Assessing the Risk of a Global Ebola Pandemic

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

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.

Virulence

Transmissibility

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 R of infection, which is the average number of additional infections one person generates over the course of illness. In order to cause an epidemic, R needs to be greater than 1.

The R 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 R of greater than 10 in an unvaccinated environment.

Transmissibility

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.

Pandemic Modeler, LifeRisks
As the principle subject matter specialist on human pandemic risk, Mary is responsible for managing the RMS Infectious Disease Model. Mary has been at RMS since 2008, where she has worked on a comprehensive upgrade to the infectious disease mortality model, development of a disease morbidity model, as well as the incorporation of the suite of excess mortality and morbidity models into the cloud-hosted LifeRisks software product. Prior to joining to RMS, she worked as a clinical researcher at UCSF. Mary has an MPH in Epidemiology and Public Health from Yale University and a Bachelor’s in Molecular & Cell Biology from UC Berkeley.

5 thoughts on “Assessing the Risk of a Global Ebola Pandemic

  1. Carol Whisler

    Travel restrictions for persons moving out of countries that have had Ebola infections seems completely scientifically sound. Quarantine and isolation of persons who are potentially incubating Ebola should be mandatory for any country in the world.

    Reply
  2. Pingback: What would be the economic impact of a global pandemic? | AEIdeas

  3. steve devitt

    I agree that Ebola will probably not become a pandemic outside of Africa (I am writing this from NIgeria), but I am surprised at the way the influenza epidemic in 1918 was presented. It killed more people than World War I and was present on every continent. Statistical tracking was not available in much of Africa and Asia, or the fatality numbers would have been much higher. Like the black plague before it, Spanish influenza arrived on boats — now we have airplanes. I lived in China during SARS and can guarantee that the Chinese government was much more competent in dealing with checking — and that for a disease that was not nearly as deadly.

    Reply
  4. Obiwan

    I am concerned that the model paints too soothing a picture of the reality.

    I have no reason to question the models assumptions about virulence. I do however question the assumptions about transmission and treatment capacity.

    TRANSMISSION: the RMS model has assumed that R = 1.5 for the current West African outbreak. That may be far too low. Reuters reports 22.8.2014 that WHO is now warning that the number of cases is far higher than estimated because many go unreported and occur in “shadow zones”.
    http://af.reuters.com/article/commoditiesNews/idAFL5N0QS1OZ20140822?pageNumber=2&virtualBrandChannel=0

    In addition, anecdotal evidence suggests that R is not 1.5 but far higher. The Liberian Patrick Saywer, who died in Lagos 29 July 2014 after flying in from Liberia via Togo managed directly to infect 11 people. For that case R=11 which is more like measles.

    Second, it is misleading to say that ebola is not airborne. Airborn infection from any virus typically happens when droplets of saliva containing viable virus come in contact with mucous membranes (eyes, mouth, nose, throat) or alternatively breaks in the skin. Typical “airborne” virus diseases like flu or measles spread widely because symptoms include coughing and sneezing. These are not typical Ebola symptoms, but it does not mean that a microdroplet of saliva propelled through the air (for example, while trying to collect a sputum sample, intubating a patient, or indeed a cough or a sneeze) will not infect: it will. Ebola is horribly infectious. 1-10 virions are enough.

    Moreover, while the highly infectious measles virus can only survive ca. 2 hours on a contaminated surface, the ebola virus can survive for days. Ebola survives days at 23 C, longer at 5C and indefinitely at -70C. Furthermore, ebola virus is found in ALL bodily fluids, including sweat and sputum as well as blood, urine, feces, vomit, sperm, vagina secretions and breast milk. This has grim implications: If for example a person with Ebola on a plane uses the toilet, then everything he has touched including the toilet seat, the faucet and the interior and exterior handles of the doors will be infectious. Anybody subsquently using the toilet will get the virus on his hands and the next time he rubs his eyes or touches his mouth he may well become infected.

    The only way to deal with Ebola is to kill it, not wait for it to die. Heat helps but it requires 60C for ca 30 minutes and 75C for ca 15 min to kill ebola. Handwashing with soap in warm water typically uses water at 40C. WHO and CDC protocols in fact recommend cleaning hands and instruments, etc. (especially where contaminated with visible organic matter) with soap and water followed by alcohol and preferably solution of .5 % chlorine bleach (about 1:10 household bleach to water).

    We want to avoid panic but downplaying the risk in a bid to spread false reassurance does an greater disservice as the West African governments have now learned, to there sorrow. Ebola is virulent, it is contagious and there is no vaccine and no treatment. This is why Ebola virus is among the very, very few viruses that can only be handled in Level 4 containment facilities. Level 4 is the highest safety level, reserved for only the most dangerous infectious agents, where staff work in sealed negative pressure laboratories accessed by multiple airlocks and additionally wear spacesuits with respirators, and not just masks and gowns and gloves.

    TREATMENT CAPACITY: The US has less capacity to safely treat Ebola cases than one thinks. It is certainly true that developed countries, including the US, have a better medical infrastructure and equipped to isolate and treat cases of hemorrhagic fever. But they are equipped to handle imported cases, like Dr. Kent Brantley. He was treated at Emory, which is one of only 4 centers in the US equipped to handle this kind of case. An ordinary hospital or clinic is not equipped to handle a Level 4 virus like Ebola and we must also remember that if an infectious case presents at a doctor’s office or emergency room, many surfaces and possibly people will be contaminated before he can be isolated, as the Patrick Sawyer case demonstrates.

    We should stay sober, but we should also be aware of just what it is we are dealing with. The RMS model needs to be revised.

    Reply

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