EbolaNumber of Ebola cases in Liberia, Sierra Leone to reach 1.4 million by mid-January 2015: CDC

A new study by the Center for Disease Control and Prevention (CDC) says that a model developed by CDC to estimate the spread of the Ebola virus shows that if current virus proliferation trends continue without additional interventions, the number of Ebola cases in Liberia and Sierra Leone will reach 1.4 million by mid-January 2015.

The model estimates that Liberia and Sierra Leone will have approximately 8,000 total Ebola cases (21,000 total cases when corrected for underreporting) by 30 September 2014 (Figure 1). Liberia will account for approximately 6,000 cases (16,000 corrected for underreporting) (Appendix [Figure 1]).

Total cases in the two countries combined are doubling approximately every twenty days (Figure 1). Cases in Liberia are doubling every 15-20 days, and those in Sierra Leone are doubling every 30-40 days (Appendix [Figure 1]).

By 30 September 2014, without additional interventions and using the described likelihood of going to an ETU, approximately 670 daily beds in use (1,700 corrected for underreporting) will be needed in Liberia and Sierra Leone (Figure 2). Extrapolating trends to 20 January 2015, without additional interventions or changes in community behavior (for example, 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) (Appendix [Figure 2]). The uncorrected estimates of cases for Liberia on 9 September 2014, were 2,618, and the actual reported cases were 2,407 (that is, model overestimated cases by +8.8 percent). The uncorrected estimates of cases for Sierra Leone on 13 September 2014, were 1,505 and the actual reported cases were 1,620 (that is, model underestimated cases by -7.6 percent).

The researchers say that results from the two illustrative scenarios provide an example of how the epidemic can be controlled and eventually stopped. If, by late December 2014, approximately 70 percent of patients were placed either in Ebola treatment units (ETUs) or home or in a community setting such that there is a reduced risk for disease transmission (including safe burial when needed), then the epidemic in both countries would almost be ended by 20 January 2015 (Appendix [Figure 3]). In the first scenario, once 70 percent of patients are effectively isolated, the outbreak decreases at a rate nearly equal to the initial rate of increase. In the second scenario, starting an intervention on 23 September 2014, such that initially the percentage of all patients in ETUs are increased from 10 percent to 13 percent and thereafter including continual increases until 70 percent of all patients are in an ETU by 22 December 2014, results in a peak of 1,335 daily cases (3,408 cases estimated using corrected data) and <300 daily cases by 20 January 2015 (Appendix [Figure 10]). Delaying the start of the intervention until 23 October 2014, results in the peak increasing to 4,178 daily cases (10,646 cases estimated using corrected data). Delaying the start further, until 22 November, results in 10,184 daily cases (25,847 estimated using corrected data) by 20 January 2015, which is the last date included in the model (Appendix [Figure 10]).

“My gut feeling is, the actions we’re taking now are going to make that worst-case scenario not come to pass,” Dr. Thomas R. Frieden, the CDC director told the New York Times. “But it’s important to understand that it could happen.”

Experts not affiliated with the CDC said the agency’s modeling figures were in line with estimates by others in the field. “It’s a nice job,” Ira Longini, a professor of biostatistics at the University of Florida who has also done computer modeling of the epidemic, told the Times. “It summarizes the extent of the problem and what has to happen to deal with it.”

Bryan Lewis, an epidemiologist at the Virginia Bioinformatics Institute at Virginia Tech, agreed that the estimates were reasonable, perhaps even a bit low compared with those generated by other models. He said that if some of the latest data from the World Health Organization (WHO) is plugged into the CDC model, “the very large numbers of estimated cases are, unfortunately, even larger.”

CDC said its new projections do not include figures for Guinea, one of the three hardest-hit countries, because case counts there have gone up and down in ways that cannot be reliably modeled.

— Read more in Estimating the Future Number of Cases in the Ebola Epidemic — Liberia and Sierra Leone, 2014–2015, CDC’s Morbidity and Mortality Weekly Report (MMWR) (23 September 2014)