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The entire corona measures of the federal, state and local governments stand and fall with the incidence value. If it is below a threshold value, it can be relaxed, if it exceeds another threshold value, the "emergency brake" must be pulled. It is no longer questioned what this incidence value actually says. That is negligent and foolish, since this size is arbitrary and not suitable to justify such serious interventions in society. Of Jens Berger

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When Corona was a new topic a year ago, politics and epidemiology justified the measures by saying that the health system had to be saved from the impending collapse. "Flatten the curve" was the political mantra at the time. The horror scenarios of the political advisor Drosten (“More than one million additional intensive care patients”) and Meyer-Herrmann (“Hundreds of thousands of intensive care patients within a few months”) did not materialize, however. To date, the health system has never come close to collapsing.

Politics switched. From then on, the motto was "The hammer and the dance" - a model that, by the way, goes back to an IT blogger. There was a lot of hammering and little dancing and after the capacities of the health system were no longer an issue, new "indicators" were needed. The so-called R-value was born, which says nothing other than the growth in the number of infected people - measured by the positive PCR tests carried out. By the summer at the latest, however, this indicator was also unusable, since when the infection rate is low, local clusters such as the one in the Tönnies meat factory can literally explode the R value. And why should a school in Garmisch be closed when the employees of a meat factory in Gütersloh become infected?

It is difficult to say today when exactly the term “incidence value” entered the political debate. The first hit in the SPIEGEL archive is a contribution from September 17, which at the time was about the question of whether the Bundesliga could play its games in front of spectators. The measures of the federal and state governments were justified with this value for the first time in November. And as we know today, this was the beginning of the current permanent lockdown, which should from now on be linked to this same value.

But what actually is the “incidence value”? The explanation is actually very simple. This value describes how many people in relation to the total population tested positive for the Sars-Cov2 virus using the PCR method in a certain period of time. Not more but also not less.

The incidence value does not provide any information about this ...

  • How many people are sick
  • How many people from the risk group have become infected
  • Whether more or fewer people have become infected compared to another period

The incidence value is not suitable to ...

  • To make statements about the infection process
  • To make statements about the occurrence of the illness
  • To spatially compare the infection or disease process
  • To compare the infection or disease process over time

These statements will come as a surprise to many readers and therefore require some explanation.

In contrast to the positive rate of the PCR tests, for example, the pure incidence value says nothing about how the relative number of infections develops. The main problem with the incidence value is that one important variable does not play a role in the overall determination: that of the number of tests. Here is a small sample calculation.

10,000 people each live in the cities of Villarriba and Villabajo, 10 of whom are infected with the Sars Cov2 virus in the period under consideration and would have a positive result in a PCR test. The mayor of Villarriba decides to test 50% of the residents with a test offensive. His counterpart in Villabajo doesn't think much of mass tests and only has 10% of the population tested. According to Adam Riese, five residents in Villarriba test positive according to the probability calculation. The village has five positive cases per 10,000 residents, which - if the period under consideration is one week - corresponds to an incidence of 50. If Villarriba were in Germany, the residents would have to be prepared for a hard lockdown.

And what about next door in Villabajo? Since there are fewer tests there, you only get a positive test - as a statistical mean - which would correspond to an incidence of 10. While Villarriba has to shut down public life, Villabajo is already celebrating again. And that although both villages in our example have exactly the same infection process!

This example already shows that the incidence value is completely unsuitable as a basis for assessment for lockdown measures. Let me point it out: if a mayor or district administrator wants to reduce the incidence value, there is a very simple instrument for doing this: test less. Because the incidence value correlates with the number of infected people if the number of tests is constant. However, it also correlates with the number of tests if the number of infected people is constant.

In the past few weeks the incidence value has become even more unusable than it already is. The reason for this are the rapid tests that are now widely used. Unlike the PCR tests, these are not included in the calculation of the incidence value. That wouldn't be tragic. However, it is problematic that the vast majority of positive rapid tests are then indirectly included in the calculation. The reason for this is that people who have a positive rapid test are asked to confirm this positive result with a PCR test. In statistics, in this case one would say that the sample is changing. Although the rapid tests are anything but reliable, one can of course assume that most of the rapid test test positives are also PCR positive. All of this is praiseworthy when it comes to recognizing as many infected people as possible so that, voluntarily or by force, they shut down their contacts and infect fewer people. However, since this procedure increases the positive rate and thus the accuracy of the tests, the incidence naturally also increases. Here, too, the following applies: the more and the more precisely we test, the higher the incidence - regardless of the occurrence of the infection.

And that seems to be the case right now. Primarily through the spread of rapid tests in kindergartens and schools, a group is moving into the sights of the tests that hardly appeared there before - children and adolescents. In these age groups, the infection is symptom-free or very mild in the vast majority of cases, has not yet been recognized at all and so could not be included in the official figures. When we talk about the unreported number of infected people, it is to a large extent about children and young people. Now, however, the quick tests are used to make a preselection. Anyone who tested positive with a rapid test in kindergarten or school is retested using a PCR test, and if this test is also positive - which is usually the case - this is also included in the incidence value and drives it up further. From a purely formal point of view, this procedure is correct; however, the significance of the lockdown question remains in the dark.

Hence, increasing incidence is neither bad nor good per se. It can indicate an increased incidence of infections. But it can also indicate that more and better - that is, more targeted - is being tested. As an indicator for the question of whether the lockdown will be ended, maintained or even tightened, however, it is not at all useful. Because why should the population be punished for testing more precisely?

When it comes to the harsh consequences of the lockdown policy, the massive collateral damage and the serious interference with the basic rights of the population, one cannot work with such vague, unreliable and meaningless values. The only variables that would come into question here - with a great deal of concession to the lockdown group - are the number of deaths and the capacities of the health system. And it is precisely these two variables that currently do not provide any justification for maintaining or even tightening the lockdown measures.

The number of deaths - despite the planning disaster in the vaccination campaign - has fallen noticeably in the last few weeks ...

... and the number of occupied intensive care beds has been constant for some time.

Extraordinary threats require extraordinary measures. But the current hard data does not allow the conclusion that the threat from Corona is so extraordinary that it could justify such severe and devastating interventions in our society and our lives.

Cover picture: Lucky Business / shutterstock.com