The W.H.O. calls for a moratorium on Covid vaccine boosters to help each country get more people vaccinated.

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 percent 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 W.H.O., 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.

W.H.O. officials went to pains to distinguish between booster shots used to shore up immunity in vaccinated populations, for which the science is not yet clear, and additional doses that may be needed by the immunocompromised to develop immunity in the first place. Officials said they objected to boosters, not to additional doses for some subgroups.