EbolaResponse modeling tool: Estimates for Liberia and Sierra Leone up to 1.4 million cases by January

The Centers for Disease Control and Prevention (CDC) built a modeling tool for called EbolaResponse, which is a spreadsheet that (1) allows user to estimate the number of Ebola Virus Disease (EVD) cases in a community, (2) tracks patients susceptibility to disease through infectivity, incubation, recovery, and death, and (3) calculates the spread of EVD and its impact for 300 days.

The federal health agency explains the EbolaResponse modeling tool in the latest Morbidity and Mortality Weekly Report (MMWR).

Some of the numbers produced by the EbolaResponse tool include:

If trends continue without scale-up of effective interventions, by September 30, 2014, Sierra Leone and Liberia will have a total of approximately 8,000 Ebola cases.

A potential underreporting correction factor of 2.5 also was calculated. Using this correction factor, the model estimates that approximately 21,000 total cases will have occurred in Liberia and Sierra Leone by September 30, 2014.

Extrapolating trends to January 20, 2015, without additional interventions or changes in community behavior (e.g., notable reductions in unsafe burial practices), the model also estimates that Liberia and Sierra Leone will have approximately 550,000 Ebola cases (1.4 million when corrected for underreporting).

The authors do point out five limitations to the findings:

First, extrapolating current trends in increase of cases to forecast all future cases might not be appropriate. Underlying factors such as a spontaneous change in contacts with ill persons or burial practices or substantial changes in movement within countries or across borders could alter future growth patterns. Therefore, limiting model-calculated projections to shorter durations such as 3 months might be more appropriate.

Second, assuming that this epidemic has similar epidemiologic parameters to previous outbreaks (e.g., incubation and infectiousness periods) might not be accurate, although anecdotal evidence to date has not indicated otherwise.

Third, reliance on expert opinion to estimate a correction factor regarding number of beds in use might not account sufficiently for factors such as patients being turned away from full ETUs.

Fourth, the correction factor could change substantially over time. Notable regional differences in underreporting might mean that using one correction factor across an entire country is inappropriate.

Finally, the illustrative scenario does not consider the logistics needed to increase the percentages of patients who are receiving care in an ETU or at home or in a community setting such that there is a reduced risk for disease transmission (including safe burial when needed).

They conclude that if conditions continue without scale-up of interventions, cases will continue to double approximately every 20 days, and the number of cases in West Africa will rapidly reach extraordinary levels. However, the findings also indicate that the epidemic can be controlled.