The United States on Thursday raised the pressure on other Group of Seven leaders to share their vaccine hoards to bring an end to the pandemic by pledging to donate 500 million doses of the Pfizer coronavirus vaccine to the world's poorest countries.

The largest ever vaccine donation by a single country will cost the United States $3.5 billion but Washington expects no quid pro quo or favours for the gift, a senior Biden administration official told reporters.

U.S. President Joe Biden's move, on the eve of a summit of the world's richest democracies, is likely to prompt other leaders to stump up more vaccines, though even vast numbers of vaccines would still not be enough to inoculate all of the world's poor.

G7 leaders want to vaccinate the world by the end of 2022 to try to halt the COVID-19 pandemic that has killed more than 3.9 million people and devastated the global economy.

A senior Biden administration official described the gesture as a "major step forward that will supercharge the global effort" with the aim of "bringing hope to every corner of the world."

"We really want to underscore that this is fundamentally about a singular objective of saving lives," the official said, adding that Washington was not seeking favours in exchange for the doses.

Vaccination efforts so far are heavily correlated with wealth: the United States, Europe, Israel and Bahrain are far ahead of other countries. A total of 2.2 billion people have been vaccinated so far out of a world population of nearly 8 billion, based on Johns Hopkins University data.

U.S. drugmaker Pfizer and its German partner BioNTech have agreed to supply the U.S. with the vaccines, delivering 200 million doses in 2021 and 300 million doses in the first half of 2022.

The shots, which will be produced at Pfizer's U.S. sites, will be supplied at a not-for-profit price.

"Our partnership with the U.S. government will help bring hundreds of millions of doses of our vaccine to the poorest countries around the world as quickly as possible," said Pfizer Chief Executive Albert Bourla.

'DROP IN THE BUCKET'

Anti-poverty campaign group Oxfam called for more to be done to increase global production of vaccines.

"Surely, these 500 million vaccine doses are welcome as they will help more than 250 million people, but that's still a drop in the bucket compared to the need across the world," said Niko Lusiani, Oxfam America's vaccine lead.

"We need a transformation toward more distributed vaccine manufacturing so that qualified producers worldwide can produce billions more low-cost doses on their own terms, without intellectual property constraints," he said in a statement.

Another issue, especially in some poor countries, is the infrastructure for transporting the vaccines which often have to be stored at very cold temperatures.

Biden has also backed calls for a waiver of some vaccine intellectual property rights but there is no international consensus yet on how to proceed.

The new vaccine donations come on top of 80 million doses Washington has already pledged to donate by the end of June. There is also $2 billion in funding earmarked for the COVAX programme led by the World Health Organization (WHO) and the Global Alliance for Vaccines and Immunization (GAVI), the White House said.

GAVI and the WHO welcomed the initiative.

Washington is also taking steps to support local production of COVID-19 vaccines in other countries, including through its Quad initiative with Japan, India and Australia.