Distinguishing epidemiological features of the 2013–2016 West Africa Ebola virus disease outbreak

ABSTRACT

The 2013–2016 West Africa Ebola virus disease epidemic was notable for its scope, scale, and complexity. This briefing presents a series of distinguishing epidemiological features that set this outbreak apart.

Compared to one concurrent and 23 previous outbreaks of the disease over 40 years, this was the only occurrence of Ebola virus disease involving multiple nations and qualifying as a pandemic. Across multiple measures of magnitude, the 2013–2016 outbreak was accurately described using superlatives: largest and deadliest in terms of numbers of cases and fatalities; longest in duration; and most widely dispersed geographically, with outbreak-associated cases occurring in 10 nations.

In contrast, the case-fatality rate was much lower for the 2013–2016 outbreak compared to the other 24 outbreaks. A population of particular interest for ongoing monitoring and public health surveillance is comprised of more than 17,000 “survivors,” Ebola patients who successfully recovered from their illness.

The daunting challenges posed by this outbreak were met by an intensive international public health response. The near-exponential rate of increase of incident Ebola cases during mid-2014 was successfully slowed, reversed, and finally halted through the application of multiple disease containment and intervention strategies.

KEYWORDS:

• Ebola
• Ebola virus disease
• epidemiology
• EVD
• outbreak
• pandemic
• PHEIC
• Public Health Emergency of International Concern

Introduction

The 2013–2016 West Africa Ebola virus disease (EVD) outbreak is a tale of the exceptional. In the 40-year history of EVD since its recognition in 1976, there have been 25 reported outbreaks, including the 2013–2016 pandemic. By the numbers, this outbreak was unmatched, with more diagnosed cases of EVD illness, more deaths, and more survivors than all other outbreaks combined. Regarding the spatial dimension, the 2013–2016 outbreak included a dense concentration of cases in the 3 contiguous nations of Guinea, Sierra Leone, and Liberia, and additional outbreak-associated cases in 7 other countries. In contrast, no previous EVD outbreak had extended beyond the borders of a single nation. In the temporal dimension, while no prior EVD epidemic lasted more than several months, the 2013–2016 Ebola episode continued for 2 y and 4 months, from index case to declaration of the cessation of viral transmission.

he 2013–2016 outbreak was of such scope and scale that it qualified as a pandemic, a natural biological disaster, and a Public Health Emergency of International Concern. The human health and security threats associated with this epidemic were unequaled in the history of Ebola. This briefing provides narrative, supported by a series of detailed tables and figures, to illustrate the defining features of the 2013–2016 EVD pandemic (Table 1).

Table 1. Distinguishing features of the 2013-2016 West Africa Ebola Virus Disease (EVD) Outbreak.

Defining features

Natural biological disaster

Pandemic: only EVD outbreak that qualified as a pandemic

Public Health Emergency of International Concern (PHEIC)

August 8, 2014 – March 29, 2016

Epidemiologic indicators

EVD cases: highest number of cases (28,652 cases through March 2016)

EVD deaths: highest number of deaths (11,325 deaths through March 2016)

EVD survivors: highest number of survivors (estimate: 17,300 survivors)

Duration: longest EVD outbreak in history (December 2013 – March 2016)

Geographic footprint:  28,616 cases reported in Guinea, Sierra Leone, Liberia

36 cases distributed in 7 additional nations

Geographic footprint: outbreak occurred in nations with no previous EVD cases

Urban spread: first EVD outbreak to spread intensively in urban centers

Spread via air travel: first EVD outbreak involving case diagnosis in developed nations as a result of air travel of infected persons from Ebola endemic areas

Impact on health care workers (HCWs)

EVD cases: highest number of HCW cases (898 HCW cases)

EVD deaths: highest number of HCW deaths (518 HCW deaths)

EVD survivors: highest number of HCW survivors (estimate: 380 survivors)

Higher case fatality rate for HCWs (57.7%) than for non-HCWs (38.9%)

Ebola virus disease

The history of EVD dates back 40 y to the initial epidemiological and clinical characterization of this disease in 1976 during 2 near-simultaneous outbreaks, caused by 2 related strains of the virus, occurring in 2 neighboring African nations.¹ These episodes were notable for both the severity of symptoms and the high case-fatality rate. The outbreak in Zaire (present-day Democratic Republic of Congo-DRC) had 318 cases and an 88 percent case-fatality rate; higher than the 53 percent death rate among 284 cases in the South Sudan epidemic.

