The omicron variant of COVID-19 has surpassed Delta in the United States, becoming the dominant strain the country, health officials announced on Monday. With a dramatic increase in omicron spread, the variant now accounts for 73% of the cases in the country.

A six-fold increase in number was reported in just one week by the United States Centers for Disease Control and Prevention.

In much of the country, omicron’s prevalence is even higher. It’s responsible for an estimated 90% of new infections in the New York area, the Southeast, the industrial Midwest and the Pacific Northwest. The national rate suggests that more than 650,000 omicron infections occurred in the United States last week.

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Dr Rochelle Walensky, the director of the Centers for Disease Control and Prevention, said that such an increase was expected and was “not surprising”.

As of November, according to the official data, the delta variant of COVID-19 was the most common in the United States, accounting for more than 99.5% of the infections.

Scientists in Africa first sounded the alarm about omicron less than a month ago and on November 26 the World Health Organization designated it as a “variant of concern.” The mutant has since shown up in about 90 countries.

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Much about the omicron variant remains unknown, including whether it causes more or less severe illness. Early studies suggest the 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.

In the week that ended December 11, the omicron variant’s share of new infections in the United States increased to 2.9% from 0.4% the week before, the Centers for Disease Control and Prevention previously reported, according to Associated Press.

(With AP inputs)