R-value for Corona: That says the number of reproductions!

R-value for Corona: That says the number of reproductions!

The so-called reproduction number, also known as the R-value, continues to play an essential role in assessing the pandemic situation and the incidence and hospitalization rate. But what is the definition of the number of reproductions, how does the calculation work and what does its development say about the current coronavirus pandemic? Everything necessary on the subject can be found in this article.

Definition: What is the reproduction number?

According to the Robert Koch Institute (RKI) definition, the reproduction number (R) describes how many people are infected on average by a person infected with a viral disease. If the number of reproductions of the SARS-CoV-2 coronavirus is 1, an infected person infects another person on average. The Robert Koch Institute publishes an average estimate of the number of reproductions daily in Germany.

When defining the R-value, a distinction is made between the primary and adequate reproduction numbers. These are defined as follows:

  1. Primary reproduction number (R0): It indicates how many people an infected person infects on average at the beginning of an epidemic or pandemic when no one has developed immunity or no measures have been taken to contain the virus – i.e. under ideal conditions for the virus. According to the RKI, the primary reproduction number for COVID-19 is 2.8 to 3.8. 1 The primary reproduction number of the delta variant is significantly higher: According to previous research results, it is between 6 and 7. For comparison, this value is between 0.9 and 2.1 for influenza and measles between 15 and 18 – that’s how many people a single person infected with measles would infect if there were no vaccine. 2
  2. Adequate reproduction number (R):  The effective reproduction number indicates how many people, on average, are infected by an infected person after measures have been taken to contain a virus, as is currently the case. Their value fluctuates – depending on the measures taken to contain the virus and how well they work.

To curb the spread of SARS-CoV-2, the adequate number of reproductions must be permanently below 1. This means that not everyone infected with the coronavirus infects another person. The number of new infections is thus permanently decreasing.


Calculation of the R-value

The adequate number of reproductions for the coronavirus is calculated by the RKI using statistical methods. The basis for this is the data on confirmed infections passed on to the Institute. The numbers of infected people reported within the last four days are compared for the calculation. The RKI lists corresponding calculation examples on its website.

You have to know that current reproduction numbers are always estimates. This estimate is known as the nowcast. The need to estimate is due to the delay when new infections are transmitted to the RKI. In some cases, when calculating the current number of reproductions, the Institute only has cases transmitted remarkably quickly. There can also be delays in the diagnosis of COVID-19, so people who are already infected are only reported to the RKI belatedly.

For this reason, the Robert Koch Institute has not published an R-value based on the last four days since July 2021. Instead, only the so-called “7-Day-R” is now published.

This is estimated using a 7-day mean value of the nowcasting curve, meaning fluctuations can be more evenly balanced. In the 7-day R, the 7-day mean value of the new cases of one day is compared with the same value four days before. The infections for these reported new cases are about four to six days before, which is why the 7-day R shows the infection process with a delay of one to two weeks.

In addition, a joint research group from the Helmholtz Institute and the Ludwig Maximilian University in Munich calculates the current number of reproductions for Germany daily. She calculates R by modelling typical disease progressions. So, while the RKI is concentrating on reports of infected people, the research group is trying to reproduce possible courses of the disease and the associated new infections.

What is the significance of the R-value in relation to the coronavirus?

In the current coronavirus pandemic, the number of reproductions is of central importance because it serves as a unit of measure for developing infection rates and, thus, for the success of protective measures. However, one should know that the current reproduction figures are based on estimates.

On the one hand, this is due to the previously mentioned delay in diagnosing and reporting infections. On the other hand, it is also because people without symptoms can also be infected with the coronavirus and thus be potential carriers of SARS-CoV-2 without being diagnosed and thus reported. The number can, therefore, never reflect the exact status but only enable the best possible assessment.

The number of corona tests carried out also influences the number of reproductions. Because the more tests are carried out, the more infections are detected, which also increases the number of reproductions. However, the number of unreported cases of infected people is falling, so the information is becoming more accurate.

The number of reproductions is nevertheless an essential factor in assessing the development of the coronavirus pandemic, but should always be viewed with the limitations mentioned above.


Hospitalization and incidence: other indicators of the pandemic

Notably, there are more criteria for dealing with the coronavirus than the number of reproductions. Other key factors are the number of hospital admissions due to COVID-19 and the number of new infections daily.

The first value is also referred to as the hospitalization incidence. To calculate this, the total number of cases reported to the RKI within the last seven days is added and converted to 100,000 people. However, there may be delays due to the long reporting times in the hospitals.

The same calculation method is used as with the so-called seven-day incidence. The number of new infections reported in the last seven days from all over Germany is added and converted to 100,000 inhabitants.

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