AWhen Chancellor Angela Merkel explained the expected scenarios for the further spread of Covid-19 in Germany at her most recent press conference, some thought that the explanations of a physicist with a doctorate could be recognized in her remarks: “We are now roughly at a reproductive factor 1. So someone infects about one. If we figure out that every 1.1 people are infected, we will be back at the performance of our health system in October, ”she explained. Then she explained that a reproductive factor of 1.2 corresponds to the situation that two out of five people and four each infect another person. Such a situation would push our health system to its limit in July, with a reproductive factor of 1.3 this would be achieved in June.
The number that Merkel put at the center of her remarks is indeed the central parameter for describing the dynamics of an epidemic. It determines their drama and the consequences for the health system. The number of reproductions is defined as the average number of infections caused by an infected person. Their value can not only change over time, but is also subject to regional variations.