Preliminary evidence indicates that COVID-19 vaccines may be less effective against infection and transmission linked to the Omicron coronavirus variant, which also carries a higher risk of reinfection, the World Health Organization (WHO) said on Wednesday.

The WHO, in its weekly epidemiological update, said that more data is needed to better understand the extent to which Omicron may evade immunity derived from either vaccines or previous infection.

"As a result of this, the overall risk related to the new variant of concern Omicron remains very high," it said.

Experts from the WHO and other countries including the U.S. have previously said that Omicron variant may cause less severe symptoms compared to Delta.

The WHO on Tuesday said the Omicron variant has been reported in 77 countries and regions, with its chief Tedros Adhanom Ghebreyesus warning that the variant is spreading faster than any previous strain and is probably present in most countries.

On Wednesday, EU chief Ursula von der Leyen also said Omicron could become dominant in Europe next month.

"If you look at the time it takes for new cases to double in number, it seems to be doubling every two or three days. And that's massive. We're told that by mid-January, we should expect Omicron to be the new dominant variant in Europe," she told the European Parliament, adding that the bloc had ample vaccines to fight the pandemic.

(With input from Reuters and AFP)