As of mid-September, fewer than four per cent of Africans have been fully immunised and most of the 5.7 billion vaccine doses administered around the world have been given in just 10 rich countries.

Chad's president Mahamat Deby warned of the dangers of leaving countries behind.

"The virus doesn't know continents, borders, even less nationalities or social statuses," he told the General Assembly.

"The countries and regions that aren't vaccinated will be a source of propagating and developing new variants of the virus.

"In this regard, we welcome the repeated appeals of the United Nations secretary general and the director general of the (WHO) in favour of access to the vaccine for all.

"The salvation of humanity depends on it."

The struggle to contain the coronavirus pandemic has featured prominently in leaders' speeches over the past few days - many of them delivered remotely because of the virus.

Country after country acknowledged the wide disparity in accessing the vaccine.

South Africa's president Cyril Ramaphosa pointed to vaccines as "the greatest defence humanity has against the ravages of this pandemic".

"It is therefore a great concern the global community has not sustained the principles of solidarity and co-operation in securing equitable access to COVID-19 vaccines," he said.

"It is an indictment on humanity that more than 82 per cent of the world's vaccine doses have been acquired by wealthy countries, while less than 1 per cent has gone to low-income countries."

He and others urged UN member states to support a proposal to temporarily waive certain intellectual property rights, to allow more countries, particularly those of low and middle income, to produce vaccines.

Earlier this year, US President Joe Biden broke with European allies to embrace the waivers, but there has been no movement toward the necessary global consensus.

Angola President Joao Lourenco said it was "shocking to see the disparity between some nations and others with respect to availability of vaccines".

"These disparities allow for third doses to be given, in some cases, while, in other cases, as in Africa, the vast majority of the population has not even received the first dose," Lourenco said.

The US, Britain, France, Germany and Israel are among the countries that have begun administering boosters or announced plans to do so.

Namibia President Hage Geingob called it "vaccine apartheid".

"There is a virus far more terrible, far more harrowing than COVID-19. It is the virus of inequality," said the president of the Indian Ocean island nation of the Seychelles, Wavel Ramkalawan.

The grim consequences of COVID-19 hit Tanzania especially hard when the East African country's then-president John Magufuli, who had insisted the coronavirus could be defeated with prayer, died in March.

The presidency went to his deputy, Samia Suluhu Hassan, who has since changed Tanzania's course on the pandemic but still sees great challenges ahead.

"We tend to forget that no one is safe until everyone is safe," she said during her speech on Thursday.

On Wednesday, Biden announced the US would double its purchase of Pfizer's COVID-19 shots to share with the world to one billion doses, with the goal of vaccinating 70 per cent of the global population within the next year.

The WHO says only 15 per cent of promised donations of vaccines - from rich countries that have access to large quantities of them - have been delivered.

The UN health agency has said it wants countries to fulfil their dose-sharing pledges "immediately" and make shots available for programs that benefit poor countries, and Africa in particular.

Australian Associated Press