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Explainer: ridding the world of polio

Polio is still not adequately controlled in Pakistan, Nigeria and Afghanistan. Wen-Yan King/Flickr, CC BY-NC-SA

Polio (poliomyelitis) is a viral disease that can lead to incurable paralysis. The World Health Organisation is coordinating a programme to eradicate this disease from the face of the earth, and we are very close to achieving this goal.

But polio is still not adequately controlled in Pakistan, Nigeria and Afghanistan. The eradication program will remain at risk until all countries have eliminated the virus.

The polio virus is an enterovirus, which means it infects the gut. But it can also occasionally affect the nervous system where its most feared symptom is permanent paralysis. This occurs in about 1% of infected people. Paralysis can be life-threatening if, for example, the muscles of breathing or swallowing are affected.

Vaccination can prevent the illness. Two highly effective and widely used vaccines are available: oral polio vaccine and inactivated (injectable) polio vaccine.

The inactivated vaccine is made from killed polio virus. And the oral vaccine is a live vaccine made from a strain of the virus known as the Sabin strain. This strain has been specially bred in the laboratory for use in the vaccine.

The oral vaccine stimulates local immunity in the gut, and is cheaper. But it also has disadvantages as it can, in rare cases, cause paralysis.

Countries choose a vaccine after considering their degree of risk for polio. Most low-risk developed countries use the inactivated polio vaccine.

The World Health Organisation (WHO) decided to eradicate polio from the globe in 1988. That year, over 350,000 cases had been reported in more than 125 countries, but there has been a tremendous reduction in the number of cases since.

In 2000, the WHO certified the Western Pacific Region (37 countries, including Australia) as being free of polio. Worldwide, between January 1 and August 6, 2014, only 135 cases were reported from ten countries.

Polio viruses are classified according to serotypes and genotypes. Genotypes are “family trees” of related viruses, and each serotype contains many different genotypes.

The immune system can distinguish the different serotypes, and the immune response is directed against each serotype separately.

There are three different serotypes of the polio virus. Because immunity to one doesn’t give immunity to another, vaccines are usually directed against all three together.

Within each of the three serotypes, there are many different genotypes. Immunity to one genotype within a serotype also means immunity to other genotypes within that serotype. Genotypes of the virus can only be distinguished from one another using nucleic acid fingerprinting techniques.

Neither serotypes nor genotypes affect the symptoms of clinical disease.

Because oral polio vaccine is a live virus vaccine, on rare occasions (about one per million recipients) it can cause paralysis, known as vaccine-associated paralytic polio.

The virus of oral vaccine can also sometimes spread from a vaccinated person to other, unvaccinated people. People infected this way can also, although rarely, develop paralysis.

If the percentage of the population vaccinated against polio is low, the vaccine virus can spread. This is known as a circulating vaccine-derived polio virus. We can identify such strains of the virus using nucleic acid fingerprinting.

Viruses found in countries where polio of local origin still occurs are known as “wild” viruses. Different genotypes of wild virus have been found in different countries.

Using this knowledge, and coupled with nucleic acid fingerprinting, we can work out the geographical origin of a wild virus. It was through this kind of analysis that we confirm whether a country has eliminated local wild virus.

The current global status of polio includes both wild and vaccine-derived viruses, both of which are circulating at very low levels in parts of the world’s population. The challenge now is to eradicate both.

Only three countries have wild polio virus of local origin – Nigeria, Afghanistan and Pakistan. Between them, these countries had reported a total of 117 cases between January 1 and August 6, 2014. They have caused a further 18 cases in seven other countries during that time.

We are now in the situation where virus derived from the vaccine may cause more paralytic polio cases than the wild virus. This poses a very serious ethical and technical challenge.

To address it, the WHO has developed a polio endgame strategy. It aims to eradicate wild and vaccine-derived polio as quickly as possible, and recommends that countries using the oral vaccine should change to the injectable one.

This changeover will work best if it’s done in a coordinated way. The plan is for all countries to start using the injectable vaccine by 2017.

We have completely eradicated two of the three serotypes of wild polio virus from the face of the earth. To protect the health of all future generations of children, it’s critically important that we maintain this achievement and go on to eradicate the last serotype.

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