Epidemics and Public Goods

In a recent article about Ebola in the Wall Street Journal, Peter Piot, the Director of the London School of Tropical Medicine & Hygiene, referred to epidemics like Ebola as what economists refer to as a public good.  For most readers of the Wall Street Journal, I’d imagine that the economic definition of public good might be familiar.  It might not be for most of those interested in public health but without economics and business training. 

Public goods have two key characteristics: being non-excludable and non-rival.  Non-excludable means that it is impossible to keep someone from experiencing it.  Non-rival means that one person’s experiencing it doesn’t limit another person’s experience.  National defense has characteristics of a public good—everyone in a country experiences it and my enjoyment of the United States’ national defense does not limit anyone else’s experience.

So, an epidemic does have these characteristics—everyone gets to experience the risk and no one is excluded from the exposure to the risk.

While the epidemic has characteristics of a public good, some solutions do not have elements of public goods but instead have externalities.  For example, for a vaccine, if I get the dose no one else can have it.  But, when I get the dose, my protection also provides some amount of protection for those around me.

The funding of public goods comes at the public expense and there must be a way to determine how much of the public good the public desires.  Externalities imply that individuals will either overconsume (in the case of negative externalities) or underconsume (in the case of positive externalities like a vaccine) the good without some type of incentive or enforcement.   

Within national borders or provincial borders within a country, it is possible to have laws that encourage or require specific behaviors, to provide monetary incentives,  or to coordinate activity through a Department of Ministry of Health.  Economists would ask about the costs and benefits of such rules or coordinating efforts.  When the epidemic is within borders the power to regulate, incentivize, and enforce is clear. 

When the epidemic is actually an international pandemic the situation is more challenging.  There is no authority that has the same type of legislative role or public health coordinating role internationally as a government within a province or nation.  As a result, efforts at coordination are, of necessity, more voluntary. 

Given the risks of the emergence of new infectious diseases and the resurgence of old ones, this is a time to consider how best to develop international cooperation for to deal with future international epidemics.  Someone may find a way to provide incentives across countries that are enforceable and that lead to better coordination and appropriate amounts of action.  If not, this is an opportunity for those who want to conduct business (and run countries) with humanity in mind to think carefully about how to encourage stakeholders to act not just with local or national humanity in mind, but with all of humanity in mind. 

Ebola Tests: Sensitivity and Specificity

Yesterday's headlines on USA Today included one that stated "Nebraska Patient Cases Raises Questions about Ebola Test."  The reason for this is the fact that the Nebraska patient was a physician treating Ebola patients who came down with symptoms and was tested and found not to be Ebola positive before other clinicians eventually realized (and found in a later test) that the man in question did have Ebola.  If the physician had been treated earlier, it is possible that his outcome--being brought back to the US and treated--may have been different from the unfortunate death he suffered.

I'd like to comment on this because it raises some really important issues about diagnostic tests.  I would have expected to find some previous writing on this, but I do not.  So here it goes.

People either have a condition or do not.  We refer to those who have the condition as positive and those who dont' as negative.  Sometimes it is good to be "positive."  With the Ebola virus, it is obvious that being positive is a very bad thing.

Test results are also positive or negative.  (Some tests are uncertain, but for the time being we will ignore that.)  So, the result at the end of a diagnostic test suggests that a person either has or does not have a condition.

What makes this most interesting is the fact that there are some people who are truly positive and for which the test identifies the person as positive.  Similarly, there are some people who are negative and whom the test identifies as negative.  These are both desirable outcomes.

However, there are also two other groups.  There are some people who are positive whom the test incorrectly identifies as negative.  And there are some people who are negative whom the test incorrectly identifies as positive.  

A test's ability to correctly identify those who are positive as positive is called sensitivity.  A test's ability to correctly identify those who are negative as negative is called specificity.

In different situations, clinicians and policy makes focus on either sensitivity or specificity.  Sometimes it is important not to over identify cases.  If the consequences of being positive are fairly small, there will be future attempts to diagnose, and further identification and treatment is costly, we can focus on making sure to identify negatives properly--specificity.  Or if there are strong negative connotations and stigmas associated with being positive it can be important to correctly identify negatives.

However there are other cases in which failing to identify a positive case can be deadly.  This is true with Ebola.  This is why in other cases sensitivity is critical.  This does not mean that there should not be concerns about specificity.  If we had a lot of people mis-identified as positive this would require an enormous amount of resources and there may be side effects of the treatment.

The main "question" I think we should asking about the Ebola test is whether there is a way to develop a test that is more sensitive.  And quickly.  And if the government can support that and someone can find it, there may be a profit to be made from an innovation for humanity.