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The study, which has not yet been peer-reviewed, also estimated that the risk of being reinfected with omicron is more than five times higher than that of delta.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

Omicron is expected to be the dominant variant in London in the next 48 hours, the Secretary of State for Health and Social Care, Sajid Javid, warned in a statement to Parliament Monday.

Omicron represents 20% of England's cases and 44% of London's cases, Javid said.

"No variant of COVID-19 has spread this fast," Javid said.

Ten people with omicron are in U.K. hospitals.

"Hospitalisations and deaths lag infections by around two weeks," Javid said, "So we can expect those numbers to dramatically increase in the days and weeks that lie ahead."

"In preparation, the UK’s four Chief Medical Officers raised the COVID Alert level to 4, its second-highest level, this was done over the weekend. And NHS England has just announced it will return to its highest level of emergency preparedness: Level 4 National Incident," Javid said. "This means the NHS response to Omicron will be a coordinated as a national effort, rather than led by individual trusts."

Javid also urged people to get booster doses, stating that 40% of adults in the U.K. have gotten boosters so far.

-ABC News' Zoe Magee