At least 17 people suspected of having the Omicron variant of coronavirus after a ‘super spreader’ Christmas party in Oslo were fully vaccinated, city health officials have said.

On Thursday, it was confirmed a fifth person in Norway had tested positive for the Omicron variant in the wake of a corporate Christmas party in Oslo last week.

Today, a city health official told the AFP news agency that 60 people of the 100-plus guests had tested positive. One case has been confirmed as Omicron with a further 16 suspected to be linked to the new variant of concern.

Officials have warned the number of Omicron-confirmed cases could increase.

“All of them had been vaccinated, none of them had symptoms and they had all done self-tests” before attending the dinner,” Tine Ravlo told AFP.

He added that most of those infected had mild symptoms and that the party organisers had broken no rules.

Norway has responded swiftly to the outbreak, reintroducing some social distancing measures and encouraging people to work from home.

From Friday, anyone arriving in Norway, whether vaccinated or unvaccinated, must take a COVID test within 24 hours of arrival.

In areas with high infection rates such as Oslo, the use of face masks in crowded places is mandatory, as is working from home for those who are able to.

Some 88% of adults in Norway, and 71% of all Norwegians, are fully vaccinated. About 11% have received a booster shot.

Austria was plunged into lockdown in November in response to the fourth wave of COVID sweeping Europe. It became the first country to announce mandatory vaccines from next year. On Friday Switzerland announced that venues could refuse entry to unvaccinated people. Germany has also announced measures that place major restrictions on unvaccinated people or those who have recently recovered from infection, including being banned from restaurants, cinemas and many shops.