Prevalence of SARS-CoV-2 antibodies in Denmark: nationwide, population-based seroepidemiological study

Abstract

Seroprevalence studies have proven an important tool to monitor the progression of the coronavirus disease 2019 (COVID-19) pandemic. We present results of consecutive population-based seroprevalence surveys performed in Denmark in 2020. In spring, late summer and autumn/winter of 2020, invitation letters including a questionnaire covering symptoms were sent to representative samples of the population above 12 years and to parents of children below 18 years in the sample. Blood samples were analysed for total Ig and seroprevalence estimates per population segment were calculated and compared to other surveillance parameters. Based on 34 081 participants (participation rate 33%), seroprevalence estimates increased from 1.2% (95%CI: 0.3-1.9%) in May to 4.1% (95%CI: 3.1-4.9%) in December 2020. Seroprevalence estimates were roughly three times higher in those aged 12-29 years compared to 65 + and higher in metropolitan municipalities. By December 2020, 1.5% of the population had tested positive by RT-PCR. Infected individuals in older age groups were hospitalised several fold more often than in younger. Amongst seropositives, loss of taste/smell were the more specific symptoms, 32-56% did not report any symptoms. In more than half of seroconverted families, we did not see evidence of transmission between generations. Seroprevalence increased during 2020; adolescents were primarily infected in the autumn/winter. Denmark has a high per capita test rate; roughly one undiagnosed infection of SARS-CoV-2 were estimated to occur for each diagnosed case. Approximately half were asymptomatically infected. The epidemic appears to have progressed relatively modestly during 2020 in Denmark.

Keywords: COVID-19 serological testing; Population register; Questionnaire; SARS-CoV-2; Seroepidemiological studies.

Fig. 1

Seroprevalence point estimates per survey period (%, grey dots), cumulative RT-PCR-confirmed SARS-CoV-2 cases (%, solid line), weekly number of SARS-CoV-2 RT-PCR tests (per 10 population, dashed line), week of invitation for each survey (+ signs above the panel) and blood sampling (- signs above the panel) by ISO week, Denmark 2020. In addition, the timeline under the figure in schematic form illustrates the strength of the national measures that were in place to reduce transmission in 2020. In short, they comprised a full lockdown (shown using dark shading) involving a close-down of normal societal activity, but without imposing a curfew in March 2020 (week 11). The lockdown was gradually lifted from late April (week 16) into May. Over the summer, only comparatively mild restrictions were in place (regulating travel, gatherings, nightlife and more, shown using light shades). Starting September (week 34) restrictions were reintroduced and with an increasing incidence of infections occurring towards the end of the year. Harder restrictions were introduced in November (week 46) followed by a full lockdown being imposed in December (from week 50)

Fig. 2

Seroprevalence and 95% confidence intervals of SARS-CoV-2 in May, August, October, and December 2020, by four age groups, Denmark