Corona - error incidence

Error incidence

"The incidence value does not only include new infections"

Photo: Imago / Rüdiger Wölk

Incidence, a medical term until spring 2020, has been on everyone's lips for a year. Daily newspapers publish the “numbers” of the “incidence”, which therefore have a threatening effect on the general public. Because the incidence, as everyone with average talent now knows, is the yardstick for lockdown.

So it is extremely important, because of this number 83 million German citizens are sent to room arrest - or not. Shops are closed, theaters remain closed and so on.

A student does the math

But what actually is the incidence? in the Duden it is defined as: "Number of newly occurring diseases within a group of people of a certain size during a certain period of time." - "Illness", mind you. For a year now, a different definition has apparently been in effect in the media in Germany, with the disease being tacitly replaced by "positive test result".

This makes the whole definition absurd, because while a sick person can be quickly filtered out of a certain amount because he shows symptoms of the disease, those who have tested positive are by no means recognizable as sick. It is therefore forbidden to speak of an incidence in this context.

So the incidence as portrayed by politics and the media is formally incorrect. But even when calculating the incidence value, errors can creep in due to incorrect initial values, as did the math student, among others Patrick Schönherr has calculated: District A and District B each have 50.000 inhabitants and one percent each. District A tests 5.000 residents and finds around 50 infected people. District B tests 2.000 residents and finds around 20 infected people. This would mean that district A would have an incidence of 100, while district B would have an incidence of 40. This even though both districts have one percent infected. “This incidence value does not allow conclusions to be drawn about the pandemic,” concludes the student. You could also say: with the number of quick tests (which can also be faulty) the “number” can be increased or decreased.

Michael Wieden, an expert in corporate health management from Ratingen, explains in one Videothat the incidence says nothing about the overall risk situation in the population. He also considers the wording used by the media when talking about the incidence value to be a problem. "The incidence value does not only include new infections," said Wieden. “It only shows the number of positive PCR tests, even including possible multiple tests per person. Furthermore, "new" implies that the person had not been infected the day before or on one of the previous days. “New” only means that the day was positive and reported, but not necessarily infected. ”Correctly, Wieden argues, the Robert Koch Institute does not have to speak of infections or new infections, but only reports of positive PCR tests .