Ebola virus disease (EVD) is a severe, communicable, viral illness in humans that is frequently fatal (Table 2).^2,3 Symptoms initially include some combination of headache, muscle pain, fever, weakness and malaise and may progress to diarrhea, vomiting, abdominal pain, and unexplained hemorrhage. Ebola virus symptoms typically appear 8 to 10 d following infection (range: 2–21 days). The percentage of ill persons who die from the disease (the “case-fatality rate”) is approximately 50 percent (range: 25–90 percent). Table 2 provides a detailed epidemiological description of the Ebola virus (the “infectious agent”) and EVD illness including the important links comprising Ebola's “chain of infection.”^2,3,4

Table 2. Epidemiological profile of Ebola Virus Disease (EVD) and the 2013–2016 West Africa Ebola Virus Disease (EVD) Outbreak.

Disaster Type	Ebola virus disease	Ebola virus disease (EVD) is a severe, communicable, viral illness in humans that is frequently fatal. Symptoms initially include some combination of headache, muscle pain, fever, weakness, and malaise and may progress to diarrhea, vomiting, abdominal pain and unexplained hemorrhage. Ebola virus symptoms typically appear 8 to 10 d following infection (range: 2–21 days). The percentage who die from the disease (case-fatality rate) is approximately 50% (range: 25–90%).
	2013–2016 West Africa EVD Outbreak Description	Multi-nation Ebola virus disease (EVD) outbreak declared to be a Public Health Emergency of International Concern (PHEIC) requiring international medical and humanitarian response.
	Pandemic outbreak	A pandemic is an epidemic that traverses international borders and sweeps across several nations. The geographic reach of the 2013–2016 EVD outbreak epidemic, with a dense concentration of cases in 3 countries, and additional cases across 7 other nations on multiple continents, officially qualifies this event as a pandemic. The 2013–2016 EVD outbreak was the only EVD pandemic among 25 EVD outbreaks from 1976–2016.
	Natural hazard taxonomy classification	This event is described as an international viral zoonotic communicable disease epidemic (pandemic) affecting large human populations on a scale that qualified as a natural biological disaster. In addition to the spread of Ebola virus and EVD illness, this pandemic ultimately escalated into a protracted complex emergency and humanitarian crisis with mass morbidity and a high case-fatality rate.
Hazard Exposure	“Chain of infection”	For communicable diseases, the primary hazard of concern is the disease agent, a necessary and essential element for disease initiation/propagation. Epidemiological investigation of infectious diseases pivots around the concept of the “chain of infection.” This chain is comprised of 6 “links:” 1) infectious disease agent (Ebola virus), 2) reservoir (where the agent principally “resides” in the environment), 3) portal of exit, 4) mode of transmission, 5) portal of entry, and 6) susceptible host.
	Infectious agent	The infectious agent that causes Ebola is an RNA virus of the family Filoviridae, genus Ebolavirus. There are 5 distinct species: Ebola virus (Zaire ebolavirus, EBOV); Sudan virus (Sudan ebolavirus, SUDV); Taï Forest virus (Taï Forest ebolavirus, TAFV), Bundibugyo virus (Bundibugyo ebolavirus, BDBV), and Reston virus (Reston ebolavirus, RESTV). Three of the species, EBOV, SUDV, and BDBV, have each caused EVD outbreaks with more than 100 cases. TAFV has caused illness in a single case, and RESTV does not produce symptomatic disease in humans. EBOV is the virus implicated in the 2013–2016 West Africa EVD outbreak and 16 of 25 total EVD epidemics through June 2016.
	Reservoir	Ebola is a “zoonotic” disease, and the primary reservoir includes a variety of small game animals and bats. Humans are actually “incidental hosts” and unnecessary for the survival of the Ebola virus. Outbreaks begin with human contact with infectious bodily fluids, meat, or wastes from a live animal or carcass. Once the “index” human case becomes infected, most or all of the serial transmission that takes place after that is human-to-human. The human element in EVD is central to human epidemics but inconsequential to the virus itself.
