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

Dr. Anthony Fauci, the nation's leading expert on infectious diseases, warned Thursday that omicron will become the dominant variant of the novel coronavirus in the United States "very soon."

"It has an extraordinary ability to transmit efficiently and spread," Fauci, the chief medical advisor to U.S. President Joe Biden, told ABC News' George Stephanopoulos in an interview on "Good Morning America."

"It has what we call a doubling time of about three days and if you do the math on that, if you have just a couple of percentage of the isolates being omicron, very soon it's going to be the dominant variant," he explained. "We've seen that in South Africa, we're seeing it in the U.K. and I'm absolutely certain that's what we're going to be seeing here relatively soon."

Fauci, who is also the director of the National Institute of Allergy and Infectious Diseases, urged Americans to "absolutely" get vaccinated against COVID-19, if they haven't already, and to also receive a booster shot when they become eligible.

"At this point, we don't believe you need an omicron-specific boost," he added. "We just need to get the boost with what you got originally for the primary vaccination."