By Sarah Toy

New York state reported a record number of Covid-19 infections for the fourth consecutive day on Monday.

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.

“It’s time to up your Covid game,” said Peter Chin-Hong, an infectious-disease physician and professor at the University of California, San Francisco.

White House press secretary Jen Psaki said on Monday night that a midlevel staffer who had been in proximity to President Biden for roughly 30 minutes last week had tested positive for Covid-19. Mr. Biden tested negative for Covid-19 earlier Monday and will undergo another test on Wednesday, Ms. Psaki said.

The staffer was in proximity to Mr. Biden during a Friday flight on Air Force One. The staffer didn’t begin experiencing symptoms until Sunday and tested positive for the virus on Monday morning, Ms. Psaki said. She didn’t name the staffer, who she said was fully vaccinated and had received a booster shot. Mr. Biden will carry on with his normal schedule, she added.

Some scientists are hopeful Omicron may not lead to as severe disease in people it infects as other variants. Salim Abdool Karim, a clinical infectious-disease epidemiologist in South Africa, said hospital admission rates in the country’s Omicron wave are much lower than during the previous Delta and Beta waves. Among those patients being admitted to the hospital, fewer are requiring ventilation, ICU care or dying, he said.

“I think the clinical picture we are seeing is one of substantially less severe disease,” he said.

Other infectious-disease experts and epidemiologists said it is still too early to tell whether Omicron causes less severe disease. They also worry that Omicron’s ascent in the U.S. is taking place as hospitals are already filling up with patients infected with the Delta variant.

Public-health officials, including Dr. Anthony Fauci, the nation’s top infectious-disease expert, have said Omicron will likely strain U.S. hospitals, especially if staff get sick and need to be isolated. Many hospitals are already grappling with widespread staffing shortages.

The Houston Methodist hospital system said Sunday that Omicron accounted for 82% of new symptomatic Covid-19 cases at its facilities. The Delta variant took three months to reach such prevalence, the hospital system said.

When the general manager of Houston’s Dandelion Café contracted Covid-19 last week, owner Sarah Lieberman thought quarantining him and testing the rest of the staff would be effective, as when virus cases had arisen before.

This time, within days, the head chef also contracted Covid-19. Then the kitchen manager tested positive. Other employees developed symptoms. Ms. Lieberman decided to close for 10 days until Dec. 27. Just after making the decision, she tested positive herself.

“In the past, we’ve always been able to stay ahead of it,” Ms. Lieberman said.