The World Health Organization called on Wednesday for a moratorium on coronavirus vaccine booster shots until the end of September, so that vaccine supplies can be focused on helping all countries vaccinate at least 10% of their populations. The agency made its appeal to the world’s wealthiest nations to address the wide disparities in vaccination rates around the world.

“I understand the concern of all governments to protect their people from the delta variant,” Tedros Adhanom Ghebreyesus, director general of the WHO, said in a briefing. “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.”

With the debate over booster shots heating up, the call highlighted a moral and scientific case long pressed by humanitarian groups: With the staggering gaps in vaccination rates around the world and cases surging as the delta variant spreads, vaccine doses should be given first to vulnerable people in poorer nations. Fully vaccinated people are protected against the worst outcomes of COVID-19 caused by the Delta variant.

Of more than 4 billion vaccine doses that have been administered around the world, more than 80% have been used in high- and upper-middle-income countries, which account for less than half of the world’s population, Tedros said.

Collectively, high-income countries have administered almost 100 doses for every 100 people, he said, while low-income countries have administered just 1.5 doses for every 100 people, mainly because of a lack of supply. African countries have administered five doses for every 100 people, compared with 88 doses per 100 people in Europe and 85 in North America, according to the Our World in Data project at the University of Oxford.

“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.

Scientists have not reached a consensus on whether booster shots are necessary. Still, with worries mounting about continuing pandemic waves and future lockdowns, an increasing number of countries, like Germany, Israel and France, are preparing to offer booster doses to segments of their populations, or have already started administering them.

Studies have indicated that the immunity generated by the Pfizer-BioNTech and Moderna vaccines is long-lasting. Researchers are still working to understand recent Israeli data suggesting that the Pfizer-BioNTech vaccine declined in efficacy months after inoculation.

Asked about the WHO’s call, Jen Psaki, the White House press secretary, said at a news conference, “We feel that it’s a false choice, and that we can do both.”

“We will have enough supply to ensure, if the FDA decides that boosters are recommended for a portion of the population, to provide those as well,” Psaki added, noting efforts by the administration to send vaccine doses to other countries.

President Joe Biden said Tuesday that the United States had donated more than 110 million vaccine doses globally, a down payment on a pledge he made to send half a billion doses of vaccine to poorer countries over the next year. Biden, who for months was under pressure to share vaccine doses, is now seeking to position his administration as a global leader in inoculating the rest of the world.

Deaths from COVID-19 have surged in African nations in recent months, while many health workers and elderly or vulnerable people in the region have remained entirely unprotected. The urgency of vaccinating more people globally has only grown as the Delta variant has spread widely; delta is considerably more contagious than other variants, and may also cause more severe illness.

That made it unacceptable, Tedros said, for millions of unvaccinated people who cannot stay at home and have to go to work and be exposed to transmission, while people who are at less risk in wealthier nations are offered booster shots.

Dr. Anthony Fauci, the top infectious disease expert in the United States and an adviser to Biden, said Tuesday that in some cases, it takes more than the usual number of shots to completely vaccinate immunocompromised people.

“Giving them an additional shot is almost not considered a booster, it’s considered part of what their original regimen should have been,” Fauci said in an interview with the Center for Strategic and International Studies. Many such patients, and “maybe most of them, have not gotten an adequate immune response to begin with,” he said.

That point was echoed by Dr. Bruce Aylward, a senior WHO adviser, who said at Wednesday’s briefing that for people like solid organ transplant patients, a third dose would be part of “their primary series” of treatment and not a booster.

But the pandemic will not end “unless the whole world gets out of it together,” Aylward said. “With the huge disparity in vaccination coverage, we are simply not going to achieve that.”