Monitoring trends and differences in COVID-19 case fatality rates using decomposition methods: Contributions of age structure and age-specific fatality

Christian Dudel; Tim Riffe; Enrique Acosta; Alyson A. van Raalte; Mikko Myrskyla

Italy is the only country for which we have a relatively long time series of data spanning several weeks. Table 3 documents how the Italian CFR evolved from March 9 to March 29. The CFR of March 9 is used as a reference, and the decomposition shows which factor is driving the trend in the CFR. From the beginning to the end of the period under study the CFR more than doubled, from 4.3% to 10.6%. This increase over time is largely driven by worsening fatality of COVID-19the fatality component explaining more than 95% of the rise in all time periodsand changes in the age structure only played a minor role, with detected cases moving to a more favorable (younger) age distribution and slightly counteracting the effect of worsening fatality. As a robustness check we changed the reference period from March 9 to March 19. This resulted in the fatality component explaining 88% of the increase in CFR.