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...