Americans vaccinated and boosted against COVID-19 can be with family over the holidays but attending large gatherings is not safe, even for those who received a booster dose, top US infectious disease expert Anthony Fauci said on Wednesday, according to Reuters.

"There are many of these parties that have 30, 40, 50 people in which you do not know the vaccination status of individuals. Those are the kind of functions in the context of Omicron that you do not want to go to," Fauci said at a White House briefing.

Early evidence indicates the Omicron variant of COVID-19 is less severe than the Delta variant, said Fauci, citing studies from South Africa and Scotland, but he also warned Americans to remain cautious.

"This is good news. However, we must wait to see what happens in our own population which has its own demographic considerations," he said.

The statements come two days after the Centers for Disease Control and Prevention (CDC) said that Omicron is now the dominant version of the coronavirus in the US, accounting for 73% of new infections last week.

Also on Monday, the US recorded its first death from the Omicron variant, an unvaccinated Houston man in his 50s with underlying health conditions.

On Sunday, Fauci warned of a “bleak winter” ahead as Omicron spurs a new wave of infections globally.

"One thing that's very clear... is (Omicron's) extraordinary capability of spreading," Fauci told NBC News. "It is just... raging through to the world."