The US Centers for Disease Control and Prevention just published a mathematical model (found here in Morbidity and Mortality Weekly Report) estimating that—worst-case scenario—Ebola cases in Sierra Leone and Liberia could reach 500,000 (1.4 million if you account for unreported cases) by January if a substantial scale-up of available interventions isn’t implemented.
In a separate recent report (found here in The New England Journal of Medicine) the World Health Organization (WHO) Ebola Response Team suggests that confirmed or probable cases could hit 20,000 by November in Sierra Leone, Guinea and Liberia—the three countries hardest hit by the Ebola outbreak. “For the medium term, at least, we must therefore face the possibility that EVD will become endemic among the human population of West Africa, a prospect that has never previously been contemplated,” the WHO team notes.
The CDC panel constructed a mathematical model that tracked patients in Sierra Leone and Liberia through different states of Ebola-related infection and disease and then used the data to calculate the future risk of transmission.
- Patients were categorized based on whether they were treated in a lower-risk setting—a hospital, clinic, home or community with proper quarantine protocols and safe burial procedures—or homes with no effective isolation. While the US government and international organizations have committed to supporting a plan to rapidly increase the number of Ebola medical facilities and the scale up of methods to rapidly isolate sick patients, the CDC model offers a sobering look at what’s in store if these plans go awry.
- If trends continue without additional interventions, the CDC model estimates that Liberia and Sierra Leone will have approximately 8,000 total Ebola cases (21,000 total cases when corrected for underreporting) by September 30. By Jan. 20, without additional interventions or changes in community behavior, such as notable reductions in unsafe burial practices, the model estimates that cases in those two countries will reach 550,000 (1.4 million when corrected for underreporting.)
- On a more positive note, if, by late December, approximately 70% of patients are in an Ebola treatment unit, home or community setting where patients are isolated and safe burial practices enforced then the epidemic in both countries would be nearly over by the end of January.
- The CDC notes that its findings are subject to number of limitations, however, such as any spontaneous changes in behavior or geographic patterns that could alter the forecasts.
The WHO team reported on clinical and epidemiologic characteristics of the epidemic in Guinea, Liberia, Nigeria and Sierra Leone over the past nine months, and offered their own forecasts for the coming weeks based upon the level of intervention.
- The porous borders of Guinea, Liberia, and Sierra Leone, where there is much cross-border traffic and relatively easy connections by road between rural towns and villages and more densely populated capitals, may have accelerated the spread of Ebola, the WHO team suggests. In contrast, the number of cases in the much more populated country of Nigeria has been far smaller, perhaps due to the speed in which rigorous control measures were implemented, the report said.
- In previous Ebola outbreaks, transmission can be interrupted, and case incidence reduced within two to three weeks after the introduction of control measures. However, the current epidemiologic outlook is bleak, the report notes. “Unless control measures — including improvements in contact tracing, adequate case isolation, increased capacity for clinical management, safe burials, greater community engagement, and support from international partners — improve quickly, these three countries will soon be reporting thousands of cases and deaths each week.