Rich nations representing a fraction of the global population have already bought up over half the promised Covid-19 vaccine stocks, a study showed, as US President Donald Trump pledged to begin inoculating Americans within weeks.

Big pharma is racing to produce an effective jab to counter a virus that has now killed more than 935,000 people around the world and infected almost 30 million.

European Union chief Ursula von der Leyen on Wednesday warned against "vaccine nationalism" that she said could put lives at risk by depriving the most vulnerable in poorer nations of immunity.

But a study released by Oxfam showed a group of wealthy countries representing just 13 percent of the world population has already secured the lion's share of doses.

"Access to a life-saving vaccine shouldn't depend on where you live or how much money you have," said Robert Silverman of Oxfam America.

"Covid-19 anywhere is Covid-19 everywhere."

The five leading vaccine candidates currently in late-stage trials will be able to supply 5.9 billion doses, enough to inoculate about three billion people, the Oxfam report said.

Some 51 percent of those jabs have been snapped up by the wealthy world, including the United States, Britain, the European Union, Australia, Hong Kong and Macau, Japan, Switzerland and Israel.

The remaining 2.6 billion have been bought by or promised to developing countries including India, Bangladesh, China, Brazil, Indonesia and Mexico.

- 'Vaccine nationalism' -
Trump on Wednesday said he would begin rolling out a vaccine in America as soon as next month, contradicting doubts expressed by a top health expert in his administration who said a jab was unlikely to be widely available until mid-2021.

"We're very close to that vaccine as you know... We think we can start sometime in October" or shortly thereafter, Trump said.

Democrats have expressed concern that Trump is pressuring government health regulators and scientists to approve a rushed vaccine in time to help his uphill bid for re-election in early November.

"We're within weeks of getting it, you know -- could be three weeks, four weeks," the president told a town hall session with voters in Pennsylvania Tuesday.

But in an implicit swipe at Trump's America First approach to the pandemic, von der Leyen said Europe would support multilateral bodies such as the World Health Organization with its attempts at a more equitable rollout.

"None of us will be safe until all of us are safe -– wherever we live, whatever we have," she said. 

"Vaccine nationalism puts lives at risk. Vaccine cooperation saves them."

- 'Going hard and early' -
The global economy has been devastated by the pandemic, with many nations still maintaining restrictions on their populations to contain fresh outbreaks.

New Zealand was added to the list of economic casualties on Thursday when it reported it had plunged into recession for the first time in a decade.

The country posted a record contraction of 12.2 percent for the April-June quarter, which coincided with a strict lockdown from March to May.

The decline follows a 1.6-percent contraction in the first three months of 2020, confirming New Zealand is in recession.

But there was optimism that the stage had been set for a strong recovery.

"Going hard and early means that we can come back faster and stronger," finance minister Grant Robertson said.

"Economists expect the current September quarter to show a record jump back to growth."

There were also positive signs in South Africa where authorities announced borders would be reopened to most countries from next month -- part of a wider easing of anti-virus curbs as infection figures improve.

Africa's most industrialised economy closed its frontiers at the start of a strict nationwide lockdown in late March.

The number of new infections has dropped from an average of 12,000 per day in July to fewer than 2,000 in recent days.

President Cyril Ramaphosa said in an address to the nation that international travel would "gradually and cautiously" resume from October 1.

"We have withstood the coronavirus storm," he said.