German Chancellor Angela Merkel has defended the EU's decision to procure coronavirus vaccines jointly as the bloc struggles with delays in rollout.
EU leaders are to hold virtual talks shortly to discuss ways of boosting vaccine supplies and improving distribution across the 27 nations.
Pressure is mounting upon them to deliver after other countries, like the UK, achieved much faster vaccination.
The European Commission is seeking added controls on vaccine exports.
Such controls could affect supply to the UK, where Prime Minister Boris Johnson has warned against imposing "blockades".
The virtual summit comes as a third wave of coronavirus infections sweeps across much of mainland Europe.
EU states have seen some of the deadliest outbreaks of the pandemic, with Italy recording more than 106,000 deaths, France 93,000, Germany 75,000 and Spain 73,000.
Yet recent figures show just 12.9 doses of vaccine have been administered per 100 people in the EU compared with 44.7 in the UK and 37.2 in the US.
The European Commission has blamed pharmaceutical companies - primarily AstraZeneca - for not delivering the promised doses to the EU.
A site in Belgium produces the Oxford-AstraZeneca vaccine, and another in the Netherlands is expected to increase supplies of the jab in the EU.
Brussels has said that of the more than 40 million doses exported from the EU over the past two months, a quarter were sent to the UK.
The UK and the EU said on Wednesday they wanted to "create a win-win situation and expand vaccine supply for all".
The EU summit leaders will also speak by video to US President Joe Biden later on Thursday.