What is a “rare disease”? Polio eradication and primary immunodeficiency

During the last two decades, surveillance and strategic vaccination campaigns deployed by the Global Polio Eradication Initiative (GPEI) have reduced polio incidence worldwide by more than 99 percent. Wild poliovirus (WPV) cases are now uncommon, with only 222 new cases reported worldwide so far in 2014.* Endemic transmission is now limited to areas of just three countries—Afghanistan, Nigeria, and Pakistan. Of the three WPV strains, only one will remain if, as now seems likely, WPV3 follows WPV2 into oblivion.

The Journal of Infectious Diseases supplement published on Nov 1, 2014, provides an overview of recent progress toward polio eradication, as well as current challenges in surveillance, risk assessment and mitigation, and post-eradication strategies to make sure polio never returns.

In The New Polio Eradication End Game, members of the World Health Organization (WHO) polio team lay out the intricate maneuvers that will be required to eliminate all polioviruses from human populations, including both wild viruses and those derived from oral polio vaccine (OPV).

OPV is a live, attenuated vaccine that is used for eradication because it is effective, inexpensive, and easy to administer. However, the weakened viruses used in the vaccine are genetically unstable; they are capable of regaining virulence and transmissibility and causing outbreaks in populations with low vaccination rates.

Vaccine-derived polioviruses (VDPVs) can be identified by divergence of their genetic sequences from those of vaccine strains. Greater divergence can suggest ongoing transmission of infection or prolonged viral replication, for example in the intestinal tracts of persons with primary immunodeficiency (PI) disorders. Unlike acquired causes of immunodeficiency, such as human immunodeficiency virus (HIV) infection, PI disorders are caused by inherited (genetic) defects in specific components of the immune system.

A study in seven countries** estimated the prevalence of poliovirus shedding in previously vaccinated persons with a PI disorder. Overall, polioviruses were found in 17/562 (3%) of persons tested; 15 of these viruses were types included in OPV; the other two were sufficiently divergent to be classified as immunodeficiency-related VDPVs, or iVDPV’s. The authors concluded:

[T]he risk of poliomyelitis caused by iVDPV may persist as long as there are persons excreting iVDPVs. The surveillance for these viruses will therefore become increasingly important, and WHO has suggested that this study series serves as a roadmap for the development of national surveillance systems to detect and respond to iVDPVs in parallel with well-established AFP [acute flaccid paralysis] surveillance systems.

As polio transmission approaches the vanishing point, PI disorders gain a higher public health profile. Data on the prevalence of PI could help inform screening and case-finding strategies; however, such data are limited, even in the developed world. A recent study used health insurance claims data to estimate the prevalence of PI disorders in the United States, finding that prevalence may be higher than previously suggested.

For more information, visit the CDC Polio Program web page.

*As of Oct 8, 2014.

** Bangladesh, China, Iran, Philippines, Russia, Sri Lanka, and Tunisia.