Nucleic acid (PCR) and antibody (IgG) tests: the course of SARS-CoV-2 infections in the German population unveiled

In early 2020, the World Health Organization (WHO) endorsed (real-time
quantitative, reverse transcription) polymeric chain reaction (PCR)
test assays as the world-wide gold standard to check individuals for
being `infected' by severe acute respiratory syndrome coronavirus 2
(SARS-CoV-2). In Germany, the consortium of authority-accredited
laboratories (ALM) that covered about 90% of all PCR tests, likewise
conducted serological mass tests for IgG antibodies, at least until
late May, 2021. We analysed the time courses of PCR and IgG tests in
terms of their respective weekly positive fractions. We related them
to each other on the grounds of the basic idea that a SARS-CoV-2
infection, as assumed to be indicated by a positive PCR test, should
evidently be followed not later than two weeks after infection by the
detection of IgG antibodies in the blood. We found that the time
course of the IgG-positive fraction as measured by ALM in their labs
can be well reproduced by the cumulative sum of the likewise
ALM-measured PCR-positive fraction, until two weeks before the IgG
value in question, only requiring a proportionality factor of
0.135. The straight-forward conclusion is that only 13.5% of those
who where tested PCR-positive actually got infected by SARS-CoV-2. In
a second analysis, we took a multiply confirmed value from the
literature, namely, the approximative, empirical factor of 10 (in
Germany) between one positive PCR test and the number of actually
infected persons, to also estimate from solely PCR-positive numbers
the fraction of infected in the whole German population. The time
course of this fraction of infected runs always a few percent below
the ALM-measured IgG-positive fraction. Both courses align well at the
end of 2021. At the turn into 2022, the Robert-Koch-Institut (RKI)
reported the IgG-positive fraction to have reached 92%, which is
almost perfectly matched by the corresponding (extrapolated) ALM value
(90%), and close to our conservative estimation (85%) of the
infected fraction. Methodologically, we propose the PCR-positive
fraction to not only constitute an adequate but a superior replacement
for the hitherto used term `incidence', which has commonly been
reported by the RKI as the absolute number of PCR positives normalised
to some (arbitrarily chosen) population size. Data-content-wise and
remarkably enough, no effect whatsoever of the SARS-CoV-2 mass
vaccination campaign (having started on 27/12/2020) can be
distinguished in the time courses of the positive fractions, whether
measured or estimated.