Emilie Koum Besson¹*, Andy Norris², Abdulla S. Bin Ghouth³, Terri Freemantle², Mervat Alhaffar¹, Yolanda Vazquez², Chris Reeve², Patrick J. Curran⁴, Francesco Checchi¹

1. Department of Infectious Disease Epidemiology, Faculty of Epidemiology and Population Health, Keppel St, London School of Hygiene and Tropical Medicine, London, United Kingdom
2. Satellite Applications Catapult, Electron Building, Didcot, United Kingdom
3. Department of Community Medicine, Hadhramout University, Hadhramout, Yemen
4. Department of Psychology, University of North Carolina, Chapel Hill, NC, United States of America

* Corresponding author: emilie.koum-besson@lshtm.ac.uk

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Abstract

Background
The burden of COVID-19 in low-income and conflict-affected countries is still unclear, largely reflecting low testing rates. In parts of Yemen, reports indicated a peak in hospital admissions and burials during May-June 2020. To estimate excess mortality during the epidemic period, we quantified activity across all identifiable cemeteries within Aden governorate in Yemen (population approximately one million) by analysing very high-resolution satellite imagery, and compared estimates to Civil Registry office records from the city.

Methods
After identifying active cemeteries through remote and ground information, we applied geospatial analysis techniques to manually identify new grave plots and measure changes in burial surface area over a period from July 2016 to September 2020. After imputing missing grave counts using surface area data, we used alternative approaches, including simple interpolation and a generalised additive mixed growth model, to predict both actual and counterfactual (no epidemic) burial rates by cemetery and across the governorate during the most likely period of COVID-19 excess mortality (from 1 April 2020), and thereby compute excess burials. We also analysed death notifications to the Civil Registry office during April-July 2020 and in previous years.

Results
We collected 78 observations from 11 cemeteries, of which 10 required imputation from burial surface area. Cemeteries ranged in starting size from 0 to 6866 graves. In all but one a peak in daily burial rates was evident from April to July 2020. Interpolation and mixed model methods estimated ≈ 1500 excess burials up to 6 July, and 2120 up to 19 September, corresponding to a peak weekly increase of 230% from the counterfactual. Satellite imagery estimates were generally lower than Civil Registry data, which indicated a peak 1823 deaths in May alone. However, both sources suggested the epidemic had waned by September 2020.

Discussion
To our knowledge this is the first instance of satellite imagery being used for population mortality estimation. Findings suggest a substantial, under-ascertained impact of COVID-19 in this urban Yemeni governorate, and are broadly in line with previous mathematical modelling predictions, though our method cannot distinguish direct from indirect virus deaths. Satellite imagery burial analysis appears a promising novel approach for monitoring epidemics and other crisis impacts, particularly where ground data are difficult to collect.

Read full pre-print here (not yet peer-reviewed)