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GENEVA (AP) — The head of the World Health Organization is calling on rich countries with large supplies of coronavirus vaccines to refrain from offering booster shots through the end of the year, expanding a call that has largely fallen on deaf ears.

WHO Director-General Tedros Adhanom Ghebreyesus also said Wednesday he was “appalled” at comments by a leading association of pharmaceutical manufacturers who said vaccine supplies are high enough to allow for both booster shots and vaccinations in countries in dire need of jabs but facing shortages.

“I will not stay silent when companies and countries that control the global supply of vaccines think the world’s poor should be satisfied with leftovers,” he told a news conference.

Tedros had previously called for a “moratorium” on booster shots through the end of September, but the United States and other countries have begun or are considering plans to offer them to their vulnerable people.

On Aug. 18, the U.S. Department of Health and Human Services announced that the Pfizer and Moderna COVID-19 vaccines weaken over time and said that a booster shot could be helpful in prolonging protection.

“Based on our latest assessment, the current protection against severe disease, hospitalization, and death could diminish in the months ahead, especially among those who are at higher risk or were vaccinated during the earlier phases of the vaccination rollout,” a team of U.S. health leaders said in a statement in August. “For that reason, we conclude that a booster shot will be needed to maximize vaccine-induced protection and prolong its durability.”

The health leaders said that Americans should get a booster shot eight months after their last dose.

Some providers were planning to begin offering booster shots starting the week of Sept. 20.