R-Zero

Criticism of the CDC handling of the first Ebola case to come to the United States and the resultant infection of two healthcare workers has reached a crescendo. At the same time, calls for a travel ban from West Africa have become more urgent. But hard-core supporters of Barack Obama seem intent on suggesting that criticism is unjustified, politically motivated, and dishonest. As James Taranto states:

The [Left leaning] New Republic’s Brian Beulter is even more afraid [of the "panic" he suggests has overtaken those of us who are less than thrilled with the handling of this situation to date]. When two American nurses were diagnosed with Ebola, he writes, it moved “the political dialogue surrounding the virus toward an unbridled opportunism”:

This week’s developments provided conservatives the psychological ammunition they needed to justify using the specter of a major Ebola outbreak as an election-year base-mobilization strategy. . . .

The competence argument is appealing because it doesn’t require dabbling in pseudoscience or xenophobia—just healthy skepticism of our governing institutions. Moreover, I’m certain this sort of skepticism does help explain why a large minority of people in the U.S. feels [sic] at risk of contracting Ebola. But they are at no great risk. That the risk is provably infinitesimal underscores
the fact that the issue with Ebola isn’t the virus itself so much as paranoia about it.

Or is the issue paranoia about paranoia about it? “I’m sort of a doom-and-gloom guy,” admits [left-wing] Salon’s Jim Newell. Meaning he’s afraid of Ebola? Nah, only that voters afraid of Ebola will enable Republican Scott Brown to succeed in his challenge against Sen. Jeanne Shaneen of New Hampshire.

But are calls for a travel ban really paranoia? Is it unreasonable to make every attempt to reduce the likelihood of another case or cases, particularly when it turns out that all of the epidemiological details concerning this virulent strain of Ebola are not fully understood?

Jonathan Last provides a few salient and troubling scientific facts that the CDC and the administration are loath to share:

Start with what we know, and don’t know, about the virus. Officials from the Centers for Disease Control (CDC) and other government agencies claim that contracting Ebola is relatively difficult because the virus is only transmittable by direct contact with bodily fluids from an infected person who has become symptomatic. Which means that, in theory, you can’t get Ebola by riding in the elevator with someone who is carrying the virus, because Ebola is not airborne.

This sounds reassuring. Except that it might not be true. There are four strains of the Ebola virus that have caused outbreaks in human populations. According to the New England Journal of Medicine, the current outbreak (known as Guinean EBOV, because it originated in Meliandou, Guinea, in late November 2013) is a separate clade “in a sister relationship with other known EBOV strains.” Meaning that this Ebola is related to, but genetically distinct from, previous known strains, and thus may have distinct mechanisms of transmission.

Not everyone is convinced that this Ebola isn’t airborne. Last month, the University of Minnesota’s Center for Infectious Disease Research and Policy published an article arguing that the current Ebola has “unclear modes of transmission” and that “there is scientific and epidemiologic evidence that Ebola virus has the potential to be transmitted via infectious aerosol particles both near and at a distance from infected patients, which means that healthcare workers should be wearing respirators, not facemasks.”

In August, Science magazine published a survey conducted by 58 medical professionals working in African epidemiology. They traced the origin and spread of the virus with remarkable precision—for instance, they discovered that it crossed the border from Guinea into Sierra Leone at the funeral of a “traditional healer” who had treated Ebola victims. In just the first six months of tracking the virus, the team identified more than 100 mutated forms of it.

Yet what’s really scary is how robust the already-established transmission mechanisms are. Have you ever wondered why Ebola protocols call for washing down infected surfaces with chlorine? Because the virus can survive for up to three weeks on a dry surface.

In an earlier post, I discussed the reasons for concern that are associated with a statistical measure called expected value. In essence, as the number of West African infections grows, the likelihood of more cases in the United States also grows even as the probability of infection remains very small. With the Obama administration issuing 100 - 150 entry visas in West Africa each week, the expected value rises. By the time the Science study was published, five of the  58 medical professionals who participated  had died from Ebola.

The key scientific indicator of an epidemic's growth rate is called R0 (R-Zero)—the reproduction number that indicates how many new cases of Ebola an infected person causes. Last provides some frightening data from West Africa:

When R0 is greater than 1, the virus is spreading through a population. When it’s below 1, the contamination is receding. In September the World Health Organization’s Ebola Response Team estimated the R0 to be at 1.71 in Guinea and 2.02 in Sierra Leone. Since then, it seems to have risen so that the average in West Africa is about 2.0. In September the WHO estimated that by October 20, there would be 3,000 total cases in Guinea, Liberia, and Sierra Leone. As of October 7, the count was 8,376.

It's not scientifically significant, but it is interesting the RO for the first US case is exactly 2.0 (so far). It's also not scientifically significant, but it is troubling that the mechanism for the transmission to those healthcare workers who are now infected is not known.

Those of us who have criticized the administration's inexplicably rigid position on a travel ban from West Africa are not in a panic. We understand the statistical probability of infection and recognize it is very small on an individual level. But we also understand that even a few more infections, coupled with the resultant media frenzy, could have significant impact on our economy and will further weaken trust in government. The expected value for infection is growing every week that R0 stays above 1.0. A travel ban can help reduce the expected value for the United States, and although it isn't a guarantee of zero infections, it's a lot better than what has been implemented to date.