	Portal of exit, mode of transmission, portal of entry	Infectious diseases spread via some mechanism that allows a non-infected person to come in contact with an infected mammal or human, or their bodily fluids, that contain the infectious agent. Human EVD outbreaks usually require initial animal-to-human transmission (“spillover”) followed by serial person-to-person transmission within human populations. 1) Wild animal-to-human transmission: Ebola does not typically reside within the human species. Initial transmission of Ebola to a single person or small number of humans is thought to involve direct contact with the bodily fluids of infected game animals or their carcasses or wastes. Animal species that may carry the Ebola virus include primates such as chimpanzees, gorillas, and monkeys; and other mammals including forest antelope, porcupines, and fruit or insectivorous bats. 2) Human-to-human direct contact transmission: Ebola next spreads from the infected index case to one or more non-infected persons through some form of direct contact transmission. The skin, blood, secretions and other bodily fluids of the infected person carry the virus that may potentially escape from the body, with the mucous membranes or broken skin serving as the “portal of exit.” The mechanism for transferring Ebola virus in the environment - the “mode of transmission” - is direct contact. Exposure of a non-infected person to the infected person's virus-contaminated skin, bodily fluids, surfaces, or materials (e.g. clothing, bedding, clothing, medical equipment) may result in infection. Infection occurs when the virus gains access to a “susceptible host” via a “portal of entry” (skin or mucous membranes). This process may happen in an instant from a caring touch or another form of direct contact. Researchers have speculated about whether the Ebola virus can be transmitted via an airborne route but there has been little support for this conjecture.^20 Sexual transmission. Sexual transmission of EBOV is very rare but verified in several cases. The WHO recommends that Ebola survivors and their sexual partners receive counseling regarding safer sexual practices (abstaining from all types of sex or using condoms consistently until the semen has twice tested negative) and receive a supply of condoms.
	EVD symptoms	Symptoms of Ebola virus disease. A constellation of signs and symptoms characterizes EVD. Early symptoms, frequently appearing with sudden onset, include fever, severe headache, nausea, muscle pain, intense weakness and malaise, and sore throat. These early symptoms are common to multiple illnesses, including malaria, and do not definitively differentiate EVD illness. Later symptoms, which typically signal higher Ebola virus titers and increased risk for death, include diarrhea, vomiting, impaired kidney and liver function with abdominal pain, and for a proportion of cases, both internal and external bleeding. The hemorrhagic symptoms, although much publicized and a source of fear regarding Ebola, appeared in only 18% of cases during the 2013–2016 outbreak. Usually laboratory results reveal low white blood cell and platelet counts and elevated liver enzymes.
	Incubation period	The incubation period, the interval from infection with Ebola virus to symptom onset, is commonly 8 to 10 d (Range: 2–21 days)
	Period of infectiousness	“Infectiousness” is the capacity of an infected person to “shed” virus that may infect another person. Persons with Ebola infection or EVD illness remain infectious as long as the Ebola virus is present in their blood or bodily fluids, a time interval that persists for up to 61 d following symptom onset.
	Period of immunity	A variable proportion of patients with EVD, particularly those who receive hydration and supportive clinical care, recover successfully. These Ebola “survivors” develop antibodies that prevent future new infection with the Ebola virus for at least 10 y.
Place Dimension	African nations	Widespread and intense transmission nations: Guinea, Sierra Leone, Liberia Other: Nigeria, Mali, Senegal
	Additional nations	United States, Italy, Spain, United Kingdom
	Countries with previous EVD outbreaks	Democratic Republic of the Congo (DRC), Gabon, South Sudan, Ivory Coast, Uganda, South Africa, Republic of the Congo (ROC) Note: No overlap with 2013–2016 outbreak nations
	2013–2016 outbreak transmission patterns	Movement of EVD: 1) forest/savannah (in reservoir) to 2) rural villages to 3) urban towns and cities (no prior urban spread) and to 4) medical facilities 5) spread to distant nations via air travel
Time Dimension	Index case	December 2013
	PHEIC	Public Health Emergency of International Concern: Declared August 8, 2014 Discontinued March 29, 2016
	Initial declaration of “disease free”	Date when nation was removed from the list of “intense transmission” nations: Liberia: May 9, 2015 Sierra Leone: November 7, 2015 Guinea: December 28, 2015

