Darin Seehafer
Darin Seehafer
Clinical Scientist, PRA Vaccine Solutions

While much progress has been made to eradicate polio, the disease remains persistent in some areas of the world. This is true in Syria where ongoing conflict is exacerbating the problem. Syria mandated vaccination in 1964 but civil war has brought several outbreaks over the past few years. In June, 17 more suspected cases were reported in areas partly held by the Islamic State. Despite permission from both the rebel group and the Syrian government to continue vaccinations, aid agencies estimate at least 400,000 Syrian children under the age of five have yet to be immunized.

Recent internal conflict in Nigera has also impacted polio eradication efforts. Despite its own intense immunization efforts, new cases appeared after Boko Haram kidnapped hundreds of young women in 2014, and, despite mass emergency vaccination campaigns, many children in northern regions controlled by that group are beyond the reach of vaccines.

Health systems under stress mean populations are not adequately vaccinated and that the opportunity for an outbreak is high. In war zones with high migratory populations, it is essential to ensure the vaccination status of all displaced refugees so that polio is not further spread beyond borders.

Polio outbreak in syria map

The discovery of more new cases in Syria is a fresh blow to hopes of eradicating the disease globally – aid agencies, who have made a final push toward eradication one of its highest priorities, called the situation “a public health emergency of international concern.” Without eradication, they say, a resurgence of polio could paralyze more than 200,000 children worldwide every year within a decade. The CDC recently called for immigrant and refugee children to be vaccinated against all three types of polio as a precaution in the United States.

But the reasons for polio’s resistance to eradication are varied and complex – on one hand, too many children are going without the required doses to protect against wild poliovirus; on the other hand, many new cases are derived from the live attenuated vaccine itself, which is shed in fecal matter. That shedding process should provide herd immunity but, as is now seen in unstable regions, the virus can mutate, spread, and cause new infections.

In Syria, health workers are concerned that vaccinating even 50 percent of the hundreds of thousands of children in immediate need there may not be enough to stop the outbreak. Immunization rates need to be closer to 80 percent to provide herd immunity and to protect an entire population.

1894: First known polio outbreak in the U.S. with 132 cases.

1916: Major epidemic with 27,000 cases and 6,000 deaths.

1950s: The injected Salk vaccine is deemed safe and church bells ring across the country.

1979: Polio in America becomes a fear of the past.

Boy receiving vaccine in syria

It’s clear that polio viruses still pose a very real threat, and that strong immunization practices, including surveillance and closing immunity gaps are necessary to make their presence a real thing of the past. If eradicated, polio would become the second disease to disappear after smallpox – and such a feat would be an enormous public health success.

The OPV Paradox

Waterborne and highly infectious, polio is caused by a virus in the throat and intestinal tract and usually spread through contact with the stool of an infected person or through oral/nasal secretions. While most infected people have no symptoms, many become paralyzed and even die.

Two types of vaccine protect against polio: inactivated poliovirus vaccine (IPV), and oral poliovirus vaccine (OPV). Jonas Salk famously developed IPV in 1955 with inactivated (killed) strains of all three poliovirus types. Safe and effective, IPV is five times more expensive than OPV. For full protection, children need four doses between the ages of two months and six years.

Developed by Albert Sabin, the oral vaccine replaced IPV for U.S. immunizations in the early 1960s until evidence appeared that it could cause vaccine-related paralysis. IPV then came back into favor and has been used in most industrialized, polio-free countries since 2000.

In addition to Syria and Nigeria, suspected polio outbreaks in recent years: Chad, Democratic Republic of the Congo, Laos, Haiti, Kenya, Ukraine

OPV is attenuated, not killed, and also produces antibodies to the three polio strains. In the event of infection, the vaccine protects against paralysis by stopping the virus from spreading to the nervous system. An immune response in the intestines where the wild, or naturally occurring, virus tends to multiply helps prevent person-to-person transmission.

But OPV is less than perfect. Too often, when the strains revert to wild poliovirus and are excreted by immunized children, OPV use can lead to vaccine-associated paralysis.

Despite concerns, OPV continues to be widely used in developing countries for both routine vaccinations and to contain outbreaks. Its low cost and ease of use mean the oral vaccine has been the primary catalyst for near eradication around the globe.

From OPV back to IPV: Key to Eradication?

Alarmed by the growing number of vaccine-derived polio cases, the WHO recently recommended that OPV be globally replaced by IPV. That radical change, if carried out, is not likely to happen right away, but it could eventually eliminate the stubborn problem of polio caused by vaccines.

In the meantime, because the type 2 component of OPV is the most likely of the three types to regain virulence, global aid agencies are conducting a maneuver called “the switch,” in which all countries using oral vaccine have been asked to stop administering the three-component type of vaccine and to adopt a bivalent version that only protects against type 1 and type 3 polioviruses. The best approach would be to use OPV judiciously while ensuring the highest level of coverage and then gradually phasing out the oral vaccine so that the disease can never be re-established.

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