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

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

The population-level case fatality rate (CFR) associated with COVID-19 varies substantially, both across countries and within countries over time. We analyze the contribution of two key determinants of the variation in the observed CFR: the age-structure of diagnosed infection cases and age-specific case-fatality rates. We use data on diagnosed COVID-19 cases and death counts attributable to COVID-19 by age for China, France, Germany, Italy, South Korea, Spain, and the United States. We calculate the CFR for each country at the latest data point and for Italy also over time. We use demographic decomposition to break the difference between CFRs into unique contributions arising from the age-structure of confirmed cases and the age-specific case-fatality. CFRs vary from 0.7% in Germany and 1.6% in South Korea to 8.6% in Spain and 10.6% in Italy. The age-structure of detected cases can explain a substantial proportion of cross-country variation in the CFR. For example, 57% of Spain's difference with respect to South Korea is explained by the observed cases being older. In Italy, the CFR increased from 4.2% to 10.6% between March 9 and March 29, 2020, and more than 95% of the change was due to increasing age-specific case fatality rates. The importance of the age-structure of infected cases likely reflects several factors, including different testing regimes and differences in transmission trajectories; while increasing age-specific case fatality rates indicate the worsening health outcomes of those infected with COVID-19. Our findings lend support to recommendations for data to be disaggregated by age, and potentially other variables, to facilitate a better understanding of population-level differences in CFRs. They also show the need for well designed seroprevalence studies to ascertain the extent to which differences in testing regimes drive differences in the age-structure of detected cases.