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The United States has promised to buy 500 million more COVID-19 vaccine doses to donate to other countries as it comes under increasing pressure to share its supply with the rest of the world.
President Joe Biden made the announcement during a virtual summit aimed at boosting global vaccination rates against the coronavirus and rallying world leaders to do more.
"To beat the pandemic here we need to beat it everywhere," Biden said as he kicked off the summit, which included leaders from the UK, Canada, Indonesia, Australia and South Africa as well as World Health Organisation head Tedros Adhanom Ghebreyesus.
"This is an all hands on deck crisis," Biden said of the pandemic that has raged since early 2020, killing at least 4,913,000 people.
The additional vaccines will bring US donations to more than 1.1 billion doses, far short of the 5 billion to 6 billion doses global health experts say is needed by poorer countries.
Delivery of the new tranche will begin in January.
Health experts say rich countries have not done enough and have criticised the United States in particular for planning booster shots for fully-vaccinated people while much of the world's population still does not have access to vaccines.
They say the planned US dose donations are too small and that the Pfizer-BioNTech vaccine is difficult to scale up and to administer in poorer countries, which lack sophisticated infrastructure for storing and shipping shots.
"We will need six to nine billion doses of vaccines" to inoculate the developing world, said Peter Hotez, dean of the National Tropical School of Medicine at Baylor University in Texas.
Biden said the United States would provide $US370 million ($A510 million) "to support administering these shots" and more than $US380 million to help the Global Vaccine Alliance (GAVI) handle vaccine distribution in regions with the greatest need.
The vaccines from Pfizer Inc and BioNTech SE will be made in the United States and shipped to low and lower middle-income countries.
A source familiar with the issue said the US government would pay $US7 per dose.
"For every one shot we've administered to date in America, we have now committed to do three shots to the rest of the world," Biden said.
In June, the Biden administration agreed to buy and donate 500 million doses.
Under the terms of that contract, the United States will pay Pfizer and BioNTech about $US3.5 billion or $US7 a dose for the shots.
The COVAX facility, backed by the World Health Organisation and GAVI, has delivered more than 286 million COVID-19 vaccine doses to 141 countries, GAVI data shows.
In September the organisations running the facility had to cut their 2021 delivery target by nearly 30 per cent to 1.425 billion doses.
Vaccination rates in some countries, including Haiti and the Democratic Republic of Congo, are less than 1 per cent, a Reuters tracker shows.