Santamaría, L. & Hortal, J. (2021) COVID-19 effective reproduction number dropped during Spain’s nationwide dropdown, then spiked at lower-incidence regions. Science of the Total Environment, 751, 142257. doi:10.1016/j.scitotenv.2020.142257


• Public health and movement restriction measures can limit the spread of infectious diseases in highly-populated regions.
• We associate governmental containment measures with the dynamics of COVID-19 transmission in Spain.
• The national lockdown decreased Rt in all regions, but the strengthened lockdown was inneffective in regions.
• Increases in transmission are associated with higher mobility within and between regions.
• Enhanced contact tracing is needed to prevent disease transmission without the need to recourse to new lockdowns.

Abstract COVID-19 pandemic has rapidly spread worldwide. Spain has suffered one of the largest nationwide bursts, particularly in the highly populated areas of Madrid and Barcelona (two of the five largest conurbations in Europe). We used segmented regression analyses to identify shifts in the evolution of the effective reproduction number (Rt) reported for 16 Spanish administrative regions. We associate these breaking points with a timeline of key containment measures taken by national and regional governments, applying time lags for the time from contagion to case detection, with their associated errors. Results show an early decrease of Rt that preceded the nationwide lockdown; a generalized, sharp decrease in Rt associated with such lockdown; a low impact of the strengthened lockdown, with a flattening of Rt evolution in high-incidence regions, and even increases in Rt at low-incidence regions; and an increase in Rt associated to the relaxation of the lockdown measures in ten regions. These results evidence the importance of generalized lockdown measures to contain COVID-19 spread, and the limited effect of the subsequent application of a stricter lockdown (restrictions to all non-essential economic activities). Most importantly, they highlight the importance of maintaining strong social distancing measures and strengthening public health control during lockdown de-escalation.