In record time, the omicron variant has become the dominant variant in the United States, accounting for 73 percent of new infections last week, the U.S. Centers for Disease Control and Prevention said Monday.

That rate suggests that there were more than 650,000 omicron infections in the United States last week, the Associated Press reported.

CDC data show nearly a sixfold rise in omicron's share of infections in just one week, and it is even higher in many parts of the country. Omicron accounts for an estimated 90 percent or more of new infections in the New York area, the Southeast, the industrial Midwest, and the Pacific Northwest. Delta became the dominant variant at the end of June, and it caused more than 99.5 percent of COVID-19 cases as recently as the end of November, CDC figures show.

The alarm about omicron was first sounded less than a month ago by South Africa, and it was designated a "variant of concern" by the World Health Organization on Nov. 26. So far, it has been detected in about 90 countries, the AP reported. Early studies suggest the fully vaccinated will need a booster shot for the best chance at preventing omicron infection, but even without the extra dose, vaccination still should offer strong protection against severe illness and death.

"All of us have a date with omicron," Amesh Adalja, M.D., a senior scholar at the Johns Hopkins Center for Health Security in Baltimore, told the AP. "If you're going to interact with society, if you're going to have any type of life, omicron will be something you encounter, and the best way you can encounter this is to be fully vaccinated."