No 37 - 2020

Reminder letter for all citizens aged 65 years or above about free influenza and pneumococcal vaccination
The National COVID-19 Prevalence Study

Reminder letter for all citizens aged 65 years or above about free influenza and pneumococcal vaccination

As in previous years, we expect that the influenza vaccination will be offered to specific risk groups and citizens aged 65 years or older from 1 October. This year, three different vaccines are used.

Two of the vaccines are the same 4-valent vaccines, Influvac Tetra and Vaxigrip, that were also used in the 2019/2020 season. Additionally, a third influenza vaccine will be used, Fluzone, which is a high-dose 3-valent vaccine. Fluzone is available to a limited extent, and we expect that it will be offered to citizens aged 85 years or older, as this vaccine is assessed as being more effective in the oldest age groups than the two 4-valent vaccines. The final drafting of the executive order is still underway, and so changes may occur.

Fluzone will be available for delivery around week 40/41.In connection with the offer of free influenza vaccination, a reminder letter will be sent to all citizens aged 65 years or older. The letter informs the citizen that from 1 October, he or she can get a free influenza vaccination which may be given by their GP or at vaccination clinics and some pharmacies, etc. Furthermore, the letter informs of the offer for free pneumococcal vaccination (Pneumovax) for all citizens aged 65 years or older. This offer came into force on 15 June 2020, EPI-NEWS 23/20.The objective of the reminder letter is to increase the coverage of this year’s influenza and pneumoccocal vaccinations.

The goal is to vaccinate 75% of the target group. Both vaccination programmes will be described in more detail in an upcoming issue of EPI-NEWS.

(L. Hallundbæk, L.S. Vestergaard, P. Valentiner-Branth, Department of Infectious Disease Epidemiology and Prevention)

The National COVID-19 Prevalence Study

In the National COVID-19 Prevalence Study, randomly chosen citizens drawn from the civil registration register (CPR Register) receive an invitation letter offering them to participate in the study. You participate by filling in a questionnaire and scheduling a sampling visit at the nearest community track test centre (The Testcenter Denmark tents). Participants can schedule a visit for blood sampling for use in antibody testing by ELISA and for throat swabs for virus determination by PCR.

In the following, we describe the main characteristics of the study, primarily to inform citizens who have been invited about the objective and implementation of the study.

Objective and background

The objective of the study is to allow us to monitor the prevalence of the epidemic in the population. The study is designed to ensure that weekly assessment of the results from PCR testing will show whether the number of positive citizens at the regional level is so high that the prevalence will reach a pre-defined threshold - which will, in turn, trigger the initiation of preventive measures.

Through periodic measurements of the level of SARS-CoV-2 antibodies in the population, the study will provide a snapshot - by region, sex and age group - of the number of citizens who have had a COVID-19 infection. Thanks to the questionnaire, this number may be assessed in conjunction with self-reported symptoms thereby contributing knowledge about the share of mild and asymptomatic infections.

The study is conducted by Statens Serum Institut by request at political level and in the context of Testcenter Denmark, i.e. in collaboration with the Danish regions and various agencies and authorities. The political agreement on expansion of phase one of the re-opening described that a representative sample of the population will be tested to facilitate monitoring of the development of COVID-19. The decision to conduct the study was made in April and the study was initiated in May 2020. A draft study design was provided by a group of experts in early May and is currently published on the SSI’s website (in Danish). Subsequently, the question about possible challenges associated with children’s participation was also assessed by experts. The conclusions made by the committees are available here (in Danish).

Practical aspects

Citizens are drawn randomly. Therefore, the citizens drawn will represent the entire population. Unfortunately, it is not possible to include interested citizens who were not drawn randomly. Participation is completely voluntary. The invitation letter is personal and will be sent to the potential participant’s e-Boks as a PDF letter. Citizens who do not have an e-Boks will receive a paper letter. Recent weeks have brought reports of a number of attempts to lure personal information from citizens by sending false e-mails (phishing) or text messages (smishing) apparently sent by SSI or Testcenter Denmark. We take this opportunity to stress that SSI and Testcenter Denmark do not send out offers of participation or test results by text or e-mail. If you receive such a message, it will be fake. In contrast, no examples have been recorded of fake e-Boks letters.

The invitation letters contain detailed information about the study, the fact that it is voluntary, how to participate, schedule a visit and how to interpret the sampling results. The letters are therefore rather long and contain quite a lot of information. We know from experience that the letters may leave some citizens with unanswered questions.

In the initial part of the letters, there is a link to the corresponding translated letters in English and Arabic for citizens who find it difficult to read Danish. Furthermore, the letters contain a personal link to an electronic questionnaire which you are encouraged to answer even if you opt out of testing. The objective of the questionnaire is primarily to enquire about previous disease episodes and symptoms that may have been caused by COVID-19.

