Both international (WHO; (1) and national are calling for increased testing for the SARS-CoV-2 virus; at present it is completely unclear how many people will actually become infected with the Covid-19 virus or have formed antibodies against it during the pandemic. The greatest uncertainty of all calculation models on hospitalisation rates, morbidity and mortality of the pandemic is the unknown number of unreported cases of citizens infected or having been infected with Covid-19.

Currently the direct detection of the SARS-CoV-2 virus from the mouth/nose/throat swab using PCR is established worldwide with high sensitivity and specificity. This direct method is expensive and not always available in sufficient quantities. Furthermore, the results are strongly influenced by the quality of the smear collection.

The detection of antibodies against the SARS-CoV-2 virus is an alternative method for determining whether a person has been infected with SARS-CoV-2 and has developed an immune response to the virus. Recently several companies have been offering such kits for sale. According to national experts, these kits have not yet been sufficiently clinically validated, which is why valid data on their sensitivity and specificity are not yet available. However, on the validation of these tests are being worked hard all over the world, so that it can be assumed (the experts also agree on this) that data on the sensitivity and specificity of the antibody tests will be available in the foreseeable future.

Probably the only reasonable and feasible method of testing the actual rate of Covid-19 infection in a given population is to detect infection (either directly or indirectly by means of antibody testing) from a sample that is as representative as possible for this population.

The selected subjects of the LEAD study, the core project of the Ludwig Boltzmann Institute for Lung Health at Otto Wagner Hospital (the hospital association is an official partner of the institute), represent an exemplary representative sample of the general population in Vienna (i.e. the sample corresponds to the Viennese population in terms of age, gender, smoking habits, place of residence, etc.). In the LEAD study, more than 11,000 subjects (aged 6-80) mainly from Vienna were subjected to a "Health examination survey" (2).

Due to the conditions described above, blood should be drawn from the subjects of the LEAD study at two points in time (as early as possible and after two to three months) and blood sera should be frozen to determine antibodies against SARS-CoV-2 and tested with a test of high sensitivity and specificity. According to the WHO this procedure (longitudinal cohort study with serial sampling of the same individuals (in the same geographic area) each time) is recommended in that provides the most comprehensive information on extent of infection.

By testing for SARS-CoV-2 antibodies twice in the subjects of the LEAD study (random sample), an accurate and representative estimate of the actual positive Covid-19 cases in the Viennese population and the rate of spread during the pandemic could be extrapolated for Vienna in the foreseeable future. Since more than 1,500 individual parameters were measured in the participants of the LEAD study, conclusions on socioeconomics, lifestyle factors and possible risk profiles (e.g. co-morbidity, smoking behaviour, etc.) can be drawn from those who tested positive (but also negative). In addition, a query regarding symptoms should be carried out at both dates in order to obtain information on the percentage of asymptomatic infections with SARS-CoV-2.