Questions and Answers: Estimating the Future Number of Cases in the Ebola Epidemic—Liberia and Sierra Leone, 2014–2015

Summary

The September 26, 2014, Morbidity and Mortality Weekly Report (MMWR), Estimating the Future Number of Cases in the Ebola Epidemic—Liberia and Sierra Leone, 2014–2015, estimates the future number of cases if current trends continue. The MMWR also adjusts the number of cases based on estimated underreported cases.

• By September 30, 2014, CDC estimates that there will be approximately 8,000 cases, or as high as 21,000 cases if corrections for underreporting are made.
  • Without additional interventions or changes in community behavior, CDC estimates that by January 20, 2015, there will be a total of approximately 550,000 Ebola cases in Liberia and Sierra Leone or 1.4 million if corrections for underreporting are made.
• Cases in Liberia are currently doubling every 15-20 days, and those in Sierra Leone and Guinea are doubling every 30-40 days.
• Halting the epidemic requires that approximately 70% of Ebola cases be cared for in Ebola Treatment Units or, if they are at capacity, at home or in a community setting in which there is a reduced risk of disease transmission and safe burials are provided.

Key Messages

• If conditions remain unchanged, the situation will rapidly become much worse.
• We know how to control and eventually stop the epidemic. Halting the epidemic requires placing up to 70% of patients into either an Ebola Treatment Unit or in a community setting in which the risk of disease transmission is reduced and safe burials are provided.
• The cost of delay will be devastating. The number of cases is doubling about every 20 days. Every month of delay in reaching the 70% target will increase the number of patients, which means more cases and more deaths and the need for even more beds and other resources.

Questions and Answers

What is the basis of these estimates?

In producing the estimates, CDC researchers assumed that current conditions of disease transmission would continue without changes. The main driver of these estimates is that, in Sierra Leone and Liberia combined, reported cases are doubling approximately every 20 days.

What does 'cases were corrected for underreporting by a factor of 2.5' mean?

For every case reported and recorded in publicly available case counts, an additional 1.5 cases are not recorded. In order to get an estimate of actual cases, we multiplied the reported number of cases by 2.5.

How was the factor of 2.5 calculated?

The factor of 2.5 was calculated by taking estimates of beds-in-use, computed by using CDC's EbolaResponse modeling tool, and comparing these estimates to an expert opinion of actual beds-in-use (for a given day: August 28, 2014). The difference between the estimated beds-in-use and the estimated actual reported beds-in-use is a potential underreporting correction factor (1.5 more beds were being used than what the model estimates).

How accurate are the estimates (using uncorrected or corrected data)?

They are only estimates, and CDC cannot guarantee their accuracy; however, the estimates illustrate that if conditions remain unchanged, the situation will rapidly become much worse. As such, the estimates are a warning and a call to action.

What must happen to ensure that these estimates do not occur?

Effective interventions need to occur quickly, including appropriate disease control methods, communication, changes in community behavior, and adequate resources (such as staff, beds, equipment, supplies). The EbolaResponse tool also estimates the number of beds that would be required in hospitals/Ebola Treatment Units. Details and resources required for enhanced protection to minimize disease transmission is not the purpose of this model.

What aspects are contributing to the spread of Ebola? Why are cases doubling approximately every 20 days?

We do not have appropriate data to say specifically why this is occurring.

We know, from earlier Ebola outbreaks, what factors contribute to the spread of Ebola and we use that information to fit the available data to our model. What we do know is that the cases are increasing rapidly, and more needs to be done quickly.

Does the model suggest that there is something different about this virus compared to other Ebola outbreaks?

We used information from past outbreaks to inform our assumptions about the period of infectivity and the time between exposure and illness, and we were able to “fit” the model to the data to date. The model does not provide any evidence that the virus is transmitted differently than during past outbreaks.

Have you evaluated what would happen if people made changes in burial practices?

Yes, the model can be used to evaluate the potential effect of a variety of changes. The outbreak could be stopped if 70% of Ebola cases are cared for in Ebola Treatment Units or, if they are at capacity, at home or in a community setting such that there is a reduced risk of disease transmission, assuming that safe burial practices are in place in both settings.