The head of the World Health Organization (WHO) is calling for a moratorium on administering booster shots of vaccines for the coronavirus disease 2019 (Covid-19) as a way to help ensure that doses are available in countries where few people have received their first shots.

WHO Director-General Tedros Adhanom Ghebreyesus made the appeal mostly to wealthier countries that have far outpaced the developing world in numbers of vaccinations.

WHO officials say the science is unproven about whether giving booster shots to people who have already received two vaccine doses is effective in preventing the spread of the coronavirus.

The United Nations health agency has repeatedly called for rich countries to do more to help improve access to vaccines in the developing world.

Tedros pointed to a WHO target set earlier this year to ensure that 10 percent of the populations in countries receive vaccines against the coronavirus.

“Accordingly, WHO is calling for a moratorium on boosters until at least the end of September to enable at least 10 percent of the population of every country to be vaccinated,” he said Wednesday.

To help take the heat out of the pandemic, WHO has been focusing on getting vaccines to older adults, health care workers and other target populations in many countries before booster shot campaigns are carried out.

Israel, France, Germany and many Middle Eastern countries have already started administering booster shots, and other nations, including the United States and Britain, are considering plans to do so in the wake of the emergence of the highly transmissible delta variant.

Dr. Katherine O’Brien, WHO’s director for immunization, vaccines and biologicals, noted that a “very limited number” of countries were giving booster doses though a larger number were contemplating it.

“The evidence is evolving. It’s moving. We don’t have a full set of evidence around whether this is needed or not,” O’Brien said, adding that the main message was that “we need instead to focus on those people who are most vulnerable.”

WHO officials reiterated their call for global “solidarity” to help battle the coronavirus pandemic and appealed to wealthy countries and corporations to help.

“We need everyone’s cooperation, especially the handful of countries and companies that control the global supply of vaccines,” Tedros said, appealing in particularl to the influential Group of 20 (G20) large economies. “The G20 has a vital leadership role to play as the countries that are the biggest producers, the biggest consumers and the biggest donors of Covid-19 vaccines.”

He urged the G20, which currently is chaired by Italy, to make “concrete commitments to support global vaccination targets.”

“We call on everyone with influence – Olympic athletes, investors, business leaders, faith leaders and every individual in their own family and community – to support our call for a moratorium on booster shots until at least the end of September,” Tedros said.