Reduced yields in the European supply chain are the reason that lower volumes of AstraZeneca's coronavirus vaccine will initially be delivered to EU member states, according to a spokesperson for the drugmaker.

The European Union expressed its “deep dissatisfaction” on Friday, after being informed by AstraZeneca that vaccine deliveries to member states — pending authorization — would not arrive before the end of the first quarter of 2021, as originally forecast.

An AstraZeneca spokesperson told CNN that reduced yields at a manufacturing site in the European supply chain have caused the lower volumes of initial deliveries, but there is no schedule delay to the start of shipments once its vaccine receives approval in Europe.

“We will be supplying tens of millions of doses in February and March to the European Union, as we continue to ramp up production volumes,” the spokesperson said, without offering details on how much lower the initial volume of vaccine supply would be.

EU's expectations: The EU was expecting 100 million doses of the AstraZeneca vaccine in the first quarter of this year. The European Medicines Agency (EMA) received an application for conditional marketing authorization for the vaccine developed by AstraZeneca and the University of Oxford on January 12. The decision on granting marketing authorization could be given by January 29.

“The European Commission will continue to insist with AstraZeneca on measures to increase predictability and stability of deliveries, and acceleration of the distribution of doses,” European Commissioner for Health and Food Safety Stella Kyriakides tweeted Friday.