Population mobility reductions associated with travel restrictions during the Ebola epidemic in Sierra Leone: use of mobile phone data

Abstract

Background: Travel restrictions were implemented on an unprecedented scale in 2015 in Sierra Leone to contain and eliminate Ebola virus disease. However, the impact of epidemic travel restrictions on mobility itself remains difficult to measure with traditional methods. New 'big data' approaches using mobile phone data can provide, in near real-time, the type of information needed to guide and evaluate control measures.

Methods: We analysed anonymous mobile phone call detail records (CDRs) from a leading operator in Sierra Leone between 20 March and 1 July in 2015. We used an anomaly detection algorithm to assess changes in travel during a national 'stay at home' lockdown from 27 to 29 March. To measure the magnitude of these changes and to assess effect modification by region and historical Ebola burden, we performed a time series analysis and a crossover analysis.

Results: Routinely collected mobile phone data revealed a dramatic reduction in human mobility during a 3-day lockdown in Sierra Leone. The number of individuals relocating between chiefdoms decreased by 31% within 15 km, by 46% for 15-30 km and by 76% for distances greater than 30 km. This effect was highly heterogeneous in space, with higher impact in regions with higher Ebola incidence. Travel quickly returned to normal patterns after the restrictions were lifted.

Conclusions: The effects of travel restrictions on mobility can be large, targeted and measurable in near real-time. With appropriate anonymization protocols, mobile phone data should play a central role in guiding and monitoring interventions for epidemic containment.

Figure 1

Temporality of Ebola virus disease, interventions, and available mobile phone data. The timespan of available mobile phone data (green) includes: the late stage of the national epidemic curve (bars); Operation Northern Push (blue); and one of the two national lockdowns (red).

Figure 2

Travel anomalies during the elimination phase of Ebola in Sierra Leone. (A) The daily number of trips between Freetown and Magbema, the largest chiefdom in the northern district of Kambia, reveal significant negative anomalies during the 2015 national lockdown (light grey) and suggest a downward trend during Operation Northern Push (darker grey). (B) The daily number of positive (grey) and negative (black) travel anomalies detected between all chiefdom pairs with an average of at least 10 trips per day.

Figure 3

District heterogeneity in lockdown impact. Reduction in travel by users from each district during the national lockdown ranges from over 70% (dark red) to nearly 30% (light red). Each dot represents 10 cumulative Ebola cases reported in each district between May 2014 and 26 March 2015, before the lockdown. Districts with larger Ebola case counts (italicized numbers adjoining to districts) tended to have larger changes in mobility during the lockdown. Dashed border outlines Magbema chiefdom, discussed in Figure 2a. Thick grey borders outline the districts targeted during Operation Northern Push.