Editorial Note: Ebola hysteria dominated popular news in 2014. However, since the outbreak began to draw attention in March 2014, only two people have been infected in the United States. Did this really warrant non-stop media coverage? Much of this attention may have been attributed in part to the worry that Ebola may “mutate to become airborne,” but is this even something we need to worry about? Join Harvard University’s Science in the News as we investigate the origins of Ebola hysteria and discuss how scientists think about viral mutations and what they tell us about the potential for an airborne strain.

This year, the Ebola virus jumped from the obscurity of African jungles to the front pages of nearly every media outlet in the United States. The virus has been — and continues to be — a major issue in West Africa, but it was given the most attention in September and October of 2014 when a man traveling to the U.S. from Liberia became diagnosed with Ebola: the first case in the U.S. Later, two nurses who treated him became infected. Despite constant reassurance from many scientists and public health officials, fear of this virus remains exceptionally high. Since there have been no new cases in the U.S., we can look back and say that those reassurances were correct. One of the biggest concerns is that the Ebola virus could mutate to become transmissible through the air and cause a global pandemic. Where does this fear come from and could it actually happen?


 Figure 1 Ebola virus is spread through direct contact with bodily fluids, and potentially through short-distance droplet transmission.

Image credit: Kaitlyn Choi.

What does “airborne” really mean?

When health professionals talk about a virus being airborne, they mean that the virus can remain suspended in the air after an infected person has released it through a cough, a sneeze, or by talking. Thus, an unsuspecting person can become infected with an airborne virus by breathing in air containing virus even when an infected person is no longer present. Measles is one virus that can be transmitted this way.

Airborne transmission is often confused with droplet transmission, but these are considered to be different modes of transmission. Droplets containing virus are too large to be airborne for long periods of time and thus do not stay suspended in the air. Droplets can be transmitted in short distances when an infected person coughs or sneezes and the droplets are ingested or enter someone else’s body through a cut.

The CDC posits that Ebola could be transmitted through droplets, but it has not been studied in detail. One can imagine a scenario in which an Ebola-infected patient is bleeding through his or her gums (a common scenario) and coughs on someone close by. While coughing is not a common symptom of Ebola (19% of cases in one study), the virus could be passed if infectious blood came into contact with another person’s eyes, nose, throat, or cut. This, however, is still very different from airborne transmission.

A terrifying true story?

When I was in high school, The Hot Zone sparked my interest in microbiology and virology. I devoured every page of the book due to the horrifying narrative of patients who suffered from filoviruses such as Ebola virus.

The Hot Zone describes a scenario that is commonly used illustrate that Ebola could become airborne: Two groups of monkeys were separated in room, prohibiting direct contact. One group was infected with a strain of Ebola virus and the other was not. The uninfected group ended up dying due to Ebola, suggesting that it spread across the room. The book’s author, Richard Preston, took this observation to mean that the virus was airborne. While some evidence points to this conclusion, the authors of the study wrote that, “the exact mode of transmission to the control group cannot be absolutely determined.” Most importantly, there was no evidence that the monkeys actually expelled Ebola virus from their lungs, which would be necessary for airborne transmission.

Scientists have long questioned these and other claims in the book. Even Preston has recently backed off on some of the depictions in the book. In a recent interview, he said he wanted to write an update “to make the clinical picture of the virus more clear and accurate” and admitted that a scene described in the book “almost certainly didn’t happen”.

An “outbreak” of conjecture

Where else could the idea of airborne transmission of Ebola come from?

In 1995, a popular movie, Outbreak, was released. Debuting at #1, the movie focuses on an outbreak of a virus discovered around 1970 in Zaire, Africa. Infection of the virus causes a hemorrhagic fever with high mortality. Sounds just like Ebola, right? However, the movie veers from reality when the virus becomes highly transmissible through the air, causing a global pandemic. Since the “movie virus” looked so similar to the actual Ebola virus, movie reviewers had a hard time determining where to draw the line between fact and fiction. After all, airborne transmission seems completely possible — until we examine what actually must happen for a virus like Ebola to become airborne.

Why isn’t Ebola airborne?

While the term “virus” is great for classifying certain types of infectious agents, it can also lead to the idea that all viruses are similar. This is not true, especially considering how viruses are transmitted. Mosquitoes spread West Nile virus. Rodents spread Hantavirus. HIV is spread through sexual contact or needle use. While measles can be transmitted from inhaling infected air particles, Ebola transmission is only possible when infected bodily fluids or droplets come into contact with the eyes, nose, throat, or cuts of an uninfected person.

For Ebola to become a substantial airborne risk , it would first have to infect the cells in our respiratory system – either in the throat or lungs. This change alone would likely require a number of mutations. Mutations can occur within individuals of a given population, so if one individual (or a small subset of individuals) become better at surviving the virus, the mutation within their viruses will persist and become more prevalent. Therefore, even if one of the many copies of Ebola in a patient mutated so that, theoretically, made the virus airborne, that mutated virus would have an advantage, compared with the Ebola that is not mutated in the same way. However, because Ebola virus is already being spread as efficiently via direct contact, there is minimal selective pressure for the virus to become airborne.

One could argue that the virus would benefit from airborne transmission because it would be able to spread more efficiently. However, if this were true, and airborne transmission was the best way for a virus to be spread, we would expect to see other viruses acquire this ability. However, this rarely happens in nature. HIV, which has infected tens of millions of people and mutates readily, is not airborne. In short, it is not as simple as having mutation X leading to airborne transmission: there are many barriers to overcome in order for airborne transmission to dominate.

Media and science perceptions

There is no precedent for any virus to drastically change its mode of transmission and Ebola is unlikely to be the first. However, the media perpetuates the unjustified fear of airborne transmission. When there is a viral outbreak, it is best to listen to the experts who have dedicated their lives to studying the science behind these issues. We should certainly pay attention to Ebola, but should not worry ourselves with highly improbable fears.

This post is cross-published courtesy of Harvard’s Science in the News’s Signal to Noise.

Featured image credit:  Georgia National Guard via Flicker