The omicron variant was estimated to be 2.9% of the COVID-19 variants circulating in the United States as of Dec. 11, according to data from the U.S. Centers for Disease Control and Prevention.

For the week ended Dec. 4, omicron variant constituted 0.4% of all variants in the country, based on the specimens sequenced.

The agency also estimated that the fast-spreading variant, first detected in South Africa, Botswana and Hong Kong in late November, was 13.1% of circulating COVID-19 variants in New Jersey, New York, Puerto Rico and Virgin Islands for the week ending Dec. 11.

The World Health Organization said on Sunday that while preliminary findings from South Africa suggest omicron may be less severe than the Delta variant, which is currently dominant worldwide, it remains unclear to what extent Omicron may be inherently less virulent.

"Are SGTF (S-gene drop-out or S-gene target failure) samples being prioritized for sequencing?" former FDA commissioner Scott Gottlieb said on Twitter .

"If that's the case, hard to derive true prevalence estimate from these percentages, and results need to be put into context. While incidence of omicron clearly rising quickly, sequence breakdown doesn't represent true prevalence."