2013–2016 West Africa EVD outbreak synopsis

The outbreak was first reported on March 21, 2014 by the Guinea Ministry of Health,⁵ after several months when multiple chains of viral transmission went unrecognized.^1,6

During the 2013–2016 Ebola pandemic, epidemiologists were able to trace the outbreak back to its source in real time.^7-9 Their investigations led to the identification of the index case, the outbreak's “Patient Zero,” a 2-year-old child from a small village of 31 households in the Guéckédou Prefecture of Guinea.⁷ The trigger event for this unprecedented EVD pandemic appears to have been a single instance of a wild animal to human viral transmission of Ebola virus to this young boy whose play area was beneath a grove of trees where infected bats were roosting.

The index case quickly became a cluster. During the brief course of the child's illness, his mother, sister, and grandmother who had cared for the children became infected; all four died.⁷ This earliest wave of case patients grew to include members of the extended family, community contacts, traditional healers, and health care workers. The virus spread along multiple, branching chains of infection that elongated and produced new sequences of infected persons who became ill and died in high proportions.^7-9 As cases proliferated and dispersed geographically over the early months of 2014, this outbreak of a “mysterious” disease developed a critical mass and activated a public health and disease control response.

It took until March 2014 to characterize the infectious disease agent and confirm its identity: Zaire ebolavirus.⁵ EVD cases increased steadily during April, May, and June 2014. The numbers inflected upward sharply during the months of July through November. By the time the outbreak was declared to be an international public health emergency on August 8, 2014, hundreds of new cases were being reported each week. The epidemic reached its zenith in October 2014 with almost 6,000 incident cases reported that month (Figs. 1 and 2).

Figure 1. Epidemic curve of Ebola virus disease cases: Guinea, Sierra Leone, Liberia, and 3-nation total by month, December 2013-March 2016.

Figure 2. Epidemic curve of Ebola virus disease deaths: Guinea, Sierra Leone, Liberia, and 3-nation total by month, December 2013-March 2016.

Meanwhile, the World Health Organization (WHO), the U.S. Centers for Disease Control and Prevention (CDC), Médecins Sans Frontières, multiple United Nations entities, and international non-governmental organizations joined forces with local public health officials on the front lines to implement a robust response.^10,11

These public health measures, coupled with an unprecedented deployment of international professionals to West Africa, ultimately succeeded in capping and containing the outbreak.^10,11 Case counts began a downward turn in the final months of 2014. In Liberia, EVD incidence dropped in November, while in Sierra Leone, the numbers of new cases remained high through December before falling steeply in January 2015 (Fig. 1). Total numbers of new outbreak-associated cases decelerated rapidly throughout 2015. Starting from 1,800 new illnesses reported in January 2015, monthly counts dropped below 400 during June and finally reached the zero mark in all 3 intense transmission nations in the final months of the year.

2013–2016 West Africa EVD outbreak classification

The 2013–2016 EVD outbreak was so noteworthy in the annals of infectious disease epidemiology that it can be differentiated from several vantage points. This was a pandemic, a biological disaster, and a Public Health Emergency of International Concern (PHEIC), one of only 4 infectious disease events ever to receive this designation from the WHO.

