Ultimate Severity of Ebola Outbreak Cannot Be Foreseen

Model used to predict eventual size of Ebola outbreak outpaced by scale of current epidemic.

Researchers are no longer able to accurately predict the eventual severity of the current Ebola outbreak in West Africa, due to the unprecedented scale of the epidemic.

Past research conducted by the University of Warwick Mathematics Institute produced a mathematical model that incorporated data from prior Ebola outbreaks to successfully replicate the epidemic’s eventual scale. In the current study published September 16, 2014, in eLife, however, researchers who applied data from the ongoing Ebola outbreak to the model found that the epidemic is expanding on an unprecedented trajectory.

“If we analyze the data from past outbreaks, we are able to design a model that works for the recorded cases of the virus spreading and can successfully replicate their eventual size,” said Thomas House, PhD, of the University of Warwick, in a press release. “The current outbreak does not fit this previous pattern and, as a result, we are not in a position to provide an accurate prediction of the current outbreak.”

One of the biggest factors in the spread of Ebola and other contagious diseases are chance events, including where individuals are located when they are most infectious, the travel patterns of those whom they come into contact with, and whether or not they are situated near medical assistance, the study noted. As a result, the mathematical model was able to accurately replicate the eventual scale of previous outbreaks by examining the initial number of individuals at the start of an outbreak and the level of infectiousness once it began.

“If we look at past Ebola outbreaks, there is an identifiable way of predicting their overall size based on modeling chance events that are known to be important when the numbers of cases of infection are small and the spread is close to being controlled,” Dr. House said. “With the current situation, we are seeing something that defies this previous pattern of outbreak severity. As the current outbreak becomes more severe, it is less and less likely that it is a chance event and more likely that something more fundamental has changed.”

The researchers found that the unprecedented scale of the current epidemic could have resulted from a number of factors, including a mutation of the virus and changes in the social contact patterns of the infected.

“It is implausible to explain the current situation solely through a particularly severe outbreak within the previously observed pattern,” Dr. House said.

The World Health Organization (WHO) has estimated that, as of September 18, 2014, the current Ebola outbreak has killed more than 2600 people, with more than 5300 confirmed and suspected cases.

The epidemic could infect more than 20,000 people before the outbreak is brought under control, according to the WHO. Under a hypothetical worst-case scenario, the current Ebola burden could expand to 277,124 patients by the end of 2014, according to researchers from Arizona State University and the University of Tokyo.

Adding to expanded relief efforts from the United States, the United Nations General Assembly and Security Council last week announced the creation of the Mission for Ebola Emergency Response to contain the outbreak.

“This unprecedented outbreak requires an unprecedented response,” said David Nabarro, the UN Senior Coordinator for the Ebola response, in a press release. “The number of cases have doubled in these countries in the last 3 weeks. To get in front of this, the response must be increased 20-fold from where it is today.”