Correlation between daily infections and fatality rate due to Covid-19 in Germany

The daily Covid-19 fatality rate is modelled with a trend line based on nominal day-to-day reproduction rates and a cosine to take account of weekly fluctuations. The fatality trajectory represented by this trend line can be projected from the number of daily infections by assuming a time lapse between symptom onset and death between 17 and 19 days and a nominal time-dependent fatality rate. The time trajectory of this fatality rate suggests a change of the infection dynamics at April 3, with an increase from 2.5% to 6% within 20 days perhaps indicating spread of infection to more vulnerable people. Later in summer, the nominal fatality rate decreases down to 1% in mid-July raising the question whether Covid-19 is intrinsically less lethal in summer. Although the time trajectories of infections and fatality are pronouncedly different, the reproduction rates obtained therefrom are similar indicating that the infection dynamics may reasonably well be deduced from the potentially biased reported infection rate if it is biased consistently, i.e. the same way, over an extended period of time. The administrative measures to contain the pandemic seem not to have an immediate effect on the infection dynamics but well the ease of restrictions. An effect of mask wearing on decreasing lethality cannot be excluded.