European Commission chief Ursula von der Leyen said Sunday the world was in a "race against time" to understand the new Covid variant and, if needed, modify vaccines to counter it.

"We know we are now in a race against time," von der Leyen said during a visit to Riga, calling for the public to take precautions to give scientists time to understand the Omicron strain.

"The scientists and manufacturers need two to three weeks to have a full picture about the quality of the mutations of this Omicron variant," she said.

"We need to buy time," she added, urging people to vaccinate, wear masks and practise social distancing.

She said that a contract struck in the summer by the European Commission with BioNTech-Pfizer for 1.8 billion vaccine doses included a clause in case of an "escape variant" -- a strain that can evade vaccine immunity.

A clause in the contract states "that if a variant turns into an escape variant... BioNTech-Pfizer is able to adapt its vaccine within 100 days," she said.

The new, heavily mutated Covid-19 variant has spread across the globe, shutting borders, renewing curbs and sparking fears for the fight against the nearly two-year-old coronavirus pandemic.