TY - JOUR

T1 - Seroprevalence and infection fatality rate of the SARS-CoV-2 Omicron variant in Denmark

T2 - A nationwide serosurveillance study

AU - Erikstrup, Christian

AU - Laksafoss, Anna Damkjær

AU - Gladov, Josephine

AU - Kaspersen, Kathrine Agergård

AU - Mikkelsen, Susan

AU - Hindhede, Lotte

AU - Boldsen, Jens Kjærgaard

AU - Jørgensen, Signe Winther

AU - Ethelberg, Steen

AU - Holm, Dorte Kinggaard

AU - Bruun, Mie Topholm

AU - Nissen, Janna

AU - Schwinn, Michael

AU - Brodersen, Thorsten

AU - Mikkelsen, Christina

AU - Sækmose, Susanne Gjørup

AU - Sørensen, Erik

AU - Harritshøj, Lene Holm

AU - Aagaard, Bitten

AU - Dinh, Khoa Manh

AU - Busch, Michael P.

AU - Jørgensen, Charlotte Sværke

AU - Krause, Tyra Grove

AU - Ullum, Henrik

AU - Ostrowski, Sisse Rye

AU - Espenhain, Laura

AU - Pedersen, Ole Birger Vesterager

N1 - Publisher Copyright: © 2022 The Authors

PY - 2022

Y1 - 2022

N2 - 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.

AB - 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.

KW - Infection fatality rate

KW - SARS-CoV-2

KW - Seroprevalence

U2 - 10.1016/j.lanepe.2022.100479

DO - 10.1016/j.lanepe.2022.100479

M3 - Journal article

C2 - 35959415

AN - SCOPUS:85135724288

VL - 21

JO - The Lancet Regional Health - Europe

JF - The Lancet Regional Health - Europe

SN - 2666-7762

M1 - 100479

ER -