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

January 8, 2015

Read the January 8, 2015 report: Updating the Estimates of the Future Number of Cases in the Ebola Epidemic

Are these estimates different from what has been published before and if so, why?

These estimates are different from those published in the MMWR on September 26, 2014. The earlier estimates were based on data gathered in August and looked at what could happen if nothing were done to slow the spread of Ebola. Since that time, many actions have been taken to prevent or slow Ebola’s spread. These include building of Ebola Treatment Units, providing home care kits, and delivering community-based education and significant behavioral change in populations affected by Ebola. As a result, the outlook for the outbreak has improved significantly.

How often does CDC plan to update these estimates?

To fully understand the impact of these interventions, periodic updating of the estimates is needed to assist in planning next steps in the response. CDC plans to update the model estimates on a regular basis to assist with planning and public health decision making.

Why are the number of future cases of Ebola expected to be much lower now compared to the estimates reported in September?

Several factors contribute to the new estimates being significantly lower. The number of Ebola cases reported weekly in Guinea and Liberia has decreased. These decreases are due, in large part, to an increase in hospitalization rates, use of Community Care Centers as well as effective home isolation, and safe burial practices. The EbolaResponse model allows for changes in clinical and behavioral factors that can affect estimates of future cases of Ebola.

Why are other estimates different from CDC’s?

Different modeling groups may use different assumptions and model constructions, thus producing different projections. Even though the estimates may differ, they all agree on the potentially devastating impact of this epidemic and the urgency of implementing a combination of interventions to reduce the number of cases.

How does the model handle those who died or recovered from Ebola?

The EbolaResponse model calculates the estimated number of Ebola cases that will occur given a set of circumstances. The model doesn’t explicitly calculate the number who will die from Ebola. However, a user can readily estimate the number of deaths by multiplying the estimated number of cases by a case fatality rate obtained from publicly-available data sets.

How did you calculate the correction factors?

In September 2014, we arrived at a correction factor of 2.5 by taking estimates of beds-in-use computed by the EbolaResponse modeling tool and comparing these estimates to actual beds-in-use reported from Liberia. For this update, we consulted with experts from all three countries, including officials from the national ministries of health, and, given current conditions, arrived at a correction factor of 2.0 for Sierra Leone and Guinea and 1.5 for Liberia.

Why have you added Guinea to the countries being analyzed in this update?

In August, the available data from Guinea did not provide enough information to conclude that the number of Ebola cases was increasing. Data now coming out of Guinea show a growth in cases, thus we included Guinea in this update.

Can the outbreak still get out of control?

Yes. Until every pocket of infection has been eliminated, the possibility that the disease may resurge cannot be ruled out. As a result, CDC and several international partners are making a combined effort to reduce all new cases to zero in all the areas afflicted by the outbreak.

What does your range of forecasts mean?

The labeled baseline provides our best estimate as to the number of future cases. Because this epidemic has proven unpredictable, we have also provided an upper and lower level to encompass a range of possibilities. These upper and lower levels show what effect even a small adjustment—for the good or for the worse—can have on the number of cases.

Why do we need forecasts for each of the three countries when data are now being collected more efficiently than before?

It is true that data collection has improved in the three highly-affected countries over the past few months. Better data collection leads to improved forecasts, which are needed by decision-makers to estimate future resource requirements and to create plans for allocating resources, medications, supplies and staff.

How reliably do these predictions reflect the truth on the ground?

Because it is built on assumptions that reflect both current understanding of Ebola transmission and field observations, the EbolaResponse model “fits” very closely to what is actually occurring in each country. It captures the spread of Ebola at the national level. The model predictions do not account for variability in geographical regions and communities; therefore, they are not meant to assist in local and regional planning.

Why is the trend for future Ebola cases in Sierra Leone different from that in Liberia?

The Ebola outbreak started a few weeks earlier in Liberia than in Sierra Leone. This means Liberia has had a few more weeks to put interventions in place. To end the epidemic faster, Sierra Leone must scale up the known effective interventions aimed at isolating and treating Ebola patients and expanding adoption of safe burial practices.