• The rapidly spreading omicron variant is now the dominant Covid strain in the U.S., representing 73% of sequenced cases, according to data published Monday by the Centers for Disease Control and Prevention.
• Omicron has displaced the previously dominant delta variant, which CDC data shows is now an estimated 26.6% of sequenced cases for the week ending December 18.

The rapidly spreading omicron variant is now the dominant Covid strain in the U.S., representing 73% of sequenced cases, according to data published Monday by the Centers for Disease Control and Prevention.

Omicron has displaced the previously dominant delta variant, which the CDC says is now an estimated 26.6% of sequenced cases for the week ending December 18. Just one week earlier, delta made up 87% of cases to omicron's 12.6%, the data shows.

The CDC had previously published data for the week ending Dec. 11 showing that omicron represented 2.9% of cases, but has revised the estimate for that period upward.

The omicron Covid-19 variant was first detected in southern Africa in late November and labeled a "variant of concern" by the World Health Organization on Nov. 26. It's not clear when exactly it first arrived in the U.S. Though California was first U.S. state to confirm an omicron case in the country on Dec. 1, the CDC has said a patient developed symptoms earlier on Nov. 15.

While the variant has proven to be extremely transmissible, much remains unknown about the severity of the illness it causes.

In some parts of the country, the share of omicron cases is higher than the nationwide figure of 73%. The CDC estimates it makes up more than 90% of cases in portions of the Northwest, South, Southeast, and Northeast.

Mayor Bill de Blasio on Monday said New York City is experiencing a rapid surge of omicron infections, though he expects the wave to peak in a matter of weeks.

Average daily case counts in New York City more than doubled over the week ending Friday Dec. 17, according to the latest data available on the the city's health department website. New York is reporting a seven-day average of more than 7,200 cases per day, up from about 3,200 the week prior, a 127% increase.

De Blasio said people infected with omicron in New York are experiencing mild symptoms so far, though he noted there remain many unanswered questions and scientists are still doing research to determine how sick people can become after contracting the variant. Even if omicron does prove to be more mild than delta, it could still strain health-care systems and potentially drive a spike in hospitalizations and deaths due to how fast it spreads, infectious disease experts have cautioned.

The U.S. is reporting a seven-day average of about 130,000 daily cases as of Dec. 19, according to data compiled by Johns Hopkins University, up 7% over the past two weeks.

Average daily cases are highest in the Northeast region adjusted for population, a CNBC analysis of Hopkins data shows. Rhode Island, New Hampshire, and New York are the three states averaging the most daily new cases per capita in the country.

About 69,000 Americans are hospitalized with Covid-19, according to a seven-day average of U.S. Department of Health and Human Services data as of Thursday, a two-week increase of 14%. Though rising, that is still lower than the delta wave's high point when more than 100,000 patients were hospitalized in early September.

CNBC's Spencer Kimball contributed reporting.