US President Joe Biden on Thursday announced the purchase and donation of 500 million Pfizer-BioNTech Covid-19 vaccine doses, bolstering America's role in fighting the pandemic.
"America will be the arsenal of vaccines in our fight against Covid-19, just as America was the arsenal of democracy in World War II," Mr Biden declared from the English town of St Ives in Cornwall.
The purchase and donation will be the largest from any country so far and augments 80 million doses the US plans to ship by the end of June.
Mr Biden's speech confirmed earlier reports of the purchase, with US media saying vaccine shipments would start as early as August with the goal of sending 200 million doses by the end of the year.
“We know the tragedy. We also know the path to recovery,” Mr Biden said.
“The key to reopening and growing economies is to vaccinate your people.”
More than 52 per cent of the adult population in the US have been fully vaccinated, with coronavirus infections and deaths decreasing to levels not seen since the start of the pandemic.
The vaccines will be distributed to about 100 lower-income countries through the World Health Organisation-backed Covax programme.
"Gavi, the Vaccine Alliance, today welcomes the decision by the United States government to procure 500 million Covid-19 vaccine doses on behalf of the Covax facility," the organisation said on Thursday.
After the first 200 million doses are distributed this year, the remaining 300 million will be sent out in the first half of 2022.
"We're a nation full of people that step up at times of need to help our fellow human beings, both at home and abroad," Mr Biden said. "We're not perfect but we step up."
He said the vaccine donations would have "no strings attached", an apparent swipe at Russia and China, which have donated vaccines internationally but with political or economic provisos.
"We won’t be asking for favours in return; we just want to save lives, to end this pandemic," Mr Biden said.
His announcement was made before the G7 summit in Cornwall, during which allied nations would announce further steps to fight the pandemic, he said.
European leaders at the conference may sign a pledge to send out 1 billion vaccine doses by the end of next year, Bloomberg has reported.
Covax has a shortfall of funding, and vaccine doses for June and July.
Some scientists suggest the world will need more than 10 billion vaccines to curb the pandemic.
"The world needs urgent new manufacturing to produce billions more doses within a year, not just commitments to buy the planned inadequate supply," Peter Maybarduk, a director at Public Citizen, tweeted in response to Mr Biden's announcement.
WHO chief Dr Tedros Adhanom Ghebreyesus has said the world would witness a "two-track" pandemic with stark differences between rich, highly vaccinated countries and poorer nations with limited access to shots.
“Our values call on us to do everything we can to vaccinate the world against Covid-19,” Mr Biden said.
“As long as the virus rages elsewhere, there’s a risk of new mutations that could threaten our people.”
Pfizer chief executive Albert Bourla appeared with Mr Biden on Thursday and spoke briefly about his company's commitment to meeting the global need for vaccines.
He said the Pfizer Covid-19 vaccine fended off all known variants and that his company was ready to address future mutations.
"We have built a process to develop, within 100 days, a new vaccine if needed, God forbid," Mr Bourla said.
More than 3.78 million people have died from Covid-19.