Pandemic

The geographic range of this epidemic, including a focal concentration of cases in the 3 contiguous countries of Guinea, Sierra Leone, and Liberia in West Africa, and a scattering of additional cases across 7 other nations on 3 continents, officially defined this event as a pandemic. A pandemic is an epidemic that traverses international borders and sweeps across several nations. On this point, the 2013–2016 EVD outbreak stands alone; none of the 24 prior or concurrent EVD episodes involved multiple nations and none achieved pandemic status (Table 3, Fig. 3)

Figure 3. Ebola virus disease outbreaks, 1976–2016 displaying outbreak size and Ebolavirus species. Source: http://www.cdc.gov/vhf/ebola/outbreaks/history/distribution-map.html

Table 3. Timeline of Ebola Virus Disease (EVD) cases, deaths, survivors, case-fatality rates, affected nations, and Ebola virus species for all reported outbreaks, 1976-2016, and comparison between the numbers/rates for the 2013-2016 outbreak and all previous outbreaks combined.

Year	EVD-Affected Nation(s)	EVD Cases	EVD Deaths	EVD Survivors	Case- Fatality Rate	Ebola Virus Species
1976	D.R. Congo	318	280	38	88.1	EBOV
1976	South Sudan	284	151	133	53.2	SUDV
1977	D.R. Congo	1	1	0	100.0	EBOV
1979	South Sudan	34	22	12	64.7	SUDV
1994	Gabon	52	31	21	59.6	EBOV
1994	Cote d’Ivoire	1	0	1	0.0	TAFV
1995	D.R. Congo	315	250	65	79.4	EBOV
1996	Gabon	37	21	16	56.8	EBOV
1996	Gabon	60	45	15	75.0	EBOV
1996	South Africa	2	1	1	50.0	EBOV
2000	Uganda	425	224	201	52.7	SUDV
2001	Gabon	65	53	12	81.5	EBOV
2001	Republic of Congo	57	43	14	75.4	EBOV
2002	Republic of Congo	143	128	15	89.5	EBOV
2003	Republic of Congo	35	29	6	82.9	EBOV
2004	South Sudan	17	7	10	41.2	EBOV
2007	D.R. Congo	264	187	77	70.8	EBOV
2007	Uganda	149	37	112	24.8	BDBV
2008	D.R. Congo	32	15	17	46.9	EBOV
2011	Uganda	1	1	0	100.0	SUDV
2012	Uganda	11	4	7	36.4	SUDV
2012	D.R. Congo	36	13	23	36.1	BDBV
2012	Uganda	6	3	3	50.0	SUDV
2014	D.R. Congo	66	49	17	74.2	EBOV
Totals: 24 EVD Outbreaks, 1976-2014		2,411	1,595	816	65.9
2013-2016 EVD Outbreak	Guinea, Sierra Leone, Liberia, Mali, Nigeria, Senegal, Italy, Spain, England, United States	28,652	11,325	17,327	39.5	EBOV
Ratios: 2013-2016 EVD outbreak to total of 24 EVD outbreaks		11.88	7.10	21.23	0.60

Notes. EBOV-Ebola virus (Zaire ebolavirus); SUDV-Sudan virus (Sudan ebolavirus); TAFV-Taï Forest virus (Taï Forest ebolavirus), BDBV-Bundibugyo virus (Bundibugyo ebolavirus)

Biological disaster

The 2013–2016 EVD outbreak qualified as a natural biological disaster. More precisely, when applying a natural hazards taxonomy,¹² this event would be definitively classified as an international viral zoonotic communicable disease epidemic (pandemic) affecting large human populations on a scale that qualified as a natural biological disaster. In addition to the spread of Ebola virus and EVD illness, this pandemic ultimately escalated into a protracted complex emergency and humanitarian crisis with mass morbidity and a high case-fatality rate.

Public health emergency of international concern

This multi-national EVD outbreak was of such severity that the Director-General of the WHO, Dr. Margaret Chan, declared the outbreak to be a “Public Health Emergency of International Concern (PHEIC)” on August 8, 2014.¹³ This declaration, one that activates medical and humanitarian response assets worldwide, remained in force for 20 months prior to being discontinued on March 29, 2016.¹⁴

Descriptive epidemiology: EVD case characteristics

Total population

The 2013–2016 West Africa Ebola virus disease (EVD) pandemic dwarfed all previous Ebola disease events.^4,15-17 The 2013–2016 West Africa EVD outbreak surpassed all previous outbreaks combined in numbers of EVD cases, deaths, and survivors (Table 3).

