A few years ago, West Africa was struck by an epidemic of Ebola, which killed more than ten thousand people in Liberia, Sierra Leone, and Guinea. The first case of Ebola was a child who had been playing in a bat-infested tree. From him, the disease rapidly spread from person to person in a chain reaction of contagion. A quite different type of hemorrhagic disease is Lassa fever, which was first identified in 1969, and named after the town in Nigeria where the first cases were observed.
Ebola is a hot topic in the media right now, with multiple cases being reported outside of West Africa and much confusion among the general public around the reality of the danger. So, are the fear and sensationalism warranted?
RMS models infectious diseases and recently developed the world’s first probabilistic model for the current West African Ebola outbreak. While Ebola is indeed a very scary and relatively deadly disease, with a case fatality rate between 69 and 73 percent according to the WHO, RMS modeling shows that it is unlikely the outbreak will become a significant threat globally.
The spread of Ebola in West Africa is in part due to misconceptions and fear surrounding the disease and a lack of public health practices. Ebola can be passed solely via bodily fluids; the risk of unknowingly contracting the disease is low.
Fear is prevalent among some West African communities that Ebola is a lie or is being used purposefully to wipe out certain ethnic groups, causing them to hide sick family members from healthcare and aid workers. Customary burial practices, in which family members kiss and interact with the dead, also have contributed to Ebola’s spread. Getting the populace in these countries to trust foreigners who are telling them to abandon their customs has been an uphill struggle.
In more developed countries where health care is more advanced and understood, the chances of transmission are exponentially smaller due to the fact that extreme containment measures are taken. Controlling the spread of the disease comes down to a question of logistics; if the medical community can control the existing cases and trace the contact made with carriers, spread is much less likely. For example, the case in Texas can be contained to one degree as long as every single person in contact with the patient is tracked.
There is also a (speculative) fear of the virus mutating into an airborne pathogen; the fact is, the chances of the virus changing the way it is transmitted, from fluid contact to airborne passage, are very low and of a similar order of magnitude to the chance of emergence of a different highly virulent novel pathogen.
Vincent Racaniello, a prominent virologist at Columbia University wrote:
“When it comes to viruses, it is always difficult to predict what they can or cannot do. It is instructive, however, to see what viruses have done in the past, and use that information to guide our thinking. Therefore, we can ask: has any human virus ever changed its mode of transmission? The answer is no. We have been studying viruses for over 100 years, and we’ve never seen a human virus change the way it is transmitted.”
The tipping point in the modeling of a virus like Ebola is the point where the resources being used to mitigate the threat outpace the increase in new cases. Trying to get ahead of the epidemic itself is like a race against a moving target, but as long as people get into treatment centers, progress will be made in getting ahead of the illness.
So, while Ebola is a very scary and dangerous illness, it is not something that we expect to become a global pandemic. However, while the current outbreak is not expected to spread significantly beyond West Africa, it still has the potential to be the most deadly infectious disease in a century and could have drastic economic impacts on the communities that suffer from Ebola breakouts. In fact, the economic impacts are likely to be worse than the actual impacts of the disease, due to negative impacts to trade and inter-community relations.
The key is to contain it where it is, reach the tipping point as quickly as possible, and to promote safety around existing infected persons. Through travel control measures and the development of several new drugs to combat the virus, the danger of epidemic should be drastically reduced in Africa and, as a result, the rest of the world.
With the current outbreak of Ebola in western Africa, as well as the recent MERS coronavirus and H7N9 avian flu outbreaks, the world is becoming increasingly concerned about the risk of emerging infectious diseases and their potential to cause the next pandemic.
As catastrophe modelers, how do we assess the risk of a pandemic?
To understand the potential dangers of Ebola, it’s helpful to look to the framework we use at RMS to model infectious disease pandemics. The RMS® LifeRisks Infectious Disease Model projects the excess mortality risk for existing infectious diseases, like influenza, as well as infectious diseases that are emerging or have recently appeared, like Ebola. When modeling a disease, we first look at two main criteria: the virulence and the transmissibility of the pathogen responsible for causing the disease. We then take into account mitigating criteria, including medical and non-medical interventions.
