Progress on Polio in Africa?

From the latest CDC Morbidity and Mortality Weekly Report: “Progress Toward Interrupting Wild Poliovirus Circulation in Countries with Reestablished Transmission — Africa, 2009-2010”

There are only four countries where polio is still “endemic” — Afghanistan, Pakistan, India, and Nigeria. Combined the four endemic countries have about 23% of the world’s population, though to be fair polio is only endemic in some portion of each country.

But the actual definition of “endemic” may not match with lay assumptions about that term. For polio, endemic countries are defined as those where transmission has never been broken. So a country where polio has been reintroduced — and is now spreading on its own, without the need for additional introductions — is by definition still not endemic. Thus, there’s essentially a three-tiered system: a) endemic countries, b) countries with reestablished transmission, and c) countries without established transmission, which may have sporadic outbreaks from imported cases or from vaccine-derived polio.

The CDC report linked above provides an overview of polio in African countries. Between 2002 and 2009 several dozen previously polio-free countries had outbreaks of polio from strains imported from India or Nigeria. (The strain of polio in each outbreak is genetically typed, which means we can determine which known strain the new one is closest too, and thus from whence the outbreak came.) Of those countries, four–Angola, Chad, Democratic Republic of the Congo (DRC), and Sudan–had persistent transmission (more than one year) after re-importation of polio that occurred before 2009. One of the milestone of the Global Polio Eradication Initiative (GPEI) was that polio transmission would be interrupted in those four countries by the end of 2010. The conclusion of the MMWR report is that it has been stopped in Sudan, but not Angola, Chad, or DRC.

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03 2011

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