Polio Still a Threat to Public Health Monday, November 7, 2011
Live virus used in polio vaccine can evolve and infect, warns TAU researcher
Health professionals and researchers across the globe believe they are on the verge of eradicating polio, a devastating virus which can lead to paralysis and death. Despite successful eradication in most countries, there are still four countries where the virus is considered endemic — and many more in which the virus still lurks.
Dr. Lester Shulman of Tel Aviv University's Sackler Faculty of Medicine and the Israeli Ministry of Health has spent years tracking isolated cases of live poliovirus infections, often discovered in countries that are supposedly polio-free. When the live-virus version of the vaccine, called Oral Polio Vaccine (OPV) evolves, he says, it can act