WASHINGTON -- The newest Omicron variant is likely to become a dominant COVID-19 variant in United States, said top US infectious disease expert Anthony Fauci on Tuesday.

"Omicron is going to be a challenge because it spreads very rapidly, and the vaccines that we use - the regular two-dose mRNA - don't do very well against infection itself," Fauci said in an interview with CNN.

In South Africa, where the Omicron variant was first identified, while there is almost a vertical spike of infection, the country is not seeing severe hospitalizations, according to Fauci, director of the US National Institute of Allergy and Infectious Diseases.

The Omicron variant, which is possibly more contagious than the Delta variant, had been found in at least 31 US states as of Monday, since the first case in the country was detected in California on Dec 1.