TAU, Health Ministry: World hasn’t wiped out polio

Live virus used in vaccine can evolve; researcher recommends use of inactivated strain for inoculations.

November 9, 2011 06:19
3 minute read.
Doctor gives vaccination [file]

Vaccination 311. (photo credit: Thinkstock)


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It is a mistake to believe that the deadly and incapacitating polio virus has been eradicated by vaccination, according to a Tel Aviv University researcher.

After years of study, Dr. Lester Shulman has found that the live virus used in the vaccine can evolve and continue to infect children and young adults, according to the Sackler Faculty of Medicine researcher.

Public health professionals and researchers around the world have been preparing to celebrate the eradication of the virus with the vaccine, which was originally available five decades ago. But, said Shulman – who has spent years tracking isolated cases of live poliovirus infections, often discovered in countries that are supposedly polio-free – there are still a handful of countries where the virus is considered endemic and many more in which the virus still lurks.

His research was recently published in One, the Public Library of Science medical journal. He has also been invited as an informal expert to the World Health Organization’s annual meeting on polio this fall.

When the live-virus version of the vaccine, called Oral Polio Vaccine (OPV) and developed by Dr. Albert Sabin, evolves, it can act like wild poliovirus and continue the threat of contagion, Shulman warned.

Medical professionals widely believe that after the wild virus is eradicated, money that went to vaccinating children against polio can be spent on some other health target. But this isn’t so, he said, recommending that public health authorities take a three-pronged approach. Vaccination policies to maintain “herd immunity” (a 95 percent polio immunization rate) should be maintained to prevent the spread of wild and evolved vaccine strains of the virus; environmental surveillance of sewage systems should continue; and a switch to Inactivated Polio Vaccine (IPV) instead of OPV should be implemented.

While the eradication of polio is seemingly within reach, this is not the time to relax, warned Shulman, who works together with Health Ministry virology experts. Most countries investigate only the possibility of poliovirus outbreaks when paralytic cases appear in the human population.

But this doesn’t take into account a potential problem posed by the live-virus vaccine.

Over time, the vaccine can mutate, and even a 1% change in the virus’s genome permits it to behave like a wild poliovirus.

If a population isn’t adequately covered by immunization, trouble is afoot.

Since 1989, Israel has been one of the few countries to practice environmental surveillance for polio. Checking designated sites along sewage systems every month for evidence of the virus allows for early detection before there are cases of paralysis. Over the past 10 years, the researchers have been trying to trace the origin of the strain that infected two individuals in central Israel.

They tracked the strains to the sewage system and have been working to pinpoint the origin.

Fortunately, because Israel maintains herd immunity for the disease, the wider population has not been threatened.

Shulman wrote that in the lab, each strain of the virus can be identified from its genomic structure and traced back to the region from which it originated.

“From the sequence of the genome, you can match it with known sequences reported by labs throughout the world,” he explained. For example, he and his colleagues traced a wild poliovirus discovered in sewage from the Gaza district to a village in Egypt.

Seeing how Israel’s environmental surveillance program has done so well, many other countries are starting to develop tracking programs of their own. As a result, they are finding evidence of vaccine-derived polio cases in humans. Paradoxically, Shulman regards this a beacon of hope in these discoveries.

As labs across the world report more cases, researchers gain a better understanding of how polioviruses establish persistent infections and can then develop effective measures to eliminate them.

These researchers are now working to develop compounds that can effectively fight these rare cases of persistent poliovirus infections. So far, they have seen promising results, noting that the mutant strains have not become resistant to the drugs under investigation.

But for now, Shulman recommends that health authorities continue immunization using inactivated vaccines to keep their populations safe.

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