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About this publication
COVID-19 is an infectious disease caused by the novel coronavirus SARS-CoV-2. Infection with SARS-CoV-2 induces antibodies to the virus, therefore, in the absence of vaccination, the presence of SARS-CoV-2 specific antibodies in a person’s blood indicates that the person has been infected with SARS-CoV-2. This is the third study measuring antibodies against SARS-CoV-2 in residual serum samples collected systematically from various geographical regions in Norway and covering all age groups. The first study with residual sera collected in April/May 2020 and August 2019, was published as a report in June 2020 (1). The second study, with residual sera collected in the late summer of 2020, was published as a report in December 2020 (2). In the present study, data on the seroprevalence in Norway in January 2021, at the end of the second wave of the pandemic, is presented. A total of 1912 residual sera were sampled from 17 microbiology laboratories in January 2020 (week number 53 in 2020/2021 to week number 6, 2021). Most of the samples (85.7%) were collected between week numbers 1-3, 2021. The sera were tested for antibodies against SARS-CoV-2 using an in-house assay established at the Department of Immunology, Oslo University Hospital. Based on these measurements, the estimated seroprevalence in the Norwegian population in January 2021 was 3.2% (95% credible interval (CrI) 2.3 - 4.1). This is an increase from the seroprevalence estimate in the late summer of 2020 (0.6% (95% CrI 0.2- 1.2))(2). Antibodies against SARS-CoV-2 were found in a total of 61 of the 1912 samples and were detected in samples from 10 of the 11 Norwegian counties. There were no significant differences between the seroprevalence estimates for each county. However, the number of samples from each county varied considerably. There were no significant differences between the seroprevalence estimates for the various age groups or between males and females.