Background: On March 17, , French authorities implemented a nationwide lockdown to respond to the COVID epidemic and curb the surge of patients requiring critical care. Assessing the effect of lockdown on individual displacements is essential to quantify achievable mobility reductions and identify the factors driving the changes in social dynamics that affected viral diffusion.
We aimed to use mobile phone data to study how mobility in France changed before and during lockdown, breaking down our findings by trip distance, user age and residency, and time of day, and analysing regional data and spatial heterogeneities.
Methods: For this population-based study, we used temporally resolved travel flows among administrative areas of mainland France reconstructed from mobile phone trajectories. Data were stratified by age class younger than 18 years, years, and 65 years or older. We distinguished between residents and non-residents and used population data and regional socioeconomic indicators from the French National Statistical Institute. We measured mobility changes before and during lockdown at both local and country scales using a case-crossover framework.
We analysed all trips combined and trips longer than km termed long trips , and separated trips by daytime or night-time, weekdays or weekends, and rush hours. Geographical heterogeneities showed anomalous increases in long-range movements even before lockdown announcement that were tightly localised in space. They were strongly associated with active populations, workers employed in sectors highly affected by lockdown, and number of hospitalisations per region, and moderately associated with the socioeconomic level of the regions.
Interpretation: Lockdown was effective in reducing population mobility across scales. Caution should be taken in the timing of policy announcements and implementation, because anomalous mobility followed policy announcements, which might act as seeding events.
