The RKI estimates the time from infection to first symptoms to be around 5 days, the time from infection to being infectious around 3 days (which results in two days during which an affected person is infectious, but might not suspect he/she is sick him/herself), and the time from infection to infecting other people around 4 days (generation time). With those assumptions (the latter one being subject to change over time as it can be influenced by political restrictions) the RKI calculates the time dependent case reproduction number R(t). It has to be kept in mind that R(t) is hence not a measure of one single day, but covers a four-day period and can only be calculated in hindsight (2).
There is more than one method to calculate R(t), with only small differences between them, however, the method used by the RKI has an inherent bias, which might be relevant when R(t) raises above 1 after restrictions are lifted (1). Another obstacle in calculating R(t) is the time lag of case reporting by local health authorities. In times of a pandemic, real-time surveillance is warranted, specifically when evaluating the effects on interventions put in place. Hence, reported case counts ideally are adjusted for occurred-but-not-yet-reported events.
Therefore, the RKI uses a methodology called nowcasting (2), which is based on a model originally developed for the outbreak of EHEC in Germany in 2011 (3, 4).