The Omicron variant caused more than 70% of recent Covid-19 cases in the U.S., the Centers for Disease Control and Prevention said Monday, highlighting its substantial increase in infectiousness compared with earlier versions of the virus.

The CDC said Monday that Omicron had overtaken the Delta variant of the coronavirus in the U.S. and accounted for an estimated 73% of infections for the week ending Dec. 18.

In many parts of the U.S., Omicron now makes up more than 90% of cases, the CDC said. Infectious-disease experts have said they believe the true share is likely even higher than that.

The CDC’s updated data showed Omicron had caused some 13% of recent infections in the week through Dec. 11.

The variant has spread rapidly in the U.S. and has reached at least 89 nations around the world since being identified in southern Africa last month.

New York state reported a record number of Covid-19 infections for the fourth consecutive day on Monday, when officials said 23,391 people had tested positive for the virus. Just over 4,000 people were hospitalized around the state, compared with peak levels of almost 19,000 in April of 2020.

Gov. Kathy Hochul said the state was setting up a system that would allow residents to order tests online. The state will also distribute 5 million rapid tests to school districts in an effort to cut down on required quarantine times starting in January.

“The kids in the classroom will be sent home with testing kits. We’re working on that supply chain now. There’s no reason why our children can’t stay in school,” the Democrat said.

The Omicron variant is causing Covid-19 cases to double every 1.5 to 3 days in places with community transmission, the World Health Organization said over the weekend. The variant is spreading rapidly even in places with high levels of immunity in the population, the United Nations agency said.

CDC officials have encouraged people to get vaccinated against Covid-19 if they haven’t already, to get booster shots if eligible and to wear masks in public indoor spaces, regardless of vaccination status. They are also recommending people take at-home Covid-19 tests before gathering with family and friends over the holidays, if possible.