Thursday, November 20, 2014

Let’s Talk About Eradication: Polio Type Three

A child receives the OPV
This week’s Morbidity and Mortality Weekly Report from the CDC indicates progress towards eradication of the third serotype of polio, wild poliovirus 3 (WPV3).

In the report, the CDC explains that regions are only designated as polio free if 3 or more years have passed since the last case of poliovirus. The Americas, Western Pacific Region, European Region, and the South-East Asia region have been certified as polio-free. Since then, the World Health Organization has turned its attention to the remaining endemic countries: Nigeria, Afghanistan, and Pakistan.  

Interestingly, no new cases of wild poliovirus 3 have been observed for two years, since November 2012. In addition to the absence of new cases, the World Health Organization has reported that the genetic diversity of collected WPV3 samples has diminished since eradication efforts first began, further suggesting that viral replication and persistence of WPV3 has similarly diminished. (Genetic diversity is characterized as having a genome that is at least 15% divergent from other strains.)

The WHO monitors polio by monitoring the number of cases of acute flaccid paralysis in the world, in addition to testing sewage samples for the poliovirus. Monitoring and surveillance programs in Pakistan and Nigeria have been improved, adding further credibility to recent evidence.

According to the report, an effective elimination strategy has been switching to bivalent polio vaccines for types 1 and 3 instead of immunizing with monovalent or trivalent vaccines for types 1, 2, and 3.

The report concludes by acknowledging that continued surveillance is necessary to confirm eradication. If successful, this will be the third time a human pathogen will have been eradicated (including smallpox and WPV2).  

-Luis Garcia

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