Poorer nations are being offered vaccines with too short a shelf life for them to be distributed before their expiry date, according to the UN's children's fund Unicef.
In December alone, more 100 million vaccines were rejected by countries unable to distribute them, Unicef's head of supply, Etleva Kadilli told members of the European Parliament on Thursday.
She said the problem was compounded by many countries' insufficient storage facilities.
Many of the world’s poorest countries, most of them in Africa, have been relying on the UN-backed Covax scheme for their vaccines.
The programme faced challenges in accessing doses early last year, but the situation significantly improved towards the end of 2021 with wealthier countries releasing doses they were holding.
However, many of the doses offered have been close to their expiry date, and have been rejected by the recipient nations.
Some countries such as Nigeria struggled with administering the vaccines forcing them to destroy expired jabs.
Only about 10% of the population on the continent has been fully vaccinated.