The EU could authorise a second coronavirus vaccine "in the coming hours", European Council chief Charles Michel said Tuesday.
Mr Michel said leaders would hold an online summit on the health crisis later this month.
National capitals have been pressuring the European Medicines Agency to issue its approval for the vaccine from US company Moderna as other advanced nations including Britain, the US and Israel press ahead.
But its human medicine committee did not approve the drug on Monday, agreeing instead to reconvene on Wednesday.
"Even if it's not certain, we hope that in the coming hours a second vaccine will be agreed on," Mr Michel said in Lisbon as Portugal took over the rotating presidency of the EU.
So far the only inoculation available in the bloc is the one produced by Pfizer-BioNTech, which the agency approved on December 21.
Mr Michel said that getting vaccines to the EU's almost 450 million people was a "gigantic challenge".
"Alongside member states, the European Commission is working night and day to make sure we can increase the number of vaccines available," he said.
But Mr Michel said this would be done while "respecting the independence of the medicines agency".