The relationship between human mobility and viral transmissibility during the COVID-19 epidemics in Italy

TL;DR

This study analyzes how reductions in human mobility, tracked via mobile data, correlate with COVID-19 transmission rates in Italy, highlighting the importance of mobility data for epidemic monitoring and intervention timing.

Contribution

It provides empirical evidence linking mobility reductions to decreases in COVID-19 transmissibility, emphasizing the utility of mobile data for epidemic control strategies.

Findings

Mobility decrease correlates with the reduction of the reproduction number below 1.

Approximately one week is needed to see the effects of mobility reduction on transmission.

A 2-week lag exists between mobility changes and their impact on the reproduction number.

Abstract

In 2020, countries affected by the COVID-19 pandemic implemented various non-pharmaceutical interventions to contrast the spread of the virus and its impact on their healthcare systems and economies. Using Italian data at different geographic scales, we investigate the relationship between human mobility, which subsumes many facets of the population's response to the changing situation, and the spread of COVID-19. Leveraging mobile phone data from February through September 2020, we find a striking relationship between the decrease in mobility flows and the net reproduction number. We find that the time needed to switch off mobility and bring the net reproduction number below the critical threshold of 1 is about one week. Moreover, we observe a strong relationship between the number of days spent above such threshold before the lockdown-induced drop in mobility flows and the total number of infections per 100k inhabitants. Estimating the statistical effect of mobility flows on the net reproduction number over time, we document a 2-week lag positive association, strong in March and April, and weaker but still significant in June. Our study demonstrates the value of big mobility data to monitor the epidemic and inform control interventions during its unfolding.