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 new omicron variant makes up nearly 100% of the strains found in wastewater samples in Orlando, Florida, officials said Thursday.

"It escalated rapidly," Orange County Utilities spokeswoman Sarah Lux told ABC News.

In its first test for the variant last Thursday, the department found no evidence of omicron in the community's wastewater, she said. On Saturday, it represented about 30% of the strains found in the samples, and by Tuesday, nearly 100%.

"So, we're talking about zero to nearly 100 in a matter of a week," Lux said.

All parts of the county are seeing an increasing presence of the omicron variant, she said. The southern area, home to the theme parks, has seen the highest amount of virus remnants, followed by the eastern area, which is home to the University of Central Florida.

-ABC News' Arielle Mitropoulos

With winter closing in and COVID-19 cases on the rise, hospitals across the United States are once again facing the pressures of caring for thousands of patients.

More than 67,000 people are currently hospitalized with COVID-19 nationwide, according to federal data.

Rebecca Long, lead nurse in a COVID-19 intensive care unit at Nebraska Medical Center in Omaha, told ABC News that she and her team "literally do not have any ICU beds" available.

"I don't want anyone else's family member or loved one to have to be in the position where we say, like, we can't help you because we don't have the resources," Long said. "As health care providers, all we want to do is help people and we can't because we physically can't."

Dr. Kyle McCarty, medical director of emergency services at both HSHS St. Mary's and HSHS St. Vincent hospitals in Green Bay, Wisconsin, told ABC News that health care workers are feeling burned out after "being asked to do more with less."

"We're exhausted by the knowledge that we are the duct tape that is preventing a complete collapse of the health care system," McCarty said. "There's a national shortage of hospital staff, which is making it difficult to take care of patients the way that we want it. There aren't enough inpatient beds for the patients that need to be admitted to the hospital."

"This is a call for reinforcements, not a warning to stay away, because we don't want this to be the new normal," he added. "If we can recruit more health care teammates, it doesn't have to be."

-ABC News' Arielle Mitropoulos