The WHO on Wednesday called for a moratorium on Covid-19 vaccine booster shots until at least the end of September to address the drastic inequity in dose distribution between rich and poor nations.
World Health Organization chief Tedros Adhanom Ghebreyesus urged the countries and companies controlling the supply of doses to change course immediately and prioritise less wealthy states.
The UN health agency has for months raged against the glaring and growing imbalance, branding it a moral outrage.
Israel last month began rolling out a booster shot for over-60s, while Germany said Tuesday it would start offering third doses of the two-shot Pfizer-BioNTech and Moderna vaccines from September.
Tedros told a press conference that he understood why countries wanted to protect their citizens from the more transmissible Delta variant of the virus, which was first identified in India.
"But we cannot accept countries that have already used most of the global supply of vaccines using even more of it, while the world's most vulnerable people remain unprotected," he said.
"We need an urgent reversal, from the majority of vaccines going to high-income countries, to the majority going to low-income countries."
The WHO wants every country to have vaccinated at least 10 percent of its population by the end of September, at least 40 percent by the end of the year, and 70 percent by the middle of 2022.
At least 4.27 billion doses of Covid-19 vaccines have now been administered globally, according to an AFP count.
In countries categorised as high income by the World Bank, 101 doses per 100 people have been injected.
That figure drops to 1.7 doses per 100 people in the 29 lowest-income countries.
"Accordingly, WHO is calling for a moratorium on boosters until at least the end of September," said Tedros.
"To make that happen, we need everyone's cooperation, especially the handful of countries and companies that control the global supply of vaccines."
Tedros said the G20 group of nations were the biggest producers, consumers and donors of Covid-19 jabs.
"The course of the Covid-19 pandemic depends on the leadership of the G20," he said.
He urged vaccine producers to prioritise Covax, the global scheme which tries to secure vaccines for nations with less financial clout, which has shipped just 177 million doses so far.
While half the European Union population has been fully vaccinated, in Africa, that figure stands at less than two percent, said the WHO's Covax frontman Bruce Aylward.
He said the booster moratorium would help to right the "extraordinary and increasing inequity," adding that the end of September target would be missed on the current trajectory.
Aylward said the world is "simply not going to be able to achieve" getting out of the pandemic if high-coverage countries start using up the available doses for third or even fourth shots.
Kate O'Brien, the WHO's vaccines chief, said there was no convincing picture yet as to whether booster doses were actually necessary, given the level of protection that the WHO-approved vaccines gave against severe disease, hospitalisation and death.
"We don't have a full set of evidence around whether this is needed or not," she said.
A spokeswoman for Germany's health ministry said Berlin was giving at least 30 million doses to Covax by the end of the year.
"We want to provide a third vaccination as a preventive measure to vulnerable people in Germany and at the same time provide our support for vaccination if possible of all populations in the world," she said.