The US will purchase 500m Covid-19 vaccines made by Pfizer to donate to other countries, Joe Biden will announce on Thursday, a significant expansion of the administration’s efforts to help increase inoculation rates around the world.

The Biden administration has agreed to buy 200m doses this year and 300m in the first half of next year to send to other countries, according to a person familiar with the deal, who confirmed an earlier report by The Washington Post.

The agreement marked a big increase on the 80m vaccine doses already pledged by the Biden administration to other countries, following criticism that the US was hoarding shots instead of helping those in need.

The White House did not comment, while Pfizer did not immediately respond to a request. The US president is expected to make the announcement in the UK, ahead of the G7 summit this weekend in Cornwall.

The US has overseen one of the fastest vaccine rollouts in the world, providing at least one dose to just over half of its population. But demand for doses has slowed in recent weeks, and more than 1m people are now receiving vaccinations each day — down from about 3.5m a day in April.

Biden had previously promised to donate 80m doses from the country’s stockpile, the first 25m of which were allocated last week. But the pledge was deemed inadequate by some experts, who warned that more doses will be needed to help countries such as India, which has suffered a devastating second wave in recent months.

All 500m doses would be allocated through the Covax scheme backed by the World Health Organization, according to the person briefed on the details, and will be given to 92 countries.

Covax has promised to deliver 2bn doses around the world by the end of the year, but the initiative has been hampered by the Serum Institute of India’s decision to stop exporting doses of the Oxford/AstraZeneca vaccine while the country battles its outbreak.

The US government’s side of the negotiations was spearheaded by Jeff Zients, the head of Biden’s Covid-19 task force, the person added. Administration officials did not say how much the doses would cost.

US officials said they were also looking at how to increase production in other countries, especially of mRNA vaccines such as those made by BioNTech/Pfizer and Moderna, which have accounted for the bulk of the US rollout.

But some campaigners want the administration to go further and force vaccine makers to hand over their technology to companies in other countries to kickstart production outside the US.

Peter Maybarduk, director of the Access to Medicines programme at consumer advocacy group Public Citizen, said in a statement: “We have yet to see a plan from the US government or the G7 of the needed ambition or urgency to make billions more doses and end the pandemic.

“President Biden and other leaders underestimate their power to set terms with the vaccine makers and co-ordinate immediate production at massive scale.”