The Contribution of the Age Distribution of Cases to COVID-19 Case Fatality Across Countries : A Nine-Country Demographic Study.

Background: There is wide variation in coronavirus disease 2019 (COVID-19) case-fatality rates (CFRs) across countries, leading to uncertainty about the true lethality of the disease. A large part of this variation may be due to the ages of individuals who are tested and identified.

Objective: To measure the contribution of distortions from the age distributions of confirmed cases to CFRs within and across populations.

Design: Cross-sectional demographic study using aggregate data on COVID-19 cases and deaths by age.

Setting: Population-based data from China, France, Germany, Italy, the Netherlands, South Korea, Spain, Switzerland, and the United States.

Participants: All individuals with confirmed COVID-19, as reported by each country as of 19 April 2020 ( n  = 1 223 261).

Measurements: Age-specific COVID-19 CFRs and age-specific population shares by country.

Results: The overall observed CFR varies widely, with the highest rates in Italy (9.3%) and the Netherlands (7.4%) and the lowest rates in South Korea (1.6%) and Germany (0.7%). Adjustment for the age distribution of cases explains 66% of the variation across countries, with a resulting age-standardized median CFR of 1.9%. Among a larger sample of 95 countries, the observed variation in COVID-19 CFRs is 13 times larger than what would be expected on the basis of just differences in the age composition of countries.

Limitation: The age-adjusted rates assume that, conditional on age, COVID-19 mortality among diagnosed cases is the same as that among undiagnosed cases and that individuals of all ages are equally susceptible to severe acute respiratory syndrome coronavirus 2 infection.

Conclusion: Selective testing and identification of older cases considerably warps estimates of the lethality of COVID-19 within populations and comparisons across countries. Removing age distortions and focusing on differences in age-adjusted case fatality will be essential for accurately comparing countries' performance in caring for patients with COVID-19 and for monitoring the epidemic over time.

Primary Funding Source: Alexander von Humboldt Foundation.