Prevalence of antibodies against SARS-CoV-2 in the Norwegian population, August 2021

Abstract

Background: One year into the COVID-19 pandemic, the cumulative number of confirmed COVID-19 cases in Norway was still low. In January 2021, when the Norwegian COVID-19 vaccination campaign started, the national seroprevalence estimate of SARS-CoV-2 antibodies was 3.2%. We have conducted a nationwide cross-sectional study in August 2021 to investigate the overall prevalence of SARS-CoV-2 antibodies in Norway after 8 months of COVID-19 mass vaccination and a third wave of SARS-CoV-2 infection.

Methods: Residual sera were collected from laboratories across Norway in August 2021. In IgG antibodies against the spike protein, the spike receptor binding domain (RBD) and the nucleocapsid protein of SARS-CoV-2 were measured by a bead-based flow cytometric assay.

Results: In total, 1926 residual sera were collected from individuals aged 0-98 years; 55.1% were from women. The overall national estimated seroprevalence from vaccination and/or infection was 62.6% (credible interval [CrI] 60.1%-65.2%) based on having antibodies against both spike and RBD. Estimated seroprevalence increased with age. Among all samples, 11.7% had antibodies against nucleocapsid. For unvaccinated children <12 years, the seroprevalence estimate due to SARS-CoV-2 infection was 12.5% (95% CrI 9.3%-16.1%). Of seropositive samples from the unvaccinated children, 31.9% lacked anti-nucleocapsid antibodies.

Conclusions: The high overall SARS-CoV-2 seroprevalence estimates are in line with Norwegian registry data. Vaccination, not infection, contributed the most to the high seroprevalence in August 2021. Lack of antibodies against nucleocapsid should not automatically be interpreted as absence of previous infection as this could lead to underestimation of COVID-19 cases in seroprevalence studies.

Keywords: COVID-19; SARS-CoV-2; infection; nucleocapsid; seroprevalence; vaccination.

FIGURE 1

Cumulative incidence of confirmed COVID‐19‐cases and COVID‐19 vaccinations in Norway, January–October 2021, and timing of the residual sera collection. Incidence of cumulative infections (in dark blue) from Beredt C19 and frequency of COVID‐19 vaccinations (first dose in green and second dose in blue) from the Norwegian Immunisation Registry (SYSVAK). The collection period for sampling of residual sera (July 19–September 12) in the present study is indicated with vertical lines. The shaded area shows the period when 85% of the samples were collected.

FIGURE 2

Percentage of seropositive study samples and percentage of COVID‐19 vaccinated individuals in Norway by age (years). The dark blue line shows the fraction of residual sera positive for antibodies against SARS‐CoV‐2 with the 95% confidence interval (grey area) (Seropositivity, not seroprevalence corrected for sensitivity and specificity). The green line shows the fraction of the Norwegian population vaccinated with the first dose of COVID‐19 vaccine and the blue line shows the fraction of the population vaccinated with the second dose of COVID‐19 vaccine by July 15, 2021 (data from SYSVAK). For individuals under 45 years, early vaccination was prioritized for the age groups 18–24 and 40–44 years over the age group 25–39 years.