AstraZeneca was expected to deliver about 80 million doses to the 27 European Union countries by the end of March, a senior official who was involved in the talks said.
At the moment, no manufacturer of COVID-19 vaccines has been able to develop a shot for children under 16, though efforts are ongoing.
"I'm sure there are many countries now, including Ireland, who are now looking at those plans and potentially having to change them because they're going to get probably less than half the vaccines that they'd expected only a couple of weeks ago".
Mali has chosen to use the AstraZeneca COVID-19 vaccine in its plan to initially immunise 4.2 million people, targeting medical staff, the elderly and those with comorbidities, the health minister said on Friday.
AstraZeneca has warned European Union countries it will cut deliveries of its Covid-19 vaccine by 60 per cent to 31 million doses in the first quarter of the year due to production problems.
EU governments "expressed deep dissatisfaction with this", EU Health Commissioner Stella Kyriakides said on Twitter after the announcement.
Taoiseach Micheál Martin previously described the AstraZeneca vaccine, which was developed in collaboration with Oxford University, as a "game changer" and predicted that the doses would arrive in Ireland by mid-February.
"Initial volumes will be lower than originally anticipated due to reduced yields at a manufacturing site within our European supply chain", an AstraZeneca spokesman said in a written statement.
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They are the only EU country to approve the vaccine, Sputnik V, which has not been approved by the European Medicines Agency.
Seychelles started rolling out its vaccination programme on January 10 with the Sinopharm vaccine, administering the first doses to leaders and essential workers with roll-out to general public planned for next week.
Pfizer has announced delays in shipments of its vaccine in the next few weeks owing to works at its main processing plant in Belgium.
The EU has yet to sign off on the vaccine, but could do so as early as Monday. The EU has ordered 600 million doses from Pfizer.
European Commission president Ursula von der Leyen and Mr Michel both say they are still aiming for the target of 70% of the EU population being vaccinated by summer.
GAVI co-runs the COVAX alliance with the World Health Organization to secure fair access to COVID-19 vaccines for developing countries.
Ireland has signed an agreement to receive 3.3m doses of the vaccine, and it was hoped this would be used to vaccinate over one million people through Global Positioning System and pharmacies.