Cumulative COVID-19 incidence indicators should account for the population-at-risk dynamics

Risk estimators for COVID-19 propagation based on the incidence rate of new cases can be misleading as they usually fail to account for the fraction of population immunized by infection or vaccination.This misconception yields different incidence rates, as we illustrate using the daily number of COVID-19 reported cases in Spain during the pre-vaccine period, between 15/01/2020 and 11/07/2021. An increase in the incidence rate of about 7% is found when properly accounting for the population at risk. Our results demonstrate that accounting for dynamic changes to the immunized fraction of the population is necessary for accurate risk estimation. We hope that our findings can lead to more effective strategies for pandemic response.