Seroprevalence of SARS-CoV-2 Antibodies in Denmark: Results of Two Nationwide Population-Based Surveys, February and May 2021

Abstract

Background

Seroprevalence studies can be used to measure the progression of national COVID-19 epidemics. The Danish National Seroprevalence Survey of SARS-CoV-2 infections (DSS) was conducted as five separate surveys between May 2020 and May 2021. Here, we present results from the two last surveys conducted in February and May 2021.

Methods

Persons aged 12 or older were randomly selected from the Danish Population Register and those having received COVID-19 vaccination subsequently excluded. Invitations to have blood drawn in local test centers were sent by mail. Samples were analyzed for whole Immunoglobulin by ELISA. Seroprevalence was estimated by sex, age and geography. Comparisons to vaccination uptake and RT-PCR test results were made.

Results

In February 2021, we found detectable antibodies in 7.2% (95% CI: 6.3–7.9%) of the invited participants (participation rate 25%) and in May 2021 in 8.6% (95% CI: 7.6–9.5%) of the invited (participation rate: 14%). Seroprevalence did not differ by sex, but by age group, generally being higher among the <50 than 50+ year-olds. In May 2021, levels of seroprevalence varied from an estimated 13% (95% CI: 12–15%) in the capital to 5.2% (95% CI: 3.4–7.4%) in rural areas. Combining seroprevalence results with vaccine coverage, estimates of protection against infection in May 2021 varied from 95% among 65+ year-olds down to 10–20% among 12–40 year-olds. In March–May 2021, an estimated 80% of all community SARS-CoV-2 infections were diagnosed by RT-PCR and captured by surveillance.

Conclusion

Seroprevalence estimates doubled during the 2020–21 winter wave of SARS-CoV-2 infections and then stabilized as vaccinations were rolled out. The epidemic affected large cities and younger people the most. Denmark saw comparatively low infections rates, but high test coverage; an estimated four out of five infections were detected by RT-PCR in March–May 2021.

Keywords:

• seroepidemiological studies
• COVID-19 serological testing
• SARS-CoV-2
• population register
• questionnaire
• ELISA

Data Sharing Statement

This work is carried out under the Surveillance auspices of the SSI as regulated in paragraph 222 of the Danish Communicable Disease Act, using personal identifiable information collected with individual consent for the purpose of the present study only. Data cannot be made publicly available for ethical and legal reasons as this would compromise violation of the rights of the participants as defined upon entry into the study.

Acknowledgments

We would like to warmly thank everyone who participated in this study by giving blood. We are very grateful to the staff in the test stations for taking blood samples. We thank all staff in TestCenter Denmark, the involved SSI departments and the expert groups that advised on the design of the study. We thank Oliver McManus for making the maps using R software.

Author Contributions

All authors made a significant contribution to the work reported, whether that is in the conception, study design, execution, acquisition of data, analysis and interpretation, or in all these areas; took part in drafting, revising or critically reviewing the article; gave final approval of the version to be published; have agreed on the journal to which the article has been submitted; and agree to be accountable for all aspects of the work.

Disclosure

The authors have declared that no competing interests exist in this work.

Additional information

Funding

In April 2020, the Danish Parliament called for the SSI to conduct a “representative population survey”.³³ The study was set-up and carried out by the SSI independent of the Parliament/Government which also had no role in the study design which was chosen following advice made by a group of independent, specifically appointed Danish scientific experts in April 2020.³⁴ This work was supported by an ad hoc grant from the Danish Government (§16.11.73 on the National budget 2021). The funders also had no role in data collection, data analysis, interpretation or writing of the report.