Seroprevalence and infection fatality rate of the SARS-CoV-2 Omicron variant in Denmark: A nationwide serosurveillance study

Christian Erikstrup*, Anna Damkjær Laksafoss, Josephine Gladov, Kathrine Agergård Kaspersen, Susan Mikkelsen, Lotte Hindhede, Jens Kjærgaard Boldsen, Signe Winther Jørgensen, Steen Ethelberg, Dorte Kinggaard Holm, Mie Topholm Bruun, Janna Nissen, Michael Schwinn, Thorsten Brodersen, Christina Mikkelsen, Susanne Gjørup Sækmose, Erik Sørensen, Lene Holm Harritshøj, Bitten Aagaard, Khoa Manh DinhMichael P. Busch, Charlotte Sværke Jørgensen, Tyra Grove Krause, Henrik Ullum, Sisse Rye Ostrowski, Laura Espenhain, Ole Birger Vesterager Pedersen

*Corresponding author for this work

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaperJournal articleResearchpeer-review


Background: Introduction of the Omicron variant caused a steep rise in SARS-CoV-2 infections despite high vaccination coverage in the Danish population. We used blood donor serosurveillance to estimate the percentage of recently infected residents in the similarly aged background population with no known comorbidity. Methods: To detect SARS-CoV-2 antibodies induced due to recent infection, and not vaccination, we assessed anti-nucleocapsid (anti-N) immunoglobulin G (IgG) in blood donor samples. Individual level data on SARS-CoV-2 RT-PCR results and vaccination status were available. Anti-N IgG was measured fortnightly from January 18 to April 3, 2022. Samples from November 2021 were analysed to assess seroprevalence before introduction of the Omicron variant in Denmark. Findings: A total of 43 088 donations from 35 309 Danish blood donors aged 17–72 years were screened. In November 2021, 1·2% (103/8 701) of donors had detectable anti-N IgG antibodies. Adjusting for test sensitivity (estimates ranging from 74%–81%) and November seroprevalence, we estimate that 66% (95% confidence intervals (CI): 63%–70%) of the healthy, similarly aged Danish population had been infected between November 1, 2021, and March 15, 2022. One third of infections were not captured by SARS-CoV-2 RT-PCR testing. The infection fatality rate (IFR) was 6·2 (CI: 5·1–7·5) per 100 000 infections. Interpretation: Screening for anti-N IgG and linkage to national registers allowed us to detect recent infections and accurately assess assay sensitivity in vaccinated or previously infected individuals during the Omicron outbreak. The IFR was lower than during previous waves. Funding: The Danish Ministry of Health.

Original languageEnglish
Article number100479
JournalThe Lancet Regional Health - Europe
Number of pages10
Publication statusPublished - Oct 2022


  • Infection fatality rate
  • SARS-CoV-2
  • Seroprevalence


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