College officials were already bracing for a potential rise in coronavirus cases when students returned to campus after Thanksgiving break. Then the world learned of the omicron variant, with its troubling mutations and a host of unknowns.

“We are concerned,” said Gerri Taylor, co-chair of the American College Health Association covid-19 task force. “So many questions are coming up because of this, for sure.”

With a matter of days or weeks left in the fall semester, most schools are sticking with the virus mitigation rules and strategies they have in place, but ramping up reminders about testing, vaccination and other safety measures. And they are monitoring the situation closely, ready to pivot to more stringent rules if needed when students return after winter break.

“We’re not out of the woods yet,” Taylor said.

The unknowns include whether omicron will be more transmissible, how widespread it might become, if existing vaccines will provide strong protection against infection, and how sick it might make people, said Sarah Van Orman, chief health officer at the University of Southern California. For many colleges, those public health questions are amplified by the large populations of students living together, and by the mass movements of people on and off campus as the academic year advances.

School leaders are wondering what will happen with the variant over the coming winter break — and how that might affect the spring semester.

All the questions swirling around the emergence of omicron are reminiscent of the confusion unleashed by the delta variant this summer. News of that variant hit not long before the fall semester began, upending plans that had been built upon the expectation that vaccines would allow college life to return almost to normal. School officials scrambled to understand the likely impact of delta, and to build in extra precautions — including vaccine mandates on many campuses — before students returned.

Now officials are waiting to see what omicron will bring.

At the University of Maryland, where cases have increased since students have returned from Thanksgiving break, officials urged people to wear a KN95 mask, get a booster shot, stay home and get tested if feeling unwell — and to reconsider international travel plans.

“Depending on what we learn about Omicron in the coming days, we may need to take additional measures,” Spyridon Marinopoulos, director of the University Health Center, wrote in a message to campus Thursday.

At Yale University, officials changed their coronavirus advisories in response to increased infections on campus and in the community and to omicron, adding increased testing requirements and asking students to avoid nonessential travel for the rest of the semester.

At other schools, officials said it was too soon to know if they will need to make changes.

“I think we’ve got to wait and see,” said Robert C. Robbins, president of the University of Arizona, monitoring everything across the university daily.

“We’re thinking strategically about how to address all these very daunting issues over the next semester,” Robbins said. “We take it sort of semester at a time, but frankly, we take it a day at a time.”

He is particularly concerned about the question of immunity with omicron, he said. People get complacent once they get vaccinated, said Robbins, who is a heart surgeon. But with a new variant, the vaccine may not be as effective, or over time antibody levels have dropped and people are more susceptible. “The only way to get a handle on that is testing,” he said, and he would like to see more of it.

He said they will be looking for the variant as part of the testing they are doing on campus.

At the University of Southern California, Van Orman said administrators are not yet requiring boosters but are strongly encouraging people to get another dose of the vaccine. Instead of scaling back surveillance testing in the spring semester, they will continue to do that as long as needed, she said

Many schools are anticipating that more international students will remain on campus over the break, given concerns about prevalence of omicron in other countries and abrupt travel restrictions.

“We’re trying to be nimble,” said Melissa Nolan, assistant professor of epidemiology at the University of South Carolina, and prepare for increased needs for meals and other services on campus over the holidays.

And for the many students who will keep travel plans in place, there’s another worry: “They will be leaving college campuses, but they’ll be going home,” Taylor said. “So we have to be concerned about them infecting people at home.”

Nolan said the school is encouraging everyone to get tested after finals before leaving campus.

They expect another surge in cases when students return, and socialize, after break, Nolan said.

But at this point in the pandemic, Robbins said, “having been through this now over a year and a half, we feel pretty confident that we have the bandwidth to handle whatever comes our way.”