A second coronavirus vaccine manufacturer has warned of supply issues to the European Union, compounding frustration in the bloc.
AstraZeneca said a production problem meant the number of initial doses available would be lower than expected.
The fresh blow comes after some nations' inoculation programmes were slowed due to a cut in deliveries of the Pfizer-BioNTech vaccine.
The EU Health Commissioner expressed "deep dissatisfaction" at the news.
Officials have not confirmed publicly how big the shortfall will be, but an unnamed EU official told Reuters news agency that deliveries would be reduced to 31m - a cut of 60% - in the first quarter of this year.
The drug firm had been set to deliver about 80 million doses to the 27 nations by March, according to the official who spoke to Reuters.
The AstraZeneca vaccine, developed with Oxford University, has not yet been approved by the EU's drug regulator but is expected to get the green light at the end of this month, paving the way for jabs to be given.
.@EU_Commission and Member States expressed deep dissatisfaction with this. We insisted on a precise delivery schedule on the basis of which Member States should be planning their vaccination programs, subject to the granting of a contitional marketing authorisation. /2— Stella Kyriakides (@SKyriakidesEU) January 22, 2021
A spokesman for AstraZeneca said on Friday that "initial volumes will be lower than originally anticipated" without giving further details.
His written statement blamed the discrepancy on "reduced yields at a manufacturing site within our European supply chain" and said the firm was continuing to ramp up production volumes.
News of the delay comes amid criticism and frustration across the region about the speed of vaccination roll-outs.
Israel, the United Arab Emirates, the UK, and the US are all well ahead of EU nations in terms of doses given per capita so far.
The European Commission has co-ordinated orders for all member states, with vaccines then distributed based on their population size.
Vaccines are increasingly seen by experts as the only way out of the Covid-19 crisis, with many European nations struggling to cope with a deadly surge of the virus over the winter period.
Threats of legal action
Austrian media have reported that only 600,000 of two million AstraZeneca doses promised by the end of March will arrive in the country on time, with the remaining 1.4m now being delivered in April.
A delay would be "completely unacceptable", Austrian Health Minister Rudolf Anschober said on Friday.
As for Pfizer, the US firm said it had to cut shipments for the next few weeks while it worked to increase capacity at its Belgian processing plant. The EU has ordered 600 million doses from Pfizer.
Across Europe, the health situation remains very serious.— Ursula von der Leyen (@vonderleyen) January 22, 2021
There is reason for hope with the vaccines. And reason for concern with the variants of the virus.
We must remain focused and determined in our response. pic.twitter.com/AvaL1XBtuf
Some regions, including Germany's most populous state North-Rhine Westphalia and parts of Italy, said earlier this week that they were suspending giving first jabs of the two-dose vaccine because of the shortages.
Italy and Poland have threatened to take legal action in response to the reduction in vaccine supply.
Meanwhile Hungary's government, which has complained over the time it is taking EU regulators to approve the Oxford-AstraZeneca vaccine, has reached a deal with Russia to buy up large quantities of its Sputnik V vaccine, even though it has not received EU approval.
European Council President Charles Michel, who led a call of EU leaders this week, said Thursday that officials were considering all ideas to try and stop future vaccine delays.
"All possible means will be examined to ensure rapid supply, including early distribution to avoid delays," he said.
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.
Borders to remain open
The total number of German Covid deaths climbed above 50,000 on Friday - a day after the country warned that it could close its borders if other EU countries were less strict in controlling the virus. Berlin sounded the alarm amid rising concern about new variants.
EU leaders agreed late on Thursday to keep their internal borders open but warned non-essential travel might need to be restricted to curb the spread of the virus.
Ms von der Leyen said Thursday that more testing and "targeted measures" were needed throughout the EU in order to keep internal and external borders open.
For its part, France said it would impose tighter travel restrictions for European arrivals from Sunday, requiring a negative PCR Covid test within three days of travel.
In the Netherlands, a ban on all flights from the UK, South Africa and South American countries came into effect on Saturday to try and prevent new coronavirus variants gaining a foothold.
Looking forward to the future, officials from EU nations reliant on tourism - including Spain and Greece - have floated the possibility of using vaccination certificates to allow for cross-border travel but there has been scepticism within the bloc.