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

The omicron variant appears to have a "growth advantage" over the delta variant, the World Health Organization said in a technical brief released Sunday.

"It is spreading faster than the delta variant in South Africa where delta circulation was low, but also appears to spread more quickly than the delta variant in other countries where the incidence of delta is high, such as in the United Kingdom," the WHO said in the brief, which was dated Friday. "Whether omicron's observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain. However, given the current available data, it is likely that omicron will outpace the delta variant where community transmission occurs."

Meanwhile, preliminary findings from South Africa suggest omicron may cause less severe illness than delta, and all cases of omicron reported in Europe to date have been mild or asymptomatic. But the WHO said "it remains unclear to what extent omicron may be inherently less virulent" and that "more data are needed to understand the severity profile."

The WHO also noted that "there are limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for omicron." However, preliminary evidence, and the considerably altered antigenic profile of the variant's spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with omicron.

"There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion," the WHO said.

The diagnostic accuracy of routinely used polymerase chain reaction (PCR) test and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be influenced by omicron. Therapeutic interventions for the management of severe or critical COVID-19 symptoms associated with omicron are also expected to remain effective, according to the WHO.

"However, monoclonal antibodies will need to be tested individually, for their antigen binding and virus neutralization and these studies should be prioritized," the WHO added.

-ABC News' Christine Theodorou