Evaluating Subcriticality during the Ebola Epidemic in West Africa

Abstract

The 2014-2015 Ebola outbreak is the largest and most widespread to date. In order to estimate ongoing transmission in the affected countries, we estimated the weekly average number of secondary cases caused by one individual infected with Ebola throughout the infectious period for each affected West African country using a stochastic hidden Markov model fitted to case data from the World Health Organization. If the average number of infections caused by one Ebola infection is less than 1.0, the epidemic is subcritical and cannot sustain itself. The epidemics in Liberia and Sierra Leone have approached subcriticality at some point during the epidemic; the epidemic in Guinea is ongoing with no evidence that it is subcritical. Response efforts to control the epidemic should continue in order to eliminate Ebola cases in West Africa.

Fig 1. Cumulative number of confirmed cases in Liberia, March 23, 2014 to July 26, 2015.

The cumulative numbers of confirmed cases over time for each of the 15 counties are shown using different colors. The cumulative number of confirmed cases over time for all 15 counties of the country is shown by the black line in the figure inset.

Fig 2. Cumulative number of confirmed cases in Sierra Leone, May 25, 2014 to July 5, 2015.

The cumulative numbers of cases over time for each of the 14 districts are shown using different colors. The black line in the figure inset shows the cumulative number of cases over time for all 14 districts of the country.

Fig 3. Cumulative number of confirmed cases in Guinea, March 2, 2014 to July 26, 2015.

The cumulative number of cases over time for each of the 25 prefectures that reported data are shown using different colors. The black line in the figure inset shows the cumulative number of cases over time for 25 prefectures in the country.

Fig 4. Estimated Reproduction Number over calendar time for Liberia.

The quantiles of the reproduction number are displayed on the last day for each week. From bottom to top, the quantiles are 0.025, 0.10, 0.25, 0.50 (black line), 0.75, 0.90, and 0.975. The black horizontal line shows the threshold for subcriticality, i.e., the reproduction number equal to 1.0. We analyzed data through June, 2015; we did not include 6 cases that were reported in the situation reports during the weeks ending on July 5, 2015 and July 12, 2015.

Fig 5. Estimated Reproduction Number over calendar time for Sierra Leone.

The quantiles of the reproduction number are displayed on the last day of each week. From bottom to top, the quantiles are 0.025, 0.10, 0.25, 0.50 (black line), 0.75, 0.90, and 0.975. The black horizontal line shows the threshold for subcriticality, i.e., the reproduction number equal to 1.0.

Fig 6. Estimated Reproduction Number over calendar time for Guinea.

The quantiles of the reproduction number are displayed on the last day of each week. From bottom to top, the quantiles are 0.025, 0.10, 0.25, 0.50 (black line), 0.75, 0.90, and 0.975. The black horizontal line shows the threshold for subcriticality, i.e., the reproduction number equal to 1.0.