In Wisconsin, only 4% of ICU beds are available.

"This is getting really scary," Dr. Jamie Hess, an emergency physician at the University of Wisconsin, told ABC News.

"We're really reaching a crisis point where we have more patients to take care of then we have beds in the hospital or staff to care for them," Hess said.

Michigan has been struggling through a similar surge for nearly three months, with the state reporting more than 6,500 new cases each day. On average, more than 500 patients are being admitted to hospitals each day.

"Where we are right now feels a lot like the first surge back in March of 2020," Erin Dicks, a nurse manager at MICU Henry Ford Hospital, told ABC News. "We don't have enough beds to be able to manage this."

Dicks said so many patients are young.

"I think one of the biggest frustrating pieces for my staff is that they look at this as, this is preventable -- people don't have to die here," Dicks said.

-ABC News' Arielle Mitropoulos

Harvard University will require boosters for all eligible members of the community, school officials said Thursday.

Earlier this week, Harvard warned of an increase in cases, saying the rise can be attributed to social events following the Thanksgiving break.

Omicron is likely already on campus, university officials added.

Harvard joins a growing list of colleges and universities moving to require third doses next semester for those eligible. Other schools include Stanford, NYU, University of Notre Dame, Syracuse, Smith College and Wellesley College.

-ABC News' Arielle Mitropoulos

In Illinois, 11,858 new cases were reported on Thursday -- the highest daily case number of all of 2021, ABC Chicago station WLS reported, citing state health officials.

Illinois confirmed its second omicron case Wednesday, detected in a suburban Chicago resident. That person is asymptomatic and vaccinated, WLS reported.

The Centers for Disease Control and Prevention has signed off on its advisory committee's recommendation that people who have a choice should get an mRNA vaccine, either Pfizer or Moderna, over the single-shot Johnson & Johnson vaccine.

The updated recommendation comes after a review of new CDC data on rare blood clots linked to the J&J vaccine.

"Today's updated recommendation emphasizes CDC's commitment to provide real-time scientific information to the American public," CDC Director Dr. Rochelle Walensky said in a statement. "I continue to encourage all Americans to get vaccinated and boosted."

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