An Omicron outbreak at a Norwegian Christmas party is providing an early, if still anecdotal, data point on the ease through which the new variant spreads between vaccinated people, and how mild its symptoms at times can be.

Before Scatec ASA, a Norway-based renewable-energy company, hosted the annual holiday party, it took all the major safety precautions, said Stian Tvede Karlsen, a company spokesman. Only vaccinated employees were invited. All had to take a rapid test the day before. The party, at Louise, an upscale Oslo...

An Omicron outbreak at a Norwegian Christmas party is providing an early, if still anecdotal, data point on the ease through which the new variant spreads between vaccinated people, and how mild its symptoms at times can be.

Before Scatec AS A, a Norway-based renewable-energy company, hosted the annual holiday party, it took all the major safety precautions, said Stian Tvede Karlsen, a company spokesman. Only vaccinated employees were invited. All had to take a rapid test the day before. The party, at Louise, an upscale Oslo restaurant serving seafood and Scandinavian fare, included about 120 people, several of whom had just returned from South Africa, where the company has a solar-panel project.

Scientists and vaccine makers are investigating Omicron, a Covid-19 variant with around 50 mutations, that has been detected in many countries after spreading in southern Africa. Here’s what we know as the U.S. and others implement travel restrictions. Photo: Fazry Ismail/EPA-EFE/Shutterstock The Wall Street Journal Interactive Edition

More than half of those present have since tested positive for Covid-19, with at least 13 confirmed to have the new variant in what appears to be the world’s biggest Omicron outbreak outside southern Africa—and a glimpse into how it fares in a highly-vaccinated population.

The Oslo municipal health department said it expects more Omicron cases to be confirmed among the 64 Covid-19 cases as medical workers test the remaining patients.

The cluster is remarkable because it took place in a bubble of immunized people, in a country where more than 80% of adults are fully vaccinated. Norway has opened up booster shots for all adults as winter sets in, a time when people begin mixing more indoors, thus providing the virus more opportunities to spread.

But it also provides signs of hope, however anecdotal and thin, that Omicron cases can turn out to be mild among healthy, vaccinated adults. So far, none of the infected employees are seriously ill, said Mr. Karlsen.

“People have different kinds of symptoms but none are severe,” he said. “No one has any serious issues.”

Still, the rapid spread, he added, had taken him aback: “That’s what’s really curious…All of a sudden, 60 people. It’s just insane.”

None of the 64 Covid-19 cases have needed hospital care, said Tine Ravlo, assistant district superintendent for the Oslo municipal government. All of them are under isolation at home, she said.

“For now, what they describe are the mild symptoms…headache, cough, sore throat and flulike symptoms,” she said. “Several have had symptoms and now say they feel better.”

Widespread vaccination has likely eased the initial burden on Norway’s hospitals as the pandemic begins its second winter there. The country is seeing more than 2,000 new cases daily, although authorities are still rolling out new lockdown-like measures to spare intensive-care wards from being overwhelmed. On Dec. 1, data compiled by Our World in Data recorded 38 confirmed deaths that day, out of a total population of 5.4 million.

From Friday morning, mask mandates that had been lifted are back in place, private indoor gatherings are limited to fewer than 100 people, office workers are encouraged to work from home, and bars are operating at reduced capacity.

The capital, Oslo, has canceled a major Christmas gathering and other holiday events are being called off as well. The country is averaging six deaths daily.

Lab results from experiments into how easily the new variant evades vaccines are still a week or two away, leaving scientists to scrutinize outbreaks like Oslo’s for early clues.

By itself, the high level of transmission at Scatec’s Christmas party might not say much about how contagious the new Omicron variant is. An hourslong party with lots of guests talking to each other in a closed, indoor and insulated environment would make for an ideal superspreader event, even for earlier variants of the coronavirus, said Alexandra Phelan, an assistant professor of global and public-health law and ethics at Georgetown University.

The mild symptoms might not tell us much, for the moment, she added. But the cases added to evidence that vaccines aren’t preventing infections, though they might still prevent the risk of severe disease in most people.

“If they were of working age and young enough to be partying into the night, they were already probably at a low risk,” she said. “The big question that this is starting to add data to, at least anecdotal data, is immune evasion.”

Write to Drew Hinshaw at drew.hinshaw@wsj.com