• Nikkil Sudharsanan, Oliver Didzun, Till Bärnighausen, and Pascal Geldsetzer.
• Heidelberg Institute of Global Health, Heidelberg University, Heidelberg, Germany (N.S., T.B.).
• Ann. Intern. Med. 2020 Nov 3; 173 (9): 714720714-720.

BackgroundThere 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.ObjectiveTo measure the contribution of distortions from the age distributions of confirmed cases to CFRs within and across populations.DesignCross-sectional demographic study using aggregate data on COVID-19 cases and deaths by age.SettingPopulation-based data from China, France, Germany, Italy, the Netherlands, South Korea, Spain, Switzerland, and the United States.ParticipantsAll individuals with confirmed COVID-19, as reported by each country as of 19 April 2020 (n = 1 223 261).MeasurementsAge-specific COVID-19 CFRs and age-specific population shares by country.ResultsThe 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.LimitationThe 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.ConclusionSelective 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 SourceAlexander von Humboldt Foundation.

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