The head of the World Health Organization (WHO) on Wednesday called for a global moratorium on booster doses, at least until the end of September, to prioritize vaccinating the most at-risk people around the world.

WHO Director-General Tedros Adhanom Ghebreyesus said 5.5 billion coronavirus vaccines have been administered so far, but 80% of those have been to upper- and middle-income countries. 

Rich countries have also offered to donate 1 billion vaccine doses to other countries, but under 15 percent of those doses have "materialized," Tedros said.