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.
Highlights
Severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) which causes COVID-19, manifests clinically ranging from asymptomatic infection to severe disease, which may lead to death
The degree to which the COVID-19 pandemic is spreading through different countries or regions may instead be assessed through population-based seroprevalence studies, which aim to quantify the proportion of the population that has developed antibodies against SARS-CoV-2
The aim of DSS was to follow the development of the COVID-19 epidemic by estimating the proportion of the population who had been infected with SARS-CoV-2, by age group, geography and sex at different time points, in order to guide preventive and control measures
Summary
Severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) which causes COVID-19, manifests clinically ranging from asymptomatic infection to severe disease, which may lead to death. The degree to which the COVID-19 pandemic is spreading through different countries or regions may instead be assessed through population-based seroprevalence studies, which aim to quantify the proportion of the population that has developed antibodies against SARS-CoV-2. Such studies have been performed in a number of countries [1,2,3,4,5,6,7,8,9]. The Danish National Seroprevalence Survey of SARS-CoV-2 infection (DSS) was initiated in the spring of 2020, following a parliamentary decision in April 2020, which called for a representative population study to be performed. We describe the set-up and results from the DSS in 2020 and relate the results to the national surveillance of RT-PCR diagnosed SARS-CoV-2-cases in Denmark
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