The US will pay for the doses at a “not-for-profit” price, according to the New York Times, which said the plan could be officially announced on Thursday.
The shots will go to 92 lower-income countries and the African Union, but Pfizer and the White House have so far not commented.
America is well advanced in its vaccine rollout but campaigners have called for richer countries to do more to help protect developing nations.
The half a billion jabs will be distributed via the World Health Organization’s (WHO) Covax scheme, which is trying to fix so-called “vaccine apartheid” by working with governments and manufacturers to ensure an equitable distribution of jabs.
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It aims to deliver two billion doses in 2021, and 1.8 billion doses by early 2022.