No 7/9 - 2021

The fourth round of the national COVID-19 prevalence study has just been launched

In the course of the next four weeks (9-12), approx. 48,000 citizens will be selected at random for participation in the prevalence study. The citizens will be invited by mail (either via e-Boks or by paper letter), and will be offered to have antibodies against COVID-19 virus (SARS-CoV-2) measured from a blood sample and to answer a questionnaire on any symptoms they may have.

The objective of the prevalence study is to continuously generate data describing how large a share of the population has become infected with COVID-19, with and without symptoms. To the extent this is possible from the data material, the objective also is to describe the prevalence of the infection regionally and in selected population groups, including different age groups.

Practical aspects of the prevalence study

The study is conducted in collaboration with TestCenter Denmark. Previously, in 2020, another three rounds were completed, EPI-NEWS 37/20. Citizens are selected at random from the Civil Registration Register (CPR). You need to be residing in Denmark and be 12 years of age or more to be selected. Thus, the study will, in principle, be representative for the entire Danish population above the age of 12.

In the course of weeks to come, Statens Serum Institut (SSI) will send an invitation letter to the citizens selected. The letter includes an offer to schedule an antibody test at one of TestCenter Denmark’s (community track) test stations.

COVID-19 vaccinated citizens

Citizens who have already been vaccinated will not be invited as they presumably already have antibodies. Citizens for whom the invitation to participate in the prevalence study coincides with initiation of COVID-19 vaccination are recommended not to participate; they may simply ignore the invitation.

Citizens who want to participate may schedule an appointment to have a blood sample taken at a test station. The blood sample is transported to the SSI and tested in the laboratory using an ELISA test (Wantai), which detects total immunoglobin against SARS-CoV-2. According to the SSI’s internal evaluation, the test has a minimum sensitivity of 97% and a specificity of 99.5%, and it is currently considered the best test on the market.

The invitation letter also contains a link to an electronic questionnaire which, among others, asks participants if they have experienced symptoms of airway infection, and if any symptoms persist.

Guidance and interpretation of the test results

The citizen him- or herself checks the results via sundhed.dk. Test results are also sent to the citizen’s GP. Furthermore, at sundhed.dk, it is now possible to check the results for any children you may have below 15 years of age. For children and young people below 18 years of age, the SSI informs by phone if the test results are positive.

In a limited number of cases, the test results are “inconclusive”. This includes errors that may have occurred anywhere in the process chain from sampling, registration of the sample, missing or incomplete samples to laboratory error and actually inconclusive analysis results. The citizen is encouraged to schedule a new appointment for repeat sampling. Up to 48 hours may pass before the option to re-order an antibody test appears on coronaprover.dk.

The GP needs to be aware of the following:

The GP is not expected to contact the citizen when the test result is received - even if the result is positive or inconclusive. If the citizen enquires about the result, the GP may inform that a positive test result indicates that you have previously had an infection that has now passed. The citizen will likely remain immune for a minimum period of some months. Even so, the scientific basis for conclusions on the association between a positive antibody test and immunity is currently weak. Therefore, the citizen still needs to adhere to the common precautions (distancing, hygiene, wearing a mask, etc.) If the test result is “inconclusive”, the citizen should schedule another appointment to have the blood sample repeated (at a test station).

The prevalence study forms part of the epidemic monitoring efforts in place

The political agreement on extension of phase one of the re-opening of Denmark as from April 2020 describes that a representative sample of the population will be tested to facilitate monitoring of the development of COVID-19. On this basis, the Danish Ministry of Health asked the SSI to conduct such studies. In April and May, an expert panel presented an overall design proposal for a population study and the study was then initiated in May 2020. The aim has been to collect representative results on resistance to COVID-19 in the Danish population, which may be used in the ongoing monitoring of the development of epidemic and in the planning of measures to curb the epidemic. The prevalence study is described in more detail here.

Results from the third round of the prevalence study

In connection with the study, 70,000 citizens aged 12 years or more and residing in Denmark were selected at random and invited to participate. Invitations were sent out in the course of a 14-week period: weeks 38-51, 2020. At that point in time, 17 test stations were capable of taking blood samples, and the study comprised citizens from all of Denmark (barring the Faeroe Islands and Greenland).

Main study results:

• The results indicate that the prevalence follows an increasing trend, and that 3.9% (95% confidence interval (CI): 3.3%-4.6%) of the Danish population aged more than 12 years had become infected by SARS-CoV-2 by the beginning of December 2020.
  
  
• Even though the infectious pressure was high in the month of December after the study was conducted and vaccination efforts have now been initiated, the overwhelming majority of the population remains at risk of becoming infected with COVID-19.

• The antibody-prevalence estimate is approx. 1.8 percentage points higher than when the second round of the study was conducted in August 2020.

• The epidemic has not affected all Danish areas equally. The highest prevalence has been observed in the Capital Region of Denmark.

• Since August 2020, it seems that COVID-19 has spread to the 12-19-year-olds where the prevalence of antibodies has increased 4-fold, leaving it on a par with the 20-29-year-olds, which is the age group recording the highest prevalence of antibodies. In people aged 20-29 years, the prevalence has increased by 2.7 percentage points. The fact that many younger persons have become affected by the condition in the autumn is also reflected in the other epidemic monitoring elements.

• The study also shows that slightly less than one third of the citizens who have become infected with SARS-CoV-2 do not recall having had any symptoms of illness in the period since 1 February. This is lower than the corresponding number recorded in the August 2020 study round.

• Compared with the number of people who had tested PCR positive, 2-3 times as many were antibody positive. This figure was lower than in previous study rounds, possibly because the PCR test reactivity was higher in the autumn than was the case previously.

• Participation in the study has declined, and is now at 25%.

Read more about the study and the results (in Danish language) in the note: Results from the 3rd round of antibody testing with 70,000 participants invited, week 38-51, 2020.
If you have any questions about the study, please contact prevalens@ssi.dk by email. Enquiries will be answered as quickly as possible.

(S. Ethelberg, S. Tribler, L. Espenhain, C. Holm Hansen, L. Wulff Krogsgaard, Department of Infectious Disease Epidemiology and Prevention)