An international effort to analyse the entire database of Ebola virus genomes from the 2013-2016 West African epidemic has revealed insights into factors that sped or slowed the rampage and calls for using real-time sequencing and data-sharing to contain future viral disease outbreaks. Published in the journalNature, the analysis found that the epidemic unfolded in small, overlapping outbreaks with surprisingly few infected travellers sparking new outbreaks elsewhere, each case representing a missed opportunity to break the transmission chain and end the epidemic sooner.
The West African Ebola epidemic dwarfed all previous central African outbreaks of the virus, sickening more than 28,000 people and killing more than 11,000 of them.
The 1,610 Ebola virus genomes analysed by the researchers represented more than 5 percent of the known cases, the largest sample analysed for a single human epidemic. The analysis is the first to look at how Ebola spread, proliferated and declined across all three countries most affected: Guinea, Sierra Leone and Liberia. Previous analyses used fewer sequences or focused primarily on either a single country or a limited time frame. The authors’ intention, they wrote, was for this comprehensive analysis to “provide a framework for predicting the behavior of future outbreaks for Ebola virus” and other human pathogens and to guide targeted, life-saving responses.
Cities aided virus’ spread, distance slowed it
The new analysis assessed 25 factors that could have contributed to the spread and duration of the West African epidemic. It confirmed the common perception that cities played a major role in the magnitude of the epidemic compared to central African outbreaks that had occurred in remote, sparsely populated regions.
Distance between cities also played a role, with the shorter the distance, the more likely that infected travelers would arrive and seed an infection. Distance was key to sparing nearby Guinea-Bissau, Senegal, Mali, Cote d’Ivoire and northern Guinea from severe and protracted epidemics. Some of these regions had large cities in which Ebola would likely have exploded had the virus been introduced.
“Essentially, it was entirely down to chance that the outbreak didn’t spread further and cause an even bigger crisis,” Dudas said. Other variables, such as shared languages, economic output and climate were not found to be significantly associated with speeding or slowing the epidemic.
The analysis did see correlations between border closure dates and virus traffic reduction; once the borders were closed, virus movement occurred mostly within countries rather than among them.
But by the time Sierra Leone, Liberia and Guinea closed their borders, cross-border travel had already seeded outbreaks in each country. And although international traffic of viruses was reduced after the closures, it didn’t stop completely. “That was part of the problem in Sierra Leone and Guinea in the final stages of the epidemic, where a particularly mobile chain [of infected people] was moving back and forth between the countries,” Dudas said.