COVID-19 updates: Elizabeth Warren tests positive

The U.K. reported a 48.59 percent single-day increase in confirmed omicron cases Sunday.

And across the U.K. in the last seven days, there's been a 51.9 percent increase in new COVID cases. Hospitalizations are up 8.1 percent during that period. However, there's been a 6.5 percent decrease in new deaths compared to the preceding seven days.

On Sunday, the British government reported 82,886 new COVID cases and 45 new deaths. The government also confirmed 12,133 new cases of the omicron variant.

London's mayor declared a major incident in the capital Sunday because of the serious threat of omicron. In an interview with the BBC on Sunday, he said that in the last 24 hours, "we're at almost 30,000 confirmed new cases, and in the last seven days almost 130,000 confirmed new cases." 

He said he's worried about the impact the increases are already having on frontline emergency services in London, adding there are a record number of absences that will hamper their ability to respond to people who are unwell.

-ABC News' Christine Theodorou

"Saturday Night Live" won't have an audience and will limit its cast and crew at tonight's taping "out of an abundance of caution," the show announced, as New York sees a spike in COVID-19 cases.

"Due to the recent spike in the Omicron variant and out of an abundance of caution, there will be no live audience for tonight’s taping of 'Saturday Night Live' and the show will have limited cast and crew," tweeted the show, which films at 30 Rockefeller Plaza in Manhattan.

"The show continues to follow all government safety guidelines in addition to a rigorous testing protocol," it added.

Charli XCX was slated to be the musical guest but is no longer performing due to the limited crew, the singer announced on Twitter, saying she was "devastated and heartbroken."

Paul Rudd is the host of the episode, which is the last one of 2021.

The move comes as New York state broke its record for daily COVID-19 cases two days in a row. Several shows in New York City, from Broadway productions to the Rockettes, also have had to cancel performances due to COVID-19 cases.

-ABC News' Joshua Hoyos

Harvard University is shifting to remote learning for the first three weeks of January due to a "rapid" rise in COVID-19 cases locally, school officials announced Saturday.

"Please know that we do not take this step lightly," school leaders said in an update to students. "It is prompted by the rapid rise in COVID-19 cases locally and across the country, as well as the growing presence of the highly transmissible Omicron variant." 

Only students with previous authorization will be allowed to remain on campus. Some programs, like essential in-person laboratory and patient-centered work, will continue as normal, the school said.

"We are planning a return to more robust on-campus activities later in January, public health conditions permitting," the school officials said. 

Several universities, including Princeton, NYU and Cornell, have shifted final exams remote and canceled events in recent days due to spikes in COVID-19 cases.

-ABC News' Arielle Mitropoulos

Enforcement of the Biden administration's vaccine-or-test mandate for private companies with more 100 employees will be delayed by a month, officials said.

The Labor Department’s Occupational Safety and Health Administration announced the change Saturday, a day after a federal appeals court lifted a stay on the rule, allowing it to proceed. 

Previously, the mandate required companies to enforce masks among unvaccinated employees beginning Dec. 6, and either see proof of vaccination or begin testing workers weekly by Jan. 4. 

The new deadlines for compliance are now Jan. 10 and Feb. 9, respectively, OSHA said.

-ABC News' Anne Flaherty