Follow-Up: Will #Ebola2014 Spread to the USA?

4.1.14 – With the CDC reporting a suspected total of 112 Ebola cases in Guinea and 7 in Libera, fear of further spread is palpable among the general public [1]. On Twitter alone, Gregory Härtl – head of PR for the WHO – has patiently fielded seemingly endless questions about whether any travel bans have yet been instated. (The answer is no; there’s just no way to justify closing Guinea’s borders and dooming its economy.)

People are scared. Zaire ebolavirus – currently considered the causative agent of the #Ebola2014 [1] – is extremely deadly; historically, it has killed up to 90% of people it’s infected [2]. It’s understandable that most folks don’t want to share a room – or even a country – with it.

And at least in the United States, we’ve got nothing to worry about yet.

I’m a stats geek, so last week I chalked up a simple mathematical model that approximates the likelihood of Ebola actually making it to the States. I’ve been updating it to reflect reports by the CDC, and as of today, the probability is still less than .001%. If you dared to follow the link, you might be wondering why I haven’t factored in the new suspected cases in Liberia. The answer’s pretty simple; all 7 cases can be traced back to Guinea and have not yet spread to other individuals within Liberia’s borders, suggesting that the disease has thus far been successfully contained [3, 4]. As long as this holds true, there’s no chance that Ebolavirus will be hopping on a plane and traveling from Liberia to the United States.

This brings up a really important point that virology PhD student Stephen Goldstein has been driving home on his Twitter feed. Exported cases of Ebola won’t necessarily lead to further transmission. This is especially the case for resource-rich countries that can effectively enforce quarantine of symptomatic patients and those who have been in contact with them. (…You know, countries like the United States.)

With this in mind, I bring our attention back to the .001% chance that a case of Ebola will surface here in America. In the unlikely event that we see a case in the States, it’ll be an example of exportation from Guinea… And given the competence of the American health system, subsequent transmission to the general public is even less likely than the exportation event itself.

While Ebola can spread from person-to-person, it requires contact with an infected individual’s bodily fluids [5]. Transmission usually happens due to unsafe care-taking and burial practices when handling the sick [6]. However, because the illness is not airborne, it’s not nearly as contagious as influenza, small pox, and other such diseases that have caused some of history’s most infamous pandemics [7].

Might Ebola make it here to the United States? It’s possible, but exceedingly unlikely. Even more importantly, an exportation event does not mean America is going to succumb to a massive outbreak of the disease.

Though certainly imperfect, our current understanding of Ebola – from its virology to its epidemiology and pathogenesis – is one of our greatest defenses against mass transmission. To some degree, we know what we’re up against – much more so than we did during the Spanish Flu and Smallpox Pandemics… And while it’s unclear where #Ebola2014 might spread to next, sufficient knowledge – paired with appropriate action by organization like MSF and able-bodied governments – should keep it from following in their footsteps.


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