Seroprevalence of antibodies against SARS-CoV-2 virus in the adult Norwegian population, winter 2020/2021: pre-vaccination period

Since early 2020, over 123 million people worldwide have been diagnosed with coronavirus disease (Covid-19), but the true number of infections with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is undoubtedly higher. The seroprevalence of antibodies against SARS-CoV-2 can provide crucial epidemiological information about the extent of infections independent of virologically detected case numbers. There is no large population-based SARS-CoV-2 seroprevalence survey from Norway; thus we estimated SARS-CoV-2 seroprevalence in Norway before the introduction of vaccines and described its distribution across demographic groups. In November-December 2020, a total of 110,000 people aged 16 years or older were randomly selected from the National Population Register and invited to complete a questionnaire and provide a dried blood spot (DBS) sample. The response rate was 30%; compliance rate for return of DBS samples was 88%. The national weighted and adjusted seroprevalence was 0.9% (confidence interval 0.7-1.0).Seroprevalence was highest among those aged 16-19 years (1.9%, 0.9-2.9), those born outside the Nordic countries 1.4% (1.0-1.9), and in the counties of Oslo 1.7 % (1.2-2.2) and Vestland 1.4% (0.9-1.8). The ratio of SARS-CoV-2 seroprevalence (0.9) to the cumulative incidence of virologically detected cases by mid-December 2020 (0.8%) was slightly above one. SARS-CoV-2 seroprevalence was low before the introduction of vaccines in Norway and was comparable to virologically detected cases, indicating that most cases in the first 10 months of the pandemic were detected. Preventive measures including contact tracing have been effective, people are complying with social distancing recommendations, and local efforts to contain outbreaks have been essential.