A Christmas party in Norway becomes a possible Omicron-spreading event.

Around half of the people who attended an office Christmas party in Oslo, where only vaccinated employees were admitted, have tested positive for the coronavirus after one guest recently returned from South Africa was found to carry the new Omicron variant, local health authorities said on Friday.

More than 120 people attended the event, held a week ago by a solar power company. Of the positive tests sequenced so far, between 15 and 20 were likely to be the Omicron variant, according to Dr. Tine Ravlo, a local chief physician involved in tracking the outbreak, who added that not all of the 60 coronavirus cases found so far had yet been fully checked for the variant.

“We are expecting more of them to likely be Omicron infections,” she said.

A spokesman for the company, Scatec, said that only vaccinated staff had been admitted to the party, and everyone had tested negative for the coronavirus before the event.

The party was held at a restaurant called Louise in central Oslo on Nov. 26, the same day that the World Health Organization labeled Omicron a “variant of concern” and many countries started closing their borders to passengers from southern Africa, where it was first identified.

The variant carries mutations that scientists say could allow it to spread more quickly and to cause more breakthrough infections, though much remains uncertain. Cases have now been reported in more than 40 countries, including in the United States.

The Scatec spokesman, Stian Tvede Karlsen, said that the employee first found to have the variant had arrived back in Norway from visiting a regional office in Cape Town before news of the variant had come out.

In order to curb further spread of the variant, the Norwegian government has introduced additional restrictions in and around Oslo, announced on Thursday and enforced from midnight.

These include the obligation to wear masks anywhere where social distance cannot be maintained, including on subways and buses or in shops, a return to working from home where possible and a cap on crowds of 100 people, except at theaters and other places with fixed seats, where up to 600 can be present. Only table service is permitted for alcoholic drinks and restaurants and events need to register guests.