemic at bay and has vowed that our nation will be the arsenal of vaccines," the statement said.