GENEVA (AP) — The head of the World Health Organization is calling for a moratorium on administering booster shots of COVID-19 vaccines as a way to help ensure that first doses are available in countries where fewer people have received 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.

WHO 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% 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% of the population of every country to be vaccinated,” he said Wednesday.