Measuring the Impact of Ebola: Will it Reach 1.4 Million?

More than six months have passed since Ebola was first identified in West Africa, and the scale of the crisis continues to grow. Over the last few weeks cases and deaths have increased significantly in the two most affected countries – Liberia and Sierra Leone – setting the stage for even more explosive growth in the weeks and months to come if further action is not taken immediately. The sense of urgency in the face of an outbreak that has grown “out of control” has been palpable among public health leaders and politicians alike, including at the United Nations last week.  Some of this urgency has been driven by stark new data and modeling projections just released by the WHO and the U.S. Centers for Disease Control and Prevention (CDC).  To help shed light on these new data, we take a deeper look at several key measures of the epidemic’s impact including estimates of current cases, prevalence and death rates from Ebola, as well as a consideration of the future projections of Ebola’s burden in the months to come.
How Many Cases?

The global health community relies on the World Health Organization (WHO) as the authoritative source on current Ebola case numbers.  WHO has published updates and situation reports about the West African Ebola epidemic, collected from the country governments, agencies, and organizations working on the ground, approximately every week since August. Even though they represent the “gold standard” of the moment, the WHO numbers cannot be taken at face value.  As WHO itself has stated, its numbers are “vast underestimates,” and the organization believes the true number of cases is two to four times greater than the official reported numbers because many cases go undetected or uncounted for a variety of reasons. 

What Share of the Population Has Already Been Affected?

Recognizing these limitations, we used these data to calculate the cumulative prevalence of Ebola (the percent of the population that has been infected to date) in Liberia and Sierra Leone as of September 22 (see Table).  Based on officially reported numbers from WHO, cumulative prevalence across both countries stands at 0.049%.  Liberia has been hardest hit with an estimated cumulative prevalence of 0.07%, more than twice the rate for Sierra Leone.  For illustrative purposes, if we take into account the underreporting factor as estimated by WHO by tripling the officially reported case numbers, the cumulative prevalence across both countries would be 0.147%; in Liberia alone, the adjusted prevalence would be 0.22%, or approximately one in every 500 people having become infected already (see Table).  Moreover, the estimated number of new cases is rising exponentially.

Why Are Cases Growing Exponentially?

These constantly upward revisions are driven by a cruel epidemiologic statistic – the reproductive rate of disease, or average number of new infections each case creates. WHO’s most recent estimates of this number are 1.51 in Liberia and 1.38 in Sierra Leone.  As long as the reproductive rate remains above 1, the epidemic will continue to expand exponentially, unless something alters its trajectory.  At the current reproductive rates, case numbers are doubling approximately every 24 days in Liberia and every 30 days in Sierra Leone according to the WHO.

Table 1. Current Case and Cumulative Prevalence Estimates fro Ebola in Liberia and Sierra Leone, including Illustrative Prevalence Rates Adjusted for Under-reporting
 Country Population Current Cumulative Reported Ebola Cases (as of Sept 22) Cumulative Reported Prevalence (%, as of Sept 22) Current Cumulative Cases if Adjusted for Under-reporting
(3x reported cases)
Current Cumulative Prevalence, based on Adjusted Case Counts (%)
Liberia  4,092,310 3022 0.074 9066 0.222
Sierra Leone  5,743,725 1813 0.032 5439 0.095
TOTAL  9,836,035 4835 0.049 14,505 0.147
Sources: CIA World Factbook July 2014 Population Estimates; World Health Organization Ebola Situation Report (22 September).
What Does the Future Hold?
CDC’s new Ebola model estimates the potential future impact of the epidemic.  According to the model’s worst case scenario, which assumes no change in the trajectory of the epidemic and also corrects for underreporting, the number of Ebola cases in Liberia and Sierra Leone together could reach 1.4 million by the end of January 2015. This translates into a whopping 14% cumulative prevalence rate. To put this in perspective, an equivalent rate in the U.S. population would be almost 45 million people nationally, and more than 90,000 in Washington, DC alone.
What’s the Relative Impact Compared to Other Diseases?
The available numbers also indicate that Ebola had, as of mid-September, already become the leading cause of death in Liberia. The WHO has estimated a case fatality rate of 70% for the West Africa Ebola outbreak; applying this rate to the officially reported cases from Liberia for the last 7 weeks, we find Ebola caused, on average, 263 deaths per week in the country.  By comparison, the top three leading causes of deaths in the country – malaria, lower respiratory infections, and diarrheal diseases (using data from the global burden of disease study for 2010) – caused an estimated 140, 89, and 88 deaths per week, respectively.  This means at its current rate, Ebola is killing people in Liberia at approximately twice the rate of the country’s previously biggest cause of death and, ominously, this rate is likely to increase for the foreseeable future as the epidemic continues to expand.
Policymaking in a Fog

One of the many challenging dimensions of the current Ebola outbreak in West Africa, and one that it shares with just about every other global health issue, has been the lack of timely and robust data.  Policymakers trying to deliver personnel, supplies, and services where they are most needed have their task made that much more difficult because we have only vague and time-delayed estimates for some of the core epidemiological aspects of the current outbreak, from exactly how many people are infected, geographic locations where transmission may rising or falling, the rate of growth or decline in cases, and how many people remain at risk of becoming infected.  These weaknesses are partly a reflection of the lack of vital statistics and general surveillance capacity in the affected countries, from before the current outbreak even began, and partly due to the rapidly changing, sometimes chaotic situation on the ground.  Arriving at such estimates typically requires coordination between governmental Ministries of Health, non-governmental organizations, multilateral institutions, and aid agencies in rural and urban areas that are spread out across multiple countries.  A difficult task under any circumstances, it has been made even more challenging in the current crisis situation.

With timely, reliable information so limited, crude estimates and projections are what the global health community has to work with at this point. Unfortunately, the story these estimates and projections tell is an exceedingly grim one, and should create an even greater incentive for the world as a whole to respond to this still-growing crisis. The numbers are stark enough, but they only hint at the full weight of the massive and growing toll that Ebola has inflicted on the people in affected countries and communities.

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