The World Health Organization (WHO) is calling for a moratorium on COVID-19 booster shots in wealthy nations through at least the end of September as poor countries struggle with access to vaccines.

WHO Director-General Tedros Adhanom Ghebreyesus said Wednesday he would like to see at least 10 percent of the population of every country vaccinated before booster shots are administered.

"I understand the concern of all governments to protect their people from the delta variant," Tedros said. "But we cannot, and we should not, 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."

Tedros said that of the more than 4 billion doses that have been administered, more than 80 percent have gone to high- and upper-middle income countries, even though they account for less than half of the world’s population.

Tedros and the WHO have long warned about vaccine inequities and have criticized wealthy countries for even discussing booster shots while some of the most vulnerable areas of the world struggle with getting even health workers vaccinated.

"We need an urgent reversal, from the majority of vaccines going to high-income countries, to the majority going to low-income countries," he said.

In May, Tedros and the WHO called for wealthy countries and vaccine manufacturers to increase efforts to supply vaccines to low-income countries, calling it a "sprint to September." He set a goal of getting 10 percent of people in all countries vaccinated by that time.

"We’re now more than halfway to that target date, but we’re not on track," Tedros said.

High-income countries have administered almost 100 doses for every 100 people; in low-income countries, they have administered only 1.5 doses for every 100 people “due to lack of supply,” he said.

In Africa, less than 2 percent of the continent is fully vaccinated, compared to 50 percent in North America and Europe.

"What we're calling for is, for at least the next two months, a complete global solidarity around the goal of catching up the rest of the world in terms of its immunization coverage," said Bruce Aylward, senior adviser to the director-general.

“The entire world is in the middle of this and as we’ve seen with the emergence of variant after variant, we cannot get out of it unless the entire world gets out of it together, and with the huge disparity in vaccination coverage we're simply not going to be able to achieve that," Aylward said.

Katherine O’Brien, WHO’s director of immunizations, vaccines and biologics, said there are a very limited number of countries that have begun administering booster doses but a far larger number of countries, including the United States, are contemplating it.

"We need instead to focus on those people who are most vulnerable, most at risk for severe disease and death, to get their first and second doses," she said.

As supply becomes more assured and there's higher coverage for first doses globally, then the discussions should start, O'Brien said.

Infectious diseases experts are cautioning against people seeking booster doses, because the evidence is not clear. But drug manufacturer Pfizer has maintained that people will need a booster shot.

"It's incredibly important that we're clear about what the data are ... if we're not really grounded in that clarity, we're going to be in a place where we have forever uncertainty about what should be done," O'Brien said.