A federal appeals court reinstated the Biden administration's nationwide workplace vaccine mandate Friday.

The Sixth Circuit Court of Appeals dissolved the stay issued by the Fifth Circuit last month on the rule, which would require employees at private companies with over 100 workers to get vaccinated or tested regularly.

The mandate "is an important step in curtailing the transmission of a deadly virus that has killed over 800,000 people in the United States, brought our healthcare system to its knees, forced businesses to shut down for months on end, and cost hundreds of thousands of workers their jobs," Judge Jane Stranch wrote in the opinion.

The rule, which was set to go into effect on Jan. 4, 2022, had been put on hold by the Fifth Circuit after it ruled that the Labor Department’s Occupational Safety and Health Administration didn’t have the legal authority to impose such a requirement on private businesses.

The Labor Department had previously said it won’t move forward with enforcing its workplace mandate until the matter was resolved in court.

Arkansas Attorney General Leslie Rutledge said Friday she plans to ask the U.S. Supreme Court to block the order.

-ABC News' Benjamin Siu and Anne Flaherty

The omicron variant could spread two to three times faster than delta, according to a new report from the Imperial College London COVID-19 response team.

The study, which has not yet been peer-reviewed, also estimated that the risk of being reinfected with omicron is more than five times higher than that of delta.

Data could change as more information on omicron's spread is gathered, though the study offers an early indication of how much more transmissible the new variant is compared to delta.

-ABC News' Cheyenne Haslett, Dr. John Brownstein and Sony Salzman

Due to a "substantial increase in cases across the league," the NFL announced that several weekend games have been pushed back.

Saturday's game between the Las Vegas Raiders and Cleveland Browns has been moved from Saturday to Monday.

The Washington Football Team vs. Philadelphia Eagles matchup and Seattle Seahawks-Los Angeles Rams game have both been moved from Sunday to Tuesday.

New York state reported 21,027 cases Thursday, setting a new single-day high, Gov. Kathy Hochul's office said.

"The winter surge in COVID-19 cases is a reminder that the pandemic is not over yet and we must take extra care to keep ourselves and each other safe," Hochul said in a statement. "The vaccine is still our best weapon to defeat the virus."

-ABC News' Josh Hoyos

School leaders at both New York University and Princeton University announced Wednesday that they will take steps to cancel end-of-year events and move winter exams online amid a surge in COVID-19 cases on their respective campuses.

"Given that we've seen an uptick in cases among undergraduates during the last 24 hours -- including suspected cases of the highly contagious Omicron variant -- we hope to avoid letting the final exam schedule interfere with students’ travel home for Winter Break," Princeton University's dean, Jill Dolan, and vice president for campus life, Rochelle Calhoun, said in a joint statement. "We certainly don’t want you remaining on campus in required isolation through the holidays."

All indoor gatherings with food and those where face coverings cannot be worn must be canceled or postponed at Princeton's New Jersey campus from Thursday through Jan. 7, according to the statement.

A statement from New York University's provost, Katherine Fleming, executive Vice President, Martin Dorph, and head of the COVID-19 prevention and response team, Dr. Carlo Ciotoli, said they "strongly encourage that final examinations and/or assessments be changed to remote/online format." All discretionary, non-essential, non-academic gatherings and events at the school's campus in New York City must be canceled "immediately," according to the statement.

"The continuous review of the data from our COVID-19 testing program has indicated a considerable acceleration in the rate of new cases in our community," they said. "It’s not a cause for alarm, but it is a cause for concern, caution, and appropriate actions."

NYU has also joined a growing list of colleges and universities across the nation that are requiring all eligible members of the school community, including faculty and students, to be fully vaccinated against COVID-19 and receive a booster shot ahead of the upcoming spring semester.

Meanwhile, officials at Cornell University in Ithaca, New York, announced Tuesday that they have moved the campus into a "level red" alert after identifying evidence of suspected cases of the omicron variant in a "significant" number of the school's COVID-19 positive samples from students.

According to data from the Centers for Disease Control and Prevention, the omicron variant is estimated to account for more than 13% of newly confirmed COVID-19 cases in the New York and New Jersey area -- the highest proportion of any region of the United States.

-ABC News' Chris Donato and Arielle Mitropoulos