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

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-nudeocapsid (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 100000 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. Copyright (C) 2022 The Authors. Published by Elsevier Ltd.

Automated summary

The Omicron variant caused a surge in SARS-CoV-2 infections in Denmark despite high vaccination coverage. Using blood donor serosurveillance, the study estimated the percentage of recently infected individuals in the background population and found that one third of infections were missed by PCR testing. The infection fatality rate was lower than previous waves.