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.