Vaccines are the best. Seriously, the first thing you would ever ask for — if charged with eradicating or decreasing the incidence of a given disease — is an effective vaccine against it, which makes anti-vaccination movements all the more perplexing, and vexing. Plenty have written about what fuels some of this animus: distrust of the medical establishment, ignorance about risk-benefit calculations, and general suspicion of the pharmaceutical industry. Yet, what makes some of these narratives occasionally difficult to refute convincingly are the nuggets of truth they sometimes contain.
For instance, vaccines can and do cause serious adverse effects, in very rare cases for the most part. Remaining vigilant about those risks is important, but so is the utilitarian concern that weighs the benefits (in lives saved or illnesses prevented) against the risks (in deaths, neurologic problems, and other adverse effects). In the vast majority of cases, that comparison works out overwhelmingly in favor of immunization as a category, even as the efficacy and risk profile varies depending on the vaccine.
Today, for World Polio Day, the World Health Organization (WHO) posted a short informational page on poliomyelitis, a highly infectious disease that results in lower limb paralysis in a number of those who contract it (about 1 in 200). Once a common infection, vaccines have drastically reduced the burden of polio in both the developed and developing worlds.
Polio cases have decreased by over 99% since 1988, from an estimated 350,000 cases in more than 125 endemic countries then, to 416 reported cases in 2013. These included only 160 cases in endemic countries; international spread from endemic areas into polio-free areas accounted for the remainder.
In 2014, only parts of 3 countries in the world remain endemic for the disease–the smallest geographic area in history. Of the 3 strains of wild poliovirus (type 1, type 2, and type 3), wild poliovirus type 2 was eradicated in 1999 and case numbers of wild poliovirus type 3 are down to the lowest-ever levels with the no cases reported since November 2012 from Nigeria.
However, the oral polio vaccine contains a weakened strain of the virus itself, which can in rare instances lead to the establishment of a new circulating strain. As I learned today, this is referred to as vaccine-derived polio. Interested in the burden of such cases, I looked it up and came across a beautiful illustration of the risk-benefit issue. Again we turn to the WHO, this time to their Question & Answer on vaccine-derived polio:
When a child is immunized with OPV [oral polio vaccine], the weakened vaccine-virus replicates in the intestine for a limited period, thereby developing immunity by building up antibodies. During this time, the vaccine-virus is also excreted. In areas of inadequate sanitation, this excreted vaccine-virus can spread in the immediate community (and this can offer protection to other children through ‘passive’ immunization), before eventually dying out.
On rare occasions, if a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. In very rare instances, the vaccine-virus can genetically change into a form that can paralyse – this is what is known as a circulating vaccine-derived poliovirus (cVDPV).
How often does this happen? According to their Q&A:
Since 2000, more than 10 billion doses of OPV have been administered to nearly 3 billion children worldwide. As a result, more than 10 million cases of polio have been prevented, and the disease has been reduced by more than 99%. During that time, 20 cVDPV outbreaks occurred in 20 countries, resulting in 758 VDPV [vaccine-derived polio virus] cases.
Do that math.
Update (10/24): I should have been a little more precise in my wording. Polio infections have been eliminated in many areas, while eradication may remain a ways off. For a brief discussion of the obstacles still standing in the way of full eradication, I point you to this editorial in the Lancet. I have updated the title of this post to reflect what I think is a better approximation of the situation, but this page on disease elimination and eradication from the Centers for Disease Control and Prevention (CDC) discusses some of the ambiguities between the terms.