An empirical estimate of the infection fatality rate of COVID-19 from the first Italian outbreak

BackgroundThe coronavirus 2019 (COVID-19) pandemic has been spreading globally for months, yet the infection fatality ratio of the disease is still uncertain. This is partly because of inconsistencies in testing and death reporting standards across countries. We provide estimates which dont rely on official cases and deaths data but only on population level statistics. MethodsWe collected demographic and death records data from the Italian Institute of Statistics. We focus on the area in Italy that experienced the initial outbreak of COVID-19 and estimated a Bayesian model fitting age-stratified mortality data from 2020 and previous years. We also assessed the sensitivity of our results to alternative assumptions on the proportion of population infected. Based on our estimates we finally studied the heterogeneity in overall lethality across countries. FindingsWe estimate an overall infection fatality rate of 1.31% (95% credible interval [CrI] 0.94 - 1.89), as well as large differences by age, with a low infection fatality rate of 0.05% for under 60 year old (CrI 0 - 0.17) and a substantially higher 4.16% (CrI 3.05 - 5.80) for people above 60 years of age. In our sensitivity analysis, we found that even under extreme assumptions, our method delivered useful information. For instance, even if only 10% of the population were infected, the infection fatality rate would not rise above 0.2% for people under 60. Finally, using data on demographics we show large expected heterogeneity in overall IFR across countries. InterpretationOur empirical estimates show a sharp difference in fatality rates between young and old people and rule out overall fatality ratios below 0.5% in populations with more than 30% over 60 years old.