The participants themselves schedule a test visit at a community track test station of their choice - i.e. at one of Testcenter Denmark’s test stations. If you do not have a NemID, you schedule a visit by calling a phone number given in the letter, depending on the region in which you want to make the appointment. You only need to schedule an appointment for the antibody test. The PCR test is then scheduled automatically. If you are experiencing symptoms, you should not use the community track, but may instead contact your GP for testing in the healthcare track. In this case, it is not possible to receive the offer of antibody testing; this part will need to wait until these participants are free of symptoms.

If you are younger than 18 years of age, you will always receive the invitation letter as a paper letter (even if you do have an e-Boks). The letter is addressed to the child, but its parents need to schedule the testing appointment. Parents are encouraged to assist their children below 18 years of age to read and complete the questionnaire. Parents who reside at the same address as their children and who accompany their children to the testing will also be offered both tests. Hereby, we hope to learn more about in-house transmission of the infection. Parents do not need to schedule an appointment for their own test, only for the test of their children.

For the test it self, participants arrange their own transportation to the selected test station, bringing their yellow health insurance card. 6 mL of blood is taken from the arm and a throat swab is made with a cotton bud. The sampling material is transported to SSI and tested at the laboratory there. The blood sample is tested for presence of COVID-19-specific antibodies (total Ig) with an ELISA test from the company WANTAI. According to SSI’s own studies, 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 throat swab is tested by PCR for the presence of virus in the same manner as any other PCR test processed in the community track.

Guidance and interpretation of the test results

The invitation letter contains information letting participants know how to find and interpret their test results. The part of the letter informing citizens of how to interpret their test results is shown here  (in Danish).

The citizen him- or herself checks the results via Test results are also communicated to the citizens’ GPs. Furthermore, at, it is now possible to check the results for any children you may have below 15 years of age. Additionally, for children below 18 years of age, SSI calls to inform of any positive antibody results. I.e. if the antibody test of a child is negative, the child does not receive the result directly from SSI; but if the child tests positive (or inconclusive), SSI informs of the result by phone.

In a limited number of cases, the test results are “inconclusive”. This term covers errors that may have occurred anywhere in the process chain from sampling, registration of the sample, missing or incomplete samples to laboratory errors and actually inconclusive analysis results. The citizen in encouraged to schedule a new appointment for repeat sampling.

The GP needs to be aware of the following:

PCR results for current infection are processed in the same manner as other SARS-CoV-2 test results. In case the citizen tests positive, the GP may contact him or her by phone. The Danish Patient Safety Authority is automatically informed of any positive test results and will contact the citizen and inform about preventive measures and contact tracing. Please note that a very limited number of citizens test PCR positive in the context of the prevalence study. If you test positive, the test results will state “confirmed” (in Danish: “påvist”). If you have been tested and have tested positive in the healthcare track, the results may also state “positive” (in Danish: “positiv”).

Antibody results. 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, which has now passed (unless the PCR test is also positive). These people will likely have achieved immunity, at least to some extent. Even so, the scientific basis for conclusions on the association between a positive antibody test and immunity is currently very weak. Out of caution, the citizen should be informed that he or she 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). Up to 48 hours may pass before the option to re-order an antibody test appears on

The previous and planned course of the study

The study was initiated in Week 19 when 2,600 citizens residing close to the five original Testcenter Denmark test stations were invited for antibody testing. As the number of test stations increased, the sample of citizens has been drawn from the entire population and, more recently, also including citizens as young as 12 years of age. Following the initial rounds of antibody testing in May, citizens have continuously been invited for throat swabs, but not for antibody measurements - this has been done by inviting 1,750 citizens for testing weekly. Recently, citizens have been allowed to undergo both tests simultaneously, and you are therefore now called in for both tests in a single invitation. In weeks 34, 35 and 36, a total of 18,000 citizens have received such invitations for testing.

We hope that this will produce updated estimation of how advanced the epidemic is in the Danish population. The intention is to continue inviting citizens for both tests throughout autumn. Currently, plans have been made to invite 4-5,000 citizens weekly in the next two months.


The results of the studies are continuously included in the epidemiologic assessment of the current level of transmission, including the dark figure which is the basis for the initiatives recommended by Danish health authorities. For the prevalence study to be useful, it is important that a sufficiently large amount of data is collected and that the study is representative. For most of the study period, the participation percentage has been lower than expected, and the study has therefore probably not been representative. In the most recent round of invitations comprising 18,000 citizens, it seems that participation may reach 30-40%. This is usable, but a higher participation rate is desirable.


By participating in the study, you assist the health authorities in understanding the scope of transmission - while you achieve an indication of whether you have already had the virus. We therefore take this opportunity to encourage anyone invited to participate in the study and urge doctors to assist when relevant, by informing participants and answering any questions about the study that they may have, thereby ensuring that the highest possible number of people become interested in participating. Questions about the study may be made via the authorities’ joint hotline on phone 7020 0233; this information is also given in the letter. If you do not get an answer on the hotline, you may contact Testcenter Denmark.

(S. Ethelberg, Department of Infectious Disease Epidemiology and Prevention)