Poorer nations last month rejected more than 100-million doses of Covid-19 vaccines distributed by the global programme COVAX, mainly due to their rapid expiry date, a UNICEF official said on Thursday.

The figure shows the difficulties of vaccinating the world despite growing supplies of jabs, with COVAX getting closer to delivering 1-billion doses to a total of nearly 150 countries.

"More than a 100-million have been rejected just in December alone," Etleva Kadilli, director of Supply Division at UN agency UNICEF told lawmakers at the European Parliament.

The main reason for rejection was the delivery of doses with a short shelf-life, she said.

Poorer nations have also been forced to delay supplies because they have insufficient storage facilities, Kadilli said, including a lack of fridges for vaccines.

UNICEF did not immediately reply to a query about how many doses have been rejected so far in total.

In addition to rejected doses, many others sit unused in storage facilities in poorer nations.

UNICEF's data on supplies and use of delivered vaccines show that 681-million shipped doses are currently unused in about 90 poorer nations across the world, according to CARE, a charity, which extracted the figures from a public database.

More than 30 poorer nations, including big states such as the Democratic Republic of Congo and Nigeria, have so far used fewer than half of the doses they have received, CARE said, citing UNICEF data.

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COVAX, the global programme co-led by the World Health Organization, has so far delivered 989-million Covid-19 vaccines to 144 countries, according to data from GAVI, a vaccine alliance, which co-manages the programme.

COVAX is the main supplier of doses to dozens of poorer nations, but is not the only one. Some countries buy doses on their own or use other regional vaccine procurement programmes.

Supplies to poorer nations have long been very limited because of lack of vaccines, as wealthier states secured most of the doses initially available from December 2020.

But in the last quarter, shipments have exponentially increased thanks to donations from rich countries that have vaccinated the majority of their populations.

In January, 67% of the population in richer nations had been fully vaccinated, whereas only 8% in poorer nations have received their first dose, WHO figures show.

The faster pace in supplies caught many receiving countries unprepared.

"We have countries that are pushing doses that are currently available towards quarter two of 2022," Kadilli said.

Of the 15-million doses from the EU that have been refused, three-quarters were AstraZeneca's shots with a shelf life of less than 10 weeks upon arrival, according to a UNICEF slide shown to EU lawmakers.

Wealthy countries donating vaccines with a relatively short shelf life has been a "major problem" for COVAX, a WHO senior official said last month, as many doses were wasted.

Reuters reported in December that up to one-million vaccines were estimated to have expired in Nigeria in November without being used.