US President Joe Biden addressed a Covid-19 summit of world leaders Wednesday with a promise to donate a "historic" extra 500 million vaccines to countries struggling to overcome the pandemic.

"This is an all-hands-on-deck crisis," Biden said. "America will become the arsenal for vaccines as we were the arsenal for democracy in World War II."

The pledge from Biden at the summit, held virtually from the White House, brings the total US commitment of donated vaccines to 1.1 billion -- more than the rest of the world combined.

"We've already shipped 160 million of these doses to 100 countries," Biden said. "For every one shot we've administered to date in America, we've now committed to do three shots to the rest of the world."

The new tranche of half a billion vaccines will be from Pfizer and will go to low-income and middle-income countries.

Biden is also challenging world leaders to vaccinate 70 percent of every country by September 2022, the White House said in a statement.

"We need other high income countries to deliver on their own ambitions," he said in his opening remarks. "We're not going to solve this crisis with half measures."

Biden stressed that the surge of vaccines must only be donated, with no "political" strings attached -- a veiled dig at China in particular.

After he spoke, Spain announced at the United Nations in New York that it was boosting its commitment to 30 million total vaccines, while Japan said it would increase its contribution to 60 million.

Wealthy countries have been criticized by the World Health Organization for their plans to roll out booster shots, with the United States on Wednesday authorizing third doses of Pfizer's vaccine for elderly and at-risk populations, while much of the world faces a severe shortage in doses.

But a senior US administration official told reporters that Washington is "proving that you can take care of your own, while helping others as well."

On Tuesday, in his first speech to the United Nations as president, Biden told delegates that Washington had put more than $15 billion towards the global Covid response.

- 70 percent target -
Despite the development of safe and highly effective vaccines in record-breaking time, huge disparities exist between countries with ample supply and others that have barely begun their immunization campaigns.

Just 3.6 percent of Africa's eligible population has been inoculated -- compared with an average of more than 60 percent in Western Europe.

Addressing the UN General Assembly, Malawi's leader urged both more vaccine doses and the patent rights to allow local production, a move supported by Biden but rejected by Germany amid fierce opposition from pharmaceutical companies.

"You can imagine our disappointment to be at an assembly like this rubbing shoulders with nations that are now administering booster shots while most of our people have yet to get their first," said President Lazarus Chakwera, who appeared virtually. 

"This form of vaccine nationalism is wrong. It is insensitive. It must end."

Biden's summit -- technically held on the sidelines of the UN General Assembly -- included foreign leaders as well as the head of Gavi, the vaccine alliance working to distribute shots to poor countries.

The White House asked governments to close the gap in financing and supply to meet a UN-backed goal of fully vaccinating 70 percent of the world by this time next year.

While the latest global coronavirus wave peaked in late August, the virus continues to spread rapidly, particularly in the United States, which is officially the worst-hit country.

Some 4.7 million people worldwide have died since the outbreak began in China in December 2019, according to an AFP tally from official sources.

Pfizer booster for elderly and high-risk

The United States on Wednesday authorized the use of boosters of Pfizer's Covid-19 vaccine for people aged over 65, as well as adults at high risk of severe disease and those in high-exposure jobs.

The announcement means a significant part of the population -- amounting to tens of millions of Americans -- are now eligible for a third shot six months after their second.

"Today's action demonstrates that science and the currently available data continue to guide the FDA's decision-making for COVID-19 vaccines during this pandemic," said Janet Woodcock, acting head of the Food and Drug Administration, in a statement.

The decision was expected and came after an independent expert panel convened by the regulatory agency last week voted in favor of recommending the move. 

The panel, however, rejected an initial plan by the White House to fully approve Pfizer boosters to everyone aged 16 and over, in what amounted to a rare rebuke of President Joe Biden's administration.

The group of vaccinologists, infectious disease specialists and epidemiologists concluded that the benefit-risk balance differed for younger people, especially young males who are more susceptible to myocarditis.

- More boosters debated -

Pfizer Covid-19 boosters are currently being debated by a separate body of experts convened by the Centers for Disease Control and Prevention (CDC), which may recommend further specifics about recipients. 

For example, if obesity is considered as putting a person "at high risk of severe Covid," that definition would cover more than 42 percent of the US population aged over 20.

The CDC may also have to define which workplaces and other settings might lead to "frequent institutional or occupational exposure to SARS-CoV-2."

For its part, the FDA indicated this would cover "health care workers, teachers and day care staff, grocery workers and those in homeless shelters or prisons, among others."

The FDA's emergency use authorization (EUA) applies to those aged 18 and up for the high risk of severe disease and high-exposure categories. It also only applies to Pfizer's vaccine.

Recipients of Moderna and Johnson & Johnson, the other US-authorized vaccines, will now await news for when they, too, might become eligible for another shot.

A number of studies have shown two doses of Pfizer or Moderna, or a single shot of J&J, continue to confer high protection against severe outcomes -- but this is slightly reduced for the elderly.

The World Health Organization has called for a moratorium on wealthy countries giving out boosters, while many countries -- especially those in Africa -- have barely begun their immunization campaigns. 

The United States argues, however, it is possible to both help middle- and lower-income nations while also protecting its own vulnerable people. 

On Wednesday, President Biden announced the United States would buy 500 million more Pfizer doses for the world, bringing its total contribution of the global supply to 1.1 billion.

Speaking to CNN on Sunday, top government scientist Anthony Fauci said that more research is needed to make a decision on whether a booster shot is warranted for the general public.

"As we said in the beginning, we would want to plan for the possibility of vaccinating all those who have gotten their initial vaccination with Pfizer," Fauci said. "And it was always pending the evaluation of all of the totality of the data from the United States, from Israel, and any bit of data that we could get by the advisory committee to the FDA."

Fauci added that the FDA panel decided against a third shoot for everybody aged 16 and over "in the proper deliberative process and they came up with a recommendation."