Federal health officials said on Monday that Omicron is already ahead of other variants and is now the main version of the US coronavirus, accounting for 73% of new infections last week.

Data from the Centers for Disease Control and Prevention shows that in just one week, the share of omicron infections has increased by nearly six times.

In most parts of the country, it is even higher. Omicron is responsible for an estimated 90% or more of new infections in the New York area, the Southeast, the Industrial Midwest, and the Pacific Northwest. National rates indicate that more than 650,000 omicron infections occurred in the United States last week.

Since the end of June, delta variants have been the main version that caused infections in the United States. According to data from the CDC, at the end of November, more than 99.5% of coronaviruses were delta.

Dr. Rochelle Varensky, director of the Centers for Disease Control and Prevention, said the new figures reflect growth in other countries.

“These numbers are obvious, but not surprising,” she said.

African scientists first issued a warning to omicron less than a month ago, and the World Health Organization designated it as a “variant of concern” on November 26. Since then, the mutant has appeared in approximately 90 countries.

Most of the content about the omicron variant is still unknown, including whether it will cause more or less serious illness. Early research shows that vaccinated people need to boost injections to get the best chance of preventing omicron infection, but even without additional doses, vaccination should still provide strong protection against serious illness and death.

“All of us have an appointment with omicron,” said Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Health and Safety Center. “If you want to interact with society, if you want to live any type of life, omicron will be the thing you encounter, and the best way for you to encounter it is to be fully vaccinated.”

Adalja said that considering the situation in South Africa, the United Kingdom and Denmark, CDC data shows that omicron surpasses delta in the United States. He is not surprised. He predicted that the holidays will spread, including breakthrough infections among vaccinated people and severe complications among unvaccinated people, which may put pressure on hospitals that are already burdened by the Delta.

Dr. Eric Topol, director of the Scripps Research and Translation Institute, said that other countries have seen rapid growth in Europe and the United States, but data from the United States shows that “significant growth has been achieved in such a short period of time.”

Topol also said that it is not yet clear how gentle omicron is compared to other variants.

“This is the biggest uncertainty right now,” Topol said. “We have to rely on omicron’s large number of hospitalizations and many serious diseases.”

The CDC estimates are based on thousands of coronavirus samples collected every week through universities and commercial laboratories and state and local health departments. Scientists analyze their genetic sequence to determine which version of the COVID-19 virus is the most abundant.

On Monday, after analyzing more samples, the CDC revised its estimate of omicron cases for the week ending December 11. Approximately 13% of the cases in that week came from omicron, instead of the 3% previously reported. In the previous week, omicron only accounted for 0.4% of cases.

CDC officials said they have not yet estimated how many people were hospitalized or died due to omicron.

Although there are still many new infections caused by delta variants, “I expect that over time, omicron will crowd out the delta,” Walensky said.