European Union governments have agreed to exercise an option to buy over 180 million doses of a version of the COVID-19 vaccine adapted for the omicron variant developed by BioNTech and Pfizer, the head of the European Commission said.

Pfizer and BioNTech began development of a prototype omicron-specific COVID-19 vaccine on November 25, and said they could have it ready in March.

"The Member States have agreed to trigger a first tranche of over 180 million extra doses of adapted vaccines, in our third contract with BioNTech-Pfizer," Ursula von der Leyen told a news conference on Thursday night at the end of a regular summit with EU leaders.

The EU's latest contract with Pfizer and BioNTech allows EU states to buy up to 1.8 billion doses through 2023, of which half have already been ordered and the other half are optional.

Pfizer said that no optional doses had been ordered yet and talks with the EU for their possible supply had so far not concerned adapted versions of the vaccine.

"At this time, discussions with the EC about the potential for additional supply pursuant to their option rights under our agreement are not specific to an adapted vaccine," Pfizer said in a statement on Friday.

"While we believe we can deliver an adapted vaccine in March 2022, we do not yet know whether this will be needed and availability will require authorization by regulatory authorities," Pfizer said.

Spokespersons for the European Commission (EC) were not immediately available to give more details about the announcement.

The European Medicines Agency (EMA) has so far said that there is no conclusive evidence an adapted vaccine is needed against micron.