The infection fatality ratio (IFR) and case fatality ratio (CFR) define the risk of death per infection and per case, respectively. The difference between IFR and CFR depends on the definition of the case. If infection is defined as case, then CFR equals IFR. It is very important to determine the IFR because it influences the control policy and individual risk perception. It would be straightforward to determine the IFR in a closed, small population, such as the Diamond Princess Cruise, where the CFR was 1.3% without age standardization (Russell et al. 2020) and 0.5% after age standardization (Faust and Del Rio 2020).