The total count of 28,652 cases, through March 2016, was 67 times larger than the second-ranking outbreak that occurred in Uganda in 2000, with 425 cases (Table 3).¹⁸ Among these cases, the total mortality was 11,325 deaths, yielding a case-fatality rate was 39.5 percent, almost 4 deaths for every 10 cases. For the 3 intensive transmission nations, the case-fatality rate ranged from 28.0 percent in Sierra Leone, to 45.0 percent in Liberia, to 66.7 percent in Guinea (Table 4). The case-fatality rate of 39.5 percent was much lower than the corresponding 65.9 percent rate computed as total deaths (1,595) among total cases (2,411) for the 24 other outbreaks combined (Table 3).

Table 4. 2013-2016 West Africa Ebola Virus Disease (EVD) Outbreak: EVD cases, deaths, and case-fatality rates for the total population and for health-care workers by nation through March 2016.

	Total Population			Health Care Workers
	Cases	Deaths	Case- Fatality Rate	Cases	Deaths	Case- Fatality Rate
Nations with Widespread and Intense Transmission
Guinea	3,804	2,536	66.7%
	10	8	80.0%
	3,814	2,544	66.7%	196	100	51.0%
Liberia	10,666	4,806	45.1%
	12	4	33.3%
	10,678	4,810	45.0%	378	192	50.8%
Sierra Leone	14,122	3,955	28.0%
	2	1	50.0%
	14,124	3,956	28.0%	307	221	72.0%
SUBTOTAL	28,616	11,310	39.5%	881	513	58.2%
Other West African Nations
Mali	8	6	75.0%	0	0	NA
Nigeria	20	8	40.0%	11	5	45.5%
Senegal	1	0	0.0%	0	0	NA
SUBTOTAL	29	14	48.3%	11	5	45.5%
Nations Outside Africa
Italy	1	0	0.0%	1	0	0.0%
Spain	1	0	0.0%	1	0	0.0%
United Kingdom	1	0	0.0%	1	0	0.0%
United States	4	1	25.0%	3	0	0.0%
SUBTOTAL	7	1	14.3%	6	0	0.0%
TOTAL	28,652	11,325	39.5%	898	518	57.7%

The total number of EVD “survivors” who recovered from illness can be approximated as the difference between total cases (28,652) and deaths (11,325), yielding more than 17,300 persons.

The final rows in Table 3 highlight the relative magnitude of the outbreak in a compelling fashion. The 2013–2016 EVD pandemic had 12 times more cases, 7 times more deaths, and 21 times more survivors compared to all 24 previous EVD outbreaks combined.

Health-care workers

Health-care workers who developed EVD were monitored intensively (Table 4). Working on the front lines with high levels of exposure to Ebola virus, and initially, without the benefit of adequate personal protective equipment, basic water and sanitation in health facilities, or sufficient training on Ebola or infection control, health workers became ill in record numbers. Among 898 health professionals who were diagnosed with EVD, 518 died. The 57.7 percent case-fatality rate for health-care workers was almost 50 percent higher than the 38.9 percent case-fatality rate for non-health-care workers (Table 4).

Descriptive epidemiology: Place and spatial characteristics

In terms of “place” or geographic distribution, the 2013–2016 West Africa EVD outbreak was differentiated from previous outbreaks in several ways. First, the outbreak took place in 10 nations that had not previously experienced Ebola virus infection or EVD illness (Table 3, Fig. 3). Examination of the line listing in Table 3 confirms that the 24 previous outbreaks occurred in 7 nations in eastern or central Africa (Fig. 3).¹ Among these, 5 nations have experienced multiple outbreaks: DRC, 7; Uganda, 5; Gabon, 4; Republic of Congo, 3; and South Sudan, 3.¹ South Africa had a 2-case outbreak and Cote d'Ivoire had a single case. None of these 7 nations was affected during the 2013–2016 outbreak while conversely, all 10 nations with outbreak-associated cases had never experienced an Ebola case previously.

