AstraZeneca has said it is standing by its coronavirus vaccine after a US agency raised concerns about trial results that showed the shot to be highly effective in preventing Covid-19.

As immunization programs gather pace around the world, another vaccine ran into trouble on Wednesday when Hong Kong and Macau suspended the Pfizer-BioNTech jab over what authorities said were packaging problems.

The uncertainties are a blow to scientists' hopes for a quick rollout of what they say is the best hope of ending a pandemic that has killed more than 2.7 million people.

AstraZeneca had published results from its US trials on Monday, showing the vaccine to be 79 percent effective in preventing Covid-19, but the US National Institute for Allergies and Infectious Diseases raised concerns that the results were outdated.

The firm backed its shot on Tuesday, saying a review found the interim results it had announced were "consistent", and that it would release new analysis and data "within 48 hours".

The AstraZeneca shot had been hailed as a potential game-changer in the fight against the pandemic, as it is cheaper and easier to store and transport than many of its rivals.

But public confidence in the drug has tumbled after more than a dozen countries temporarily suspended its rollout because of isolated cases of blood clots -- even though the World Health Organization and regulators have found no link with the shot.

AstraZeneca has also struggled for months with production and supply chains, delivering only 30 percent of the doses it promised the European Union for the first quarter and sparking fury in Brussels.

The European Commission on Wednesday will revise trade rules to strengthen its hand when it comes to preventing exports of vaccines produced in the bloc.

The already stumbling vaccination programmes in Hong Kong and Macau also ran into trouble on Wednesday, as authorities temporarily stopped giving Pfizer-BioNTech shots over concerns about the packaging of vials.

Health officials did not explain what exactly was wrong with them, but insisted there were no safety issues.

 'It was like a war'

Despite these issues, rich nations have accelerated their drives but fears are growing about supplies for the rest of the world.

"I am deeply concerned that many low-income countries have not yet received a single #COVID19 vaccine dose," United Nations Secretary-General Antonio Guterres tweeted Wednesday.

"Vaccines must be a global public good. The world must unite to make this a reality."

The pandemic has taken a turn for the worse in many nations, with known infections worldwide approaching 124 million.

Hard-hit Brazil's daily death count crossed 3,000 for the first time, as the South American nation's healthcare infrastructure was pushed to the brink by an explosion of cases.

Supplies of medical oxygen for Covid-19 patients have fallen to "worrying" levels in six of Brazil's 27 states, officials said Tuesday.

The warning raised fears of a repeat of horrific scenes in the northern city of Manaus in January, when oxygen shortages left dozens of Covid-19 patients to suffocate to death.

"You have no idea what it is to see families running around to find oxygen canisters, the fights outside places selling oxygen," Manaus-based doctor Adele Benzaken told AFP.

"It was like a war -- the chaos of a bombing, when people are running around desperately without knowing what to do."

 'We have a new virus'

The surges have forced governments to reimpose unpopular and economically punishing restrictions, including lockdowns, despite the availability of vaccines.

German chancellor Angela Merkel has ordered a nationwide shutdown for five days starting April 1 to fight a spike fuelled by the variant first detected in Britain.

"Essentially, we have a new virus," she said. "It is much deadlier, much more infectious and infectious for much longer."

The Netherlands extended coronavirus restrictions until April 20, while Norway will ban alcohol sales at bars and restaurants as part of new anti-virus rules starting Thursday.

The Belgian government was to hold an emergency meeting Wednesday to decide how to fight a worrying spike in cases.

Those on the pandemic frontlines in hard-hit Britain, under immense strain throughout the crisis, got a boost Tuesday when a painting by street artist Banksy sold for a record 16.75 million pounds ($23.1 million) in an auction by Christie's to raise money for health workers.