The US envoy in charge of vaccine aid will leave her post after leading an effort in which President Joe Biden has promised more than 1.1 billion doses, an official said Friday.

Gayle Smith, named by Secretary of State Antony Blinken in April, will return to her position as president of ONE, the campaign backed by pop star Bono to fight extreme poverty.

A senior State Department official said that Smith would step down at the end of the month and had always intended the position to be temporary.

Smith "took on the temporary assignment at the request of Secretary Blinken in order to build out our global response and set the stage for the longer-term thinking about resilience to public health threats," the official told AFP.

She will be replaced in the interim by Mary Beth Goodman, like Smith a veteran of former president Barack Obama's administration, until a new coordinator is announced, the official said.

The Biden administration has vowed to show global leadership in defeating the Covid-19 pandemic by donating doses with no strings attached, drawing a difference from geopolitical rivals China and Russia which have heavily promoted their own vaccines.

Biden has promised at least 1.1 billion doses to the world.

More than 212 million doses have been shipped so far, with the Philippines, Pakistan, Bangladesh and Vietnam as the top recipients, according to a tracker by the Kaiser Family Foundation.

Despite the efforts, the vast majority of vaccinations remain in developed nations, China and India, with only tiny fractions of the population vaccinated in much of sub-Saharan Africa, even as wealthy countries administer boosters.