Preliminary findings from study after Christmas superspreader party in Oslo

As of 8 December, around 70 % of just over 100 participants at a Christmas party held on 26 November at a restaurant in Oslo have been subsequently diagnosed with SARS-CoV-2. Of those who are infected, 17 are confirmed as being infected with the omicron variant. It is assumed that the majority of cases without sequencing results available at this time are also infected with the same variant. In addition, more than 60 people who visited the restaurant the same evening as the Christmas party have been confirmed as infected with SARS-CoV-2.

The preliminary findings come from a study into the outbreak by the Norwegian Institute of Public Health (NIPH) in collaboration with Oslo Municipality.

Information was collected through telephone interviews with the participants conducted over the weekend, with responses from 95% of the participants.

All the participants at the Christmas party were asked to take a rapid antigen test before the party. Everyone reported a negative result (PCR test or rapid antigen test) 1–3 days before the Christmas party. Most of the participants were aged between 30 and 50 years and had received their second vaccine dose between May and November 2021.

Preliminary analyses

The preliminary analyses show:

• Of 111 participants at the Christmas party who were interviewed, 73% (80 people) were subsequently diagnosed with SARS-CoV-2. Of these, 17 are confirmed with the omicron variant by sequencing as of December 8, 2021. Analysis of additional samples is ongoing, but it is assumed that most people who are ill are infected with the omicron variant.
• All except one of those who were diagnosed with SARS-CoV-2 reported symptoms. Most reported symptom onset three days after the party.
• Over 70% of cases reported cough, lethargy, headache, sore throat and over half of them reported fever. No hospital admissions have been reported.
• Eight cases reported travel outside Norway during the two weeks before the Christmas party (different countries in Africa and Europe).
• In addition, infection has been detected among more than 60 people who visited the restaurant on the same evening as the party.

The figures may be updated at a later date as the study is still ongoing.

Widespread transmission

The preliminary results from the study show that there was widespread transmission at this event, even though the vast majority of participants were vaccinated with two doses of an mRNA vaccine. It is well-known that situations where many people are gathered closely together indoors give a high risk of transmission, plus high noise levels, talking loudly and singing will increase the risk. In addition, the omicron variant may have increased transmissibility, which would have contributed to further spread in this context.

Even though most of the cases have not had a severe disease course at this time, almost all developed symptoms relatively quickly after the Christmas party. The attendees were young and fully vaccinated and would not typically develop serious illness after SARS-CoV-2 infection. It is therefore difficult to comment on the severity of the disease with this variant based on these preliminary results.

Further work

The NIPH is continuing with the analyses, to study the transmission, symptom development and vaccination. The NIPH also plans to publish an article with more detailed results soon.