What is R_t?

The R_t, also known as the reproduction rate, describes how many people an infected person ends up infecting. Together with the number of new infections, the reproduction rate R_t is an important figure to follow to understand how fast the disease spreads.
We can distinguish three different scenarios:

• R_t>1: the number of new infections increases
• R_t=1: the number of new infections remains the same
• R_t<1: the number of new infections decreases

For more information, you can take a look at this page to begin with.

Where do the data come from?

• Belgium: The daily data comes from the EPISTAT COVID-19 dataset , of "Siensano", the national public health institute of Belgium. The Belgian data loader was contributed by Lode Nachtergaele.

• Germany: We collect positive tests numbers thanks to the official dashboard of the RKI, which is based on the underlying ArcGIS system.
  We also collect daily tests results by region. They are kindly provided to us by the RKI. A summary of these figures is also published by the RKI in its weekly report.
  The number of tests carried out is difficult to analyze in the current reporting system. Thus, we currently use the data compiled by the ARS team of the Robert Koch Institute. Their figures are based on figures provided voluntarily by test laboratories. Today, for small federal states in particular, the data may therefore not be representative of the real number of tests carried out. The German data loader was contributed by Laura Helleckes and Michael Osthege.

• France: The daily data come from the French government open-data repository. Note that data by regions only contain tests for which residence regions of tested people could be known. Hence, countrywide data contain more tests than the sum of all regions. Moreover, data transmission can sometimes excess 9 days. Indicators are updated daily on test results reception. The French data loader was contributed by Alexandre Andorra.

• Italy: The data is sourced from the data repository (CC-BY-4.0). of the Dipartimento della Protezione Civile (the department of civil protection). The Italian data loader was contributed by Davide Ferrero.

• United States: All the data come from covidtracking.org. The US data loader originates from the original project.

• All other countries: Data come from ourworldindata.org.

Visit github.com/rtcovidlive/rtlive-global for all the data sources and a detailed explanation of how the model is programmed.

What do the superscript and subscript numbers mean?

The large number describes the median value, while the small numbers indicate the range in which our analysis suspects the true value is, with a 90% probability. X = 0.94 _0.78 ^1.11 can therefore be read as "Our analysis has shown that there is a 90% probability that X lies between 0.78 and 1.11.".

How are the colors of the risk indicators chosen?

For the reproduction number, we choose the color based on the probability that R > 1.

Color	Probability range
green	0-25 %
gray	25-50 %
orange	50-75 %
red	75-100 %

What are the weaknesses of the method?

The model requires the number of daily performed COVID-19 tests, which are only available within a delay for all countries. We therefore use Facebook's Prophet library to predict the number of tests that will be performed.
Our forecast is shown in the detailed views, but, in a nutshell, the older the latest test numbers are, the less reliable this forecast becomes.
No model can exactly describe reality and the more we learn about the pandemic, the more "flaws" in the model we get to discover. The model also makes assumptions that are informed by literature sources, but do not necessarily apply exactly to every countries we model here. In addition, these literature sources are only snapshots that apply to a greater or lesser extent in the course of the pandemic.

Importantly, two probability distributions are key to the model:

• The time lag between an infection and an observed positive test.
• The time lag between the infection of a person A and the infection of another person B by this person A.

Who is behind this page?

We, Laura Helleckes and Michael Osthege, are doctoral students at the Institute for Biotechnology (IBG-1) at the Jülich Research Center. We have adapted the original open-source model for cross-country compatibility. But we're not alone here! As you'll see at github.com/rtcovidlive/rtlive-global, there are many other people, who made a contribution to this project. If you have any questions or comments, please contact us by email.