Improved response to disasters and outbreaks by tracking population movements with mobile phone network data: a post-earthquake geospatial study in Haiti

Abstract

Background: Population movements following disasters can cause important increases in morbidity and mortality. Without knowledge of the locations of affected people, relief assistance is compromised. No rapid and accurate method exists to track population movements after disasters. We used position data of subscriber identity module (SIM) cards from the largest mobile phone company in Haiti (Digicel) to estimate the magnitude and trends of population movements following the Haiti 2010 earthquake and cholera outbreak.

Methods and findings: Geographic positions of SIM cards were determined by the location of the mobile phone tower through which each SIM card connects when calling. We followed daily positions of SIM cards 42 days before the earthquake and 158 days after. To exclude inactivated SIM cards, we included only the 1.9 million SIM cards that made at least one call both pre-earthquake and during the last month of study. In Port-au-Prince there were 3.2 persons per included SIM card. We used this ratio to extrapolate from the number of moving SIM cards to the number of moving persons. Cholera outbreak analyses covered 8 days and tracked 138,560 SIM cards. An estimated 630,000 persons (197,484 Digicel SIM cards), present in Port-au-Prince on the day of the earthquake, had left 19 days post-earthquake. Estimated net outflow of people (outflow minus inflow) corresponded to 20% of the Port-au-Prince pre-earthquake population. Geographic distribution of population movements from Port-au-Prince corresponded well with results from a large retrospective, population-based UN survey. To demonstrate feasibility of rapid estimates and to identify areas at potentially increased risk of outbreaks, we produced reports on SIM card movements from a cholera outbreak area at its immediate onset and within 12 hours of receiving data.

Conclusions: Results suggest that estimates of population movements during disasters and outbreaks can be delivered rapidly and with potentially high validity in areas with high mobile phone use.

Figure 1. Estimated net changes of the PaP population compared to the population of PaP on the earthquake day (7-d moving average shown in red).

The earthquake caused a sharp net outflow from PaP. The PaP population then gradually grew until the end of the study period. The wavy shape of the curve is caused by net inflows during working days and corresponding net outflows during weekends. Sharp but temporary increases and decreases in the PaP population occurred during major holidays.

Figure 2. Estimated distribution of persons who were in PaP on the day of the earthquake but outside PaP 19 d after the earthquake.

Circles are shown for communes that received at least 500 persons.

Figure 3. Estimates of the proportion of PaP persons who had left PaP by February 17, 2010, divided per province.

Red, mobile phone network data; grey, National Civil Protection Agency (NCPA) estimates used by the Office for the Coordination of Humanitarian Affairs (OCHA) and other relief agencies.

Figure 4. Distribution of PaP persons who left and returned to the city following the earthquake.

UNFPA household survey (beige) and population estimates based on SIMs that spent a minimum of 2 and 7 d outside PaP respectively (red and green).

Figure 5. Average daily numbers of SIMs moving out of the cholera outbreak area.

October 15 to October 23, 2010, divided per communal section of destination. The data were disseminated to relief agencies at the outset of the outbreak (October 24, 2010).