Virulence is a measure of how deadly a disease is, typically measured by the case-fatality rate (CFR), which is the proportion of people who die from the disease to those who do not. The current Ebola CFR is 55 percent. For comparison, the CFR for bubonic plague typically ranges from 25 to 60 percent. CFR for flu is typically less than 0.1 percent.
Transmissibility refers to how likely an infected person is to transmit the disease to another person, and is measured in terms of the basic reproductive number, or R₀ of infection, which is the average number of additional infections one person generates over the course of illness. In order to cause an epidemic, R₀ needs to be greater than 1.
The R₀ for the current Ebola outbreak is greater than 1, and the disease will continue to spread. Past Ebola outbreaks have been estimated to be in the 1.3 to 1.6 range, but have occasionally been greater than 5, which is why there is cause for concern. However, Ebola is less transmissible than many other infectious diseases. For example, measles, which is highly transmissible, has an R₀ of greater than 10 in an unvaccinated environment.
Societal and Environmental Factors
Societal and environmental factors can play a large role in transmissibility. In this case, societal and environmental factors in West Africa have contributed to the disease’s spread. For example, traditional burial practices in which families wash the deceased can expose additional people to the virus.
However, the risk of Ebola developing into a pandemic that extends beyond the region is low, due to the standard public health and infection control practices in place in many countries globally. Ebola can only be transmitted via direct contact with bodily fluids, especially blood, which means that caregivers are the primary people who might be exposed to the virus. In many countries including the U.S., the general practice is to treat all blood as potential sources of infection, due to experience with HIV and other blood-borne diseases. In quarantine situations, such as those being used with the American Ebola cases in Atlanta, the likelihood of transmission from a single person is miniscule.
Medical and Non-Medical Interventions
Medical and non-medical interventions mitigate the risk of an infectious disease pandemic. Typical medical interventions for infectious disease include pharmaceuticals and vaccines. Often, there is no specific therapy or drug available for new or emerging diseases. In these cases, we model the effect of supportive care, which includes management of blood pressure, oxygen, and fluid levels. As we’ve seen with the current outbreak, supportive care and the access to healthcare can vary substantially, depending on the region or population. With the exception of experimental treatments, there are no pharmaceutical interventions available for Ebola. Experimental Ebola drugs are not applicable to large populations at this time.
If there are high enough immunization rates, vaccines can reduce or stop the spread of diseases like measles or whooping cough. Unfortunately, a vaccine isn’t currently available for Ebola. Ebola outbreaks occur sporadically and are caused by different virus strains, making vaccine development more difficult.
In addition to vaccines and medical interventions, we account for non-medical interventions when modeling the impact of pandemics. Non-medical interventions include quarantines, school closures, and travel restrictions. Various countries in Africa have begun to implement these methods in hopes of stopping the spread of Ebola. However, these types of countermeasures can often be difficult to time or enforce properly. Ebola can have an incubation period from two days to as long as 21 days.
So, what is the pandemic potential of Ebola?
The current outbreak is now the largest outbreak of Ebola to date, and the World Health Organization (WHO) has designated the outbreak as a Public Health Emergency of International Concern. However, while cases will continue to develop, a global pandemic is unlikely. Even if the disease were to spread to other regions of the world, Ebola is still considered a rare disease and the transmissibility is likely to be much lower due to quarantine and infection-control measures, even if the CFR remains high. We have not seen any community transmission outside of Africa, and this is expected to continue. Ebola is a very serious disease, with devastating consequences to impacted communities. As risk managers, we aim to improve understanding of catastrophes such as pandemic disease so that as a society we can be better prepared to mitigate risk and recover from catastrophes.
Rebecca Vessenes contributed to this post. As a Senior Quantitative Modeler at RMS, Rebecca is involved in the development and parameterization of the LifeRisks longevity models. She recently completed the longevity model for Japan and has worked on determining the correlation structure for mortality improvement between countries. Prior to working for RMS, she led the Financial Modeling group at AIR. Rebecca earned a Ph.D. in mathematics from California Institute of Technology and is an actuary with the Society of Actuaries.