Seroprevalence of SARS-CoV-2 in the Norwegian population measured in residual sera collected in late summer 2020
This is the second study measuring antibodies against SARS-CoV-2 in serum samples collected systematically from various geographical regions in Norway and covering all age groups. A total of 1812 residual sera from 16 laboratories were sampled in week numbers 30-37, 2020, and tested for antibodies against SARS-CoV-2.
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 the presence of these antibodies in a person’s blood indicates that the person has been infected with SARS-CoV-2. This is the second 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 from April/May 2020 and August 2019, was published as a report in June 2020 (1). In the present study, a total of 1812 residual sera were sampled from 16 microbiology laboratories from late July to mid-September 2020 (week numbers 30-37). The sera were tested for antibodies against SARS-CoV-2 using a novel in-house assay established at the Department of Immunology, Oslo University Hospital.
Based on these measurements, the estimated seroprevalence in the Norwegian population in the late summer was 0.6% [95% credible interval (CrI) 0.2 - 1.2]. This is not significantly different from the seroprevalence estimate based on the 900 samples collected in the spring (1.0% [95% CrI 0.1- 2.4])(1). Antibodies against SARS-CoV-2 were found in samples from 6 of the 11 counties, but there were no significant differences between the seroprevalence estimates for each county. However, the sample size from each county varied considerably (from 20 to 387) and the number of positive samples were limited (11 in total). There were no significant differences between the seroprevalence estimates for the various age groups or between males and females.
During the summer of 2020, the pandemic was quite contained in Norway. This may explain why the estimated seroprevalence for the Norwegian population was similar in the spring and in the late summer. In this first follow-up study, more laboratories contributed, and a greater number of samples was tested, leading to better precision of the seroprevalence estimate. New follow-up studies of antibodies against SARS-CoV-2 in future collections of residual sera will provide further information about the development of the COVID-19 pandemic in Norway.