Second, the 2013–2016 outbreak was the only multi-national EVD “pandemic” in history, affecting 6 nations in Africa, 3 in Europe, and the United States. The other 24 outbreaks were limited to a single nation. Fully 99.9 percent of the EVD cases were concentrated in the 3 neighboring countries of Guinea, Sierra Leone, and Liberia; designated by the World Health Organization (WHO) as the “widespread and intense transmission” nations (Fig. 4, Table 4).¹⁸ Only 36 additional cases were diagnosed across the remaining 7 nations. These included 29 cases in the African nations of Nigeria, Mali, and Senegal; one case each in Italy, Spain, and England; and four cases in the United States (Table 4).¹⁸

Figure 4. Distribution of Ebola virus disease cases for the widespread and intensive transmission nations of Guinea, Sierra Leone, and Liberia through March 2016. Source: Centers for Disease Control and Prevention. http://www.cdc.gov/vhf/ebola/images/west-africa-distribution-map.jpg; Webpage: http://www.cdc.gov/vhf/ebola/outbreaks/2014-west-africa/distribution-map.html

Third, this outbreak was distinguished by virus penetration into susceptible, impoverished, densely-populated metropolitan centers. While previous EVD outbreaks had been rural, relatively short-lived, and sputtering, the West Africa EVD pandemic intensified into a firestorm of urban viral spread.

Fourth, this was the only outbreak where outbreak-associated cases were introduced via air travel into several highly-developed nations.

Descriptive epidemiology: Temporal characteristics

The 2013–2016 outbreak was the longest-duration EVD epidemic, continuing for almost 28 months, the time period from the date of diagnosis of the index case in December 2013 to the end of the PHEIC declaration. Within the 28-month span of the outbreak, the PHEIC was in force for 20 months, from August 8, 2014 to March 29, 2016.^13,14

As displayed in Fig. 1, the majority of EVD cases were diagnosed from August 2014 to February 2015. The time course varied across the 3 widespread transmission nations. Liberia displayed both the steepest rise in cases and the most rapid decline and became the first of the 3 nations to be declared “Ebola-free” on May 9, 2015. This declaration officially removed Liberia from the list of intense transmission nations. Sierra Leone achieved this status on November 7, 2015 and Guinea on December 28, 2015. However, there were brief bursts of “flare-up” cases (24 total cases) reported in these nations prior to March 29, 2016 when the WHO officially discontinued the public health emergency.¹⁴

Public health response

Containing and halting disease transmission posed extraordinary challenges to local, regional, and international response capabilities, prompting Thomas R. Frieden, Director of the US. Centers for Disease Control and Prevention (CDC), to remark retrospectively on “how close the world came to a global catastrophe.”^10,17

CDC professionals described the interlocking elements that comprised the disease control intervention: 1) conducting aggressive community-based surveillance to identify and isolate persons with EVD infection; 2) implementing “contact tracing” for all contacts of EVD cases and monitoring these individuals for the full 21-day Ebola incubation period; 3) investigating current and past EVD cases with trace-backs to identify active “chains” of transmission; 4) tracking deaths and bringing trained burial teams to safely handle the cadavers of the deceased; 5) maintaining daily case reporting; 6) educating and updating health-care workers about infection control practices; and 7) training health workers on safe procedures for donning, doffing, and working in personal protective equipment, for the mutual protection of staff and patients.^6,19

This multi-faceted approach, drawing upon local, regional, and international assets, was able to stop the epidemic spread of the Ebola virus. The epidemic curves, Figures 1 and 2, serve as a testament to both the extreme surge in EVD cases and deaths in mid-2014, and the speed with which a concerted public health response was able to reverse these upward trends into plummeting declines.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.

Author note

This briefing on the descriptive epidemiology of the 2013–2016 EVD epidemic is a spinoff of work that is currently underway, exploring the mental health and psychosocial dimensions of the outbreak, and specifically the role of fear-related behaviors in potentiating disease spread.^4,15-17 These epidemiologic patterns have guided our collaborators and are shared for the benefit of investigators for whom this information provides useful context and for a broader readership with interest in the dynamics of this outbreak.