The European Medicines Authority (EMA) was meeting on Wednesday to decide on whether to approve Moderna's Covid-19 vaccine after failing to reach a decision earlier this week.
"The meeting of EMA's human medicines committee (CHMP) to discuss Covid-19 vaccine Moderna has started," the regulator said in a tweet.
The CHMP called an unscheduled meeting on Monday to discuss Moderna's vaccine but failed to reach a conclusion.
The medicines authority did not specify at the time why it could not approve the vaccine, but on Tuesday said its experts were "working hard to clarify outstanding issues with the company".
The Dutch national medicines authority told Reuters it was preparing for both a positive decision on Wednesday and for a scenario in which a conclusion still could not be reached.
The EMA has set a Jan. 12 deadline for whether to recommend Moderna's vaccine.
It recommended a Covid-19 vaccine from Pfizer and its German partner BioNTech on Dec. 21.
A positive decision would mark the second regulatory authorisation for Moderna's vaccine after the United States and further validation of its messenger RNA technology, just over a year since the first outbreak of the virus was identified in Wuhan, China.
It was about 95% effective at preventing illness in clinical trials that found no serious safety issues.
Since its rollout in the European Union began a week ago, hundreds of thousands of Europeans have received the Pfizer-BioNTech vaccine.
More than a million shots have been distributed in Britain in just under a month, and the uneven start of the campaign has led to criticism of health authorities in Germany and France, among others.
The Netherlands on Wednesday start vaccinations, giving its first injection to a 39-year old care home nurse.