October 17, 2014
Fear of Ebola is spreading rapidly in the United States. Fanned by media outlets with hours of programming to fill and graphics budgets to burn through, people in the US are afraid of getting Ebola entirely out of proportion to the risk. I have heard of people coming into the emergency room with a fever because their “roommate is from Africa.” I don’t blame people for being scared. Throughout human history, there has always been a fear of the unknown and the unseen and the sensationalization of novel threats. Though you are much more likely to die from the flu, people are not lining up at their local doctors’ offices for flu shots. You are also more likely to die in a car accident or a plane crash, and yet people continue driving and flying with little hysteria. But this is not a post about statistics, it is a post about epidemiology — and why you shouldn’t be afraid of getting Ebola.
If you have been reading about Ebola in the news, or you saw the movie Contagion, you’ve probably heard about a concept called R-naught, or R0. This is a simple concept, but it is central to the field of epidemiology and to understanding the potential for Ebola to spread. All you need to understand about R0 is that if an individual with Ebola (or any disease) is infecting more than one other more person on average (R0>1), the disease will spread. If they are infecting less than one person on average (R0<1), the disease will not spread. You may ask, but the Liberian man in Texas infected two nurses and one of those nurses got on a plane with 131 other people—so is R0 > 1? The short answer is that for the US, the answer is no. Remember that the key term is “on average,” and neither of those nurses is likely to spread the disease to anyone else. Why not? The answer is related to how Ebola is transmitted.
The R0 of viruses like Ebola is made up of two factors: how much virus an individual is shedding (i.e., how many individual viruses are coming out of the person), and how much contact that individual has with other people. How much virus an individual is shedding varies with time, but you have to be shedding virus to transmit the virus. Patterns of shedding look a bit like a mountain: once you start shedding, it increases until you hit a peak, and then as your immune system combats the virus, your shedding amount decreases. However, the manner in which you shed virus is critical. The flu, for instance, infects your respiratory system (i.e., your nose and airways to your lungs). If you have the flu and you sneeze (or even talk), you are expelling millions to billions of tiny virus particles. These virus particles can hang in the air for hours and float around and are capable of infecting someone. This is called airborne transmission. Ebola cannot be transmitted this way. Yes, if an Ebola patient sneezes into your face, droplets containing virus may be expelled and could cause an infection, but they do not hang in the air, they fall to the ground. As long as someone with Ebola doesn’t sneeze directly in your face, it isn’t likely to infect you through the air. That’s why it is considered a contact-transmitted disease. You need to physically touch a person or their bodily fluids. The timing of when you are ‘infectious’—or when you start shedding virus—is also important. For flu, you probably start shedding virus before you start showing symptoms, increasing the likelihood of spreading it. Individuals with Ebola don’t start shedding virus until they show symptoms—and even then viral shedding starts out slow, increases with symptoms and is at its peak at the point of death.
That leads to the second part of R0: contact with infected individuals. An individual with any disease that sneezes alone in a forest is not going to be heard, and more importantly is not going to transmit that virus to anyone else, because no one else is there. And that is why you should not worry about Ebola. Here in the US we have a functioning health system and a functioning government, and any individual diagnosed with Ebola will be quarantined. They will be placed in isolation and have no contact with other people that are not protecting themselves properly (more on the Texas situation below). In addition, the peak time at which an individual is infectious with Ebola is generally when the symptoms are at their worst and you are very, very unlikely to be near or caring for an individual who is that sick with Ebola. Even the nurse who traveled on a plane with other people is unlikely to have been shedding much virus if any at all, and she was not spreading bodily fluids (i.e., vomiting in the aisle or on the seat), so even people sitting next to her or who occupied her seat after her are not at much risk.
Thus, the R0 of Ebola in the US is less than one, significantly less than one. That is not the case in West Africa. When someone is sick with Ebola in West Africa, their health system is often unable to care for them properly due to a lack the infrastructure and resources, so either the virus spreads in the hospital to other patients or the patient is at home and cared for by relatives, who then become infected. Isolated cases that are brought into the United States (realistically this is probably not the last one), will not spread. Thus you need not worry.
But two nurses were infected—isn’t that worrying? No, I say. I’ll first point to the fact that the only people who seem to have contracted the virus are healthcare workers that had direct contact with the patient. No one else prior to his going to the hospital has of yet become infected, including the man’s friends and family. Is it tragic that these healthcare workers were infected? Of course. The complete story has yet to come out, but whatever the facts, it is clear that this hospital was not fully prepared to handle this case. There have been other patients in specialized hospitals in the US that have not infected any healthcare workers, so I think this is an isolated situation, and hopefully by catching it early, these workers will survive.
What about the possibility of the virus going airborne? Yes, viruses do mutate, so purely in theory this is possible – but, and I say this very strongly, this is extremely unlikely to happen. But, you say, I saw Dr. Oz or some other person say this could happen, and it happened in the movie Outbreak. As you surely know, not everything you see in movies is true. There are a number of reasons why this is unlikely, but the main reason is that the target of the virus is not the respiratory tract of humans. Ebola is already very good at transmitting via bodily fluids—so it doesn’t currently have significant selection pressure to evolve. In addition, a virus evolving to change the way it transmits is immensely difficult. HIV, which has infected millions of people over more than 50 years, is still only transmitted sexually. So I would not worry about the Ebola virus evolving to become airborne at all.
I would not and do not worry about getting Ebola and neither should you. We have a robust healthcare system and a functioning government that is more than capable of containing isolated cases of Ebola, especially now that everyone is on high alert. But beyond the fact that hospitals and the government should be (and are) planning and training for these cases (and since the Dallas case they have been doing so overtime) – the main point here is that an average person walking around is in no way going to contract Ebola.
If you are still really concerned about Ebola, there are thousands of people in West Africa that are suffering. You can help them. The situation there is a slow-moving natural disaster, and experts and medical professionals from the CDC, the Red Cross and Doctors Without Borders are putting their lives on the line to help. So please, stop worrying about Ebola in the US. And consider donating to help people who really have something to worry about.
Here is a list of places where you can donate and hopefully save lives: cidi.org/ebola-ngos
Eili Klein is a fellow at CDDEP and an assistant professor in the Center for Advanced Modeling in the Social, Behavioral and Health Sciences (CAM) in the Department of Emergency Medicine at Johns Hopkins University. His research focuses on the role of individuals in the spread of infectious diseases.
Image via Wikimedia commons.