One of the most important questions on the COVID-19 pandemic is ascertaining the correct timing to introduce non-pharmaceutical interventions (NPIs), based mainly on mobility restrictions, to control the rising of the daily incidence in a specific territory. Here, we make a retrospective analysis of the first wave of the epidemic in Spain and provide a set of useful insights to optimize actions in the near future. We have reconstructed the exposure times, from infection to detectability, to

more »

... ectly estimate the reproduction number Rt. This enables us to analyze counterfactual scenarios to understand the impact of earlier or later responses, decoupling containment measures from natural immunity. Our results quantify the differences in the number of fatalities for earlier and later responses to the epidemic in Spain.Teaser"We propose a backward analysis of pandemic incidence in a region to determine the correct timing of authorities' non-pharmaceutical interventions to fight COVID-19"