Prevalence of SARS-CoV-2 antibodies in Denmark 2020: results from nationwide, population-based sero-epidemiological surveys

  1. Laura Espenhain
  2. Siri Tribler
  3. Charlotte Sværke Jørgensen
  4. Christian Holm Hansen
  5. Ute Wolff Sönksen
  6. Steen Ethelberg

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Abstract

Seroprevalence studies have proven an important tool to monitor the progression of the coronavirus disease 2019 (COVID-19) epidemic. We present results of consecutive population-based seroprevalence surveys performed in Denmark in 2020.

Methods

Invitation letters including a questionnaire covering symptoms were sent to representatively drawn samples of the population in spring, late summer and autumn/winter of 2020. Blood samples from participants taken at public test-centers were analyzed for total Ig and seroprevalence estimates per population segment calculated and compared to other surveillance parameters.

Results

From 34,081 participating individuals (response rate 33%), we obtained seroprevalence estimates increasing from 1.1% (95%CI: 0.7%–1.7) in May to 4.0 % (95%CI: 3.4%–4.7%) in December 2020. By December 2020, 1.5% of the population 12 years and older had tested positive by PCR. Seroprevalence estimates were roughly 3 times higher in those aged 12-29 compared to 65+ and higher in metropolitan municipalities. Among seropositives, loss of taste/smell were the more specific symptoms, 32%-56% did not report any symptoms. In half of seroconverted families, we did not see evidence of transmission between generations. Infected individuals in older age groups were hospitalized several fold more often than in younger.

Conclusions

Seroprevalence increased during 2020; younger age groups were primarily infected in the autumn/winter surge. Approximately half were asymptomatically infected. Denmark has a high per capita test rate; roughly two undiagnosed infections of COVID-19 were estimated to occur for each diagnosed case. The epidemic appears to have progressed relatively modestly during 2020 in Denmark.

summary

We describe population-based COVID-19 seroprevalence surveys performed in Denmark in 2020. The seroprevalence increased during the year, particularly in adolescents and young adults, but was overall low. Roughly two undiagnosed cases per PCR-confirmed case were detected by December 2020.

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  1. ScreenIT

    SciScore for 10.1101/2021.04.07.21254703: (What is this?)

    Please note, not all rigor criteria are appropriate for all manuscripts.

    Table 1: Rigor

    Institutional Review Board Statementnot detected.
    Randomizationnot detected.
    Blindingnot detected.
    Power Analysisnot detected.
    Sex as a biological variablenot detected.

    Table 2: Resources

    No key resources detected.

    Results from OddPub: We did not detect open data. We also did not detect open code. Researchers are encouraged to share open data when possible (see Nature blog).

    Results from LimitationRecognizer: We detected the following sentences addressing limitations in the study:
    The study had several limitations. When interpreting the findings, the suboptimal participation rates should be taken into account. Participation decreased from 48% in DSS-I, through 39% in DSS-II to 26% in DSS-III, and even fewer replied to the questionnaire concerning symptoms. Even though the drawn sample is representative of the population, participation may not be. We do not know if certain subgroups of the population were underrepresented in the study, but may presume that hard to reach populations would be so. The seroprevalence estimates were stable but slightly underestimated when taking the non-response by age group, sex and region of residence into account. In conclusion, our study provides estimates of SARS-CoV-2 dissemination in Denmark at four time points, based on a representative sample of the population and relate it to the number of PCR-confirmed COVID-19 cases in the national surveillance system. We found that the epidemic had predominantly affected the capital and metropolitan areas and saw indications of a higher seroprevalence in young adults throughout the epidemic, although children 12-17 years old were mainly affected in the second surge of the epidemic. Overall, the estimated seroprevalence in Denmark throughout 2020 was low, compared to other countries. The results seem to support that the measures introduced in Denmark in the spring of 2020 and onwards have been effective in keeping the epidemic from developing rapidly in the community, however als...

    Results from TrialIdentifier: No clinical trial numbers were referenced.

    Results from Barzooka: We did not find any issues relating to the usage of bar graphs.

    Results from JetFighter: We did not find any issues relating to colormaps.

    Results from rtransparent:
    • Thank you for including a conflict of interest statement. Authors are encouraged to include this statement when submitting to a journal.
    • Thank you for including a funding statement. Authors are encouraged to include this statement when submitting to a journal.
    • No protocol registration statement was detected.

    About SciScore

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  2. Version published to 10.1101/2021.04.07.21254703v1 on medRxiv