  The infection fatality rate (IFR) of the Coronavirus Disease 2019 (COVID-19)
is one of the most discussed figures in the context of this pandemic. Using
German COVID-19 surveillance data and age-group specific IFR estimates from
multiple international studies, this work investigates time-dependent
variations in effective IFR over the course of the pandemic. Three different
methods for estimating (effective) IFRs are presented: (a) population-averaged
IFRs based on the assumption that the infection risk is independent of age and
time, (b) effective IFRs based on the assumption that the age distribution of
confirmed cases approximately reflects the age distribution of infected
individuals, and (c) effective IFRs accounting for age- and time-dependent dark
figures of infections. Results show that effective IFRs in Germany are
estimated to vary over time, as the age distributions of confirmed cases and
estimated infections are changing during the course of the pandemic. In
particular during the first and second waves of infections in spring and
autumn/winter 2020, there has been a pronounced shift in the age distribution
of confirmed cases towards older age groups, resulting in larger effective IFR
estimates. The temporary increase in effective IFR during the first wave is
estimated to be smaller but still remains when adjusting for age- and
time-dependent dark figures. A comparison of effective IFRs with observed CFRs
indicates that a substantial fraction of the time-dependent variability in
observed mortality can be explained by changes in the age distribution of
infections. Furthermore, a vanishing gap between effective IFRs and observed
CFRs is apparent after the first infection wave, while a moderately increasing
gap can be observed during the second wave. Further research is warranted to
obtain timely age-stratified IFR estimates.
