Measuring the Impact of Ebola: Will it Reach 1.4 Million?
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 (%)|
|Sources: CIA World Factbook July 2014 Population Estimates; World Health Organization Ebola Situation Report (22 September).|
What Does the Future Hold?
What’s the Relative Impact Compared to Other Diseases?
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.