At least one person has died in the United Kingdom after being infected with the omicron variant, British Prime Minister Boris Johnson said Monday.

It's the first publicly confirmed death globally from the new variant of the novel coronavirus, which was initially identified in southern Africa last month and has since spread rapidly around the world. Deaths from omicron may have already occurred in other countries but no others have been publicly confirmed yet.

Speaking to reporters during a visit to a vaccination clinic in London, Johnson said the death was a patient who had been diagnosed at a hospital but gave no further details.

"Sadly, at least one patient has now been confirmed to have died with omicron," the prime minister said. "So I think the idea that this is somehow a milder version of the virus, I think that's something we need to set on one side and just recognize the sheer pace at which it accelerates through the population. So the best thing we can do is all get our boosters."

The National Football League will require that all its vaccinated Tier 1 and 2 employees receive COVID-19 booster shots by Dec. 27, according to a memo sent to all 32 teams within the professional American football league on Monday.

This group of employees includes coaches and other employees who work closely with NFL players and help in essential league operations.

"On November 29, the CDC issued a study showing that the effectiveness of the approved COVID-19 vaccines may decrease over time and has recommended that all eligible vaccinated individuals over the age of 18 should receive a booster shot," the memo states. "Given the increased prevalence of the virus in our communities, our experts have recommended that we implement the CDC’s recommendation."

The memo came as 36 players were added to the NFLS's COVID-19 reserve list. Players are currently not required to receive boosters; but in Monday’s memo, the NFL said teams should consider making boosters available for players and their families.

"Any individual who is not currently subject to the requirement for boosters will be required to obtain the booster within 14 days of becoming eligible," the memo states.

-ABC News’ Katie Conway

The U.S. Air Force gave administrative discharges to 27 personnel who refused to get vaccinated against COVID-19 and comply with the federal government's mandate.

Over 7,000 Air Force members are unvaccinated against COVID-19, which includes about 4,500 who are waiting to see if they are eligible for religious exemption. More than 1,500 religious exemption requests have been denied and those individuals remain unvaccinated.

The Air Force has not granted a single religious exemption, according to officials.

Over 1,000 members have outright refused to get their shots.

More discharges over vaccine refusal are likely, according to an official.

-ABC News' Luis Martinez

California is reinstating an indoor mask mandate from Dec. 15 to Jan. 15.

Half of the state is already under a mask mandate per local rules, according to California Health and Human Services Secretary Mark Ghaly.

California is also tightening restrictions on unvaccinated people at large events. At events with more than 1,000 people, anyone who isn’t vaccinated must show proof of a negative antigen test within 24 hours or a negative PCR test within 48 hours.

State officials also recommend that travelers get tested three to five days before arriving in California.

-ABC News' Matt Fuhrman

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