The World Health Organization condemned Wednesday the rush by wealthy countries to provide Covid vaccine booster shots, while millions around the world have yet to receive a single dose.

Speaking before US authorities announced that all vaccinated Americans would soon be eligible to receive additional doses, WHO experts insisted there was not enough scientific evidence that boosters were needed and said providing them while so many were still waiting to be immunised was immoral.

"We're planning to hand out extra life jackets to people who already have life jackets, while we're leaving other people to drown without a single life jacket," WHO's emergency director Mike Ryan told reporters from the UN agency's Geneva headquarters.

"The fundamental, ethical reality is we're handing out second life jackets while leaving millions and millions of people without anything to protect them."

WHO called earlier this month for a moratorium on Covid vaccine booster shots to help ease the drastic inequity in dose distribution between rich and poor nations.

But that has not stopped a number of countries from moving forward with plans to add a third jab, as they struggle to thwart the Delta variant.

US authorities warned Wednesday that Covid-19 vaccination efficacy was decreasing over time, and said they had authorised booster shots for all Americans from September 20 starting eight months after an individual has been fully vaccinated.

The officials said that while the vaccines remain "remarkably effective" in reducing the risk of severe disease, hospitalisation and death from the effects of Covid, protection could diminish in the months ahead without boosted immunisation.

Washington had already authorised an extra dose for people with weakened immune systems.

Israel has also begun administering third doses to Israelis aged 50 and over.

'Shame on all humanity'

But WHO experts insisted that the science was still out on boosters and stressed that ensuring that people in low-income countries where vaccination is lagging received jabs was far more important.

"What is clear is that it's critical to get first shots into arms and protect the most vulnerable before boosters are rolled out," WHO chief Tedros Adhanom Ghebreyesus told Wednesday's press conference.

"The divide between the haves and have nots will only grow larger if manufacturers and leaders prioritise booster shots over supply to low- and middle-income countries," he said.

"The virus is evolving and it is not in the best interests of leaders just to focus on narrow nationalistic goals when we live in an interconnected world and the virus is mutating quickly."

Tedros voiced outrage at reports that the single-dose J&J vaccine currently being filled and finished in South Africa was being shipped for use in Europe "where virtually all adults have been offered vaccines at this point".

"We urge J&J to urgently prioritise distribution of their vaccines to Africa before considering supplies to rich countries that already have sufficient access," he said.

"Vaccine injustice is a shame on all humanity and if we don't tackle it together, we will prolong the acute stage of this pandemic for years when it could be over in a matter of months."