Up to one million COVID-19 vaccines expired in Nigeria in November, according to Reuters.
Citing at least two sources, Reuters said the development shows one of the biggest single losses of doses that highlights the difficulty African nations have getting shots in arms.
African governments have been pushing for more vaccine deliveries as inoculation rates lag richer regions.
In Nigeria, Africa's most populous nation and home to more than 200 million people, fewer than 4% of adults have been fully vaccinated, according to the World Health Organization (WHO).
The expired doses were made by AstraZeneca (AZN.L) and delivered from Europe, the sources with direct knowledge of vaccine delivery and use told Reuters.
They were supplied via COVAX, the dose-sharing facility led by the GAVI vaccine alliance and the WHO which is increasingly reliant on donations.
A third source with knowledge of the delivery said some of the doses arrived within four-to-six weeks of expiry and could not be used in time, despite efforts by health authorities.
A count of the expired doses is still underway and an official number is yet to be finalised, the sources said.
"Nigeria is doing everything it can. But it's struggling with short shelf life vaccines," one source told Reuters. "Now (supply is) unpredictable and they're sending too much."
A spokesperson for the National Primary Health Care Development Agency, said the number of vaccines received and used is still being tallied and it would share its findings in the coming days.
The WHO said doses had expired, but declined to give a figure. It said 800,000 additional doses that had been at risk of expiry in October were all used in time.
"Vaccine wastage is to be expected in any immunization programme, and in the context of COVID-19 deployment is a global phenomenon," the WHO said in a statement responding to Reuters' questions. It said vaccines delivered with "very short" shelf lives were a problem.
Nigeria's vaccine loss appears to be one of the largest of its kind over such a short time period, even outstripping the total number of vaccines that some other countries in the region have received.
It is not alone in wasting vaccines, however.