Syrian outbreak exposes challenges to polio eradication

The world was alarmed by a recent polio outbreak in Syriawhich used to be one of the many polio-free nations. Nigeria, Pakistan and Afghanistan are the last three remaining countries in which the disease is still present, but the re-emergence of polio in other parts of the globe raises concerns on current polio eradication programs and which forward steps need to be taken.

Poliovirus (PV), the virus that causes polio, reappeared in Syria most likely due to the civil war. Vaccines that protect individuals from disease and prevent the spread of virus within a population are currently available for polio. But the civil war has triggered a drop in the vaccination rate, thereby causing at least 17 reported polio cases in Syria. In addition to that, many non-vaccinated people are carriers of PV even though they do not appear to have the disease.The Syrian plight has caused civilians to flee the country, and among them are PV carriers who may apprehensively spread the disease in neighboring countries and even beyond. In fact, dozens of environmental samples in Israel, a country next door to Syria, tested positive for wild-type PV last year, which was alarming for a developed and rich country with a high vaccination rate (>95%). A closer look into the vaccination program of Israel provides some explanation. Since 2005, Israel adopted an inactivated polio vaccine (IPV)-only vaccination strategy, mainly because IPV has a better safety profile than its counterpart, the oral polio vaccine (OPV). IPV offers excellent protection against disease, but unlike OPV, it does not prevent viral transmission. Recognizing this weakness in the IPV-only vaccination program and its potential involvement in the recent silent spread of PV throughout the country, OPV was reintroduced into Israel’s routine vaccination program in October 2013.

Despite OPV’s popularity, especially in developing countries, as a stable, cheap and conveniently administered vaccine, it may soon outlast its usefulness. In rare cases (1 in 2.7 million), the “live” attenuated virus present in the vaccine may acquire mutations by which it becomes virulent again. As a result, the vaccinee may become paralyzed, a condition known as vaccine-associated paralytic poliomyelitis. Another drawback of OPV are vaccine-derived polioviruses, which may start to circulate and cause disease in under-immunized populations upon excretion in the stool of vaccinated children. This is further complicated by prolonged excretion (up to 22 years), which has already been documented for 33 immunocompromised people who received OPV.Polio map

Once transmission of wild poliovirus has been halted, OPV will be the only source of PV in the community. Therefore the use of OPV will eventually be stopped and IPV will be the only “weapon” available to battle future outbreaks. However, the recent events in Israel illustrate that IPV alone is not enough to eradicate the virus during this post-OPV era. It is about time that effective alternatives are sought out. For example, antivirals that treat chronic PV excreters may hold the key to stop future outbreaks and completely eradicate the virus. Thus, although the world is now 99% free of polio, we must remain vigilant against this disease and continue to pursue better options for polio eradication.

Reference List

1.    Eichner, M. and S. O. Brockmann. 2013. Polio emergence in Syria and Israel endangers Europe. Lancet 382:1777. doi:S0140-6736(13)62220-5 [pii];10.1016/S0140-6736(13)62220-5 [doi].

2.    MacLennan, C., G. Dunn, A. P. Huissoon, D. S. Kumararatne, J. Martin, P. O’Leary, R. A. Thompson, H. Osman, P. Wood, P. Minor, D. J. Wood, and D. Pillay. 2004. Failure to clear persistent vaccine-derived neurovirulent poliovirus infection in an immunodeficient man. Lancet 363:1509-1513.

3.    Roberts, L. 2013. Infectious disease. Israel’s silent polio epidemic breaks all the rules. Science 342:679-680. doi:342/6159/679 [pii];10.1126/science.342.6159.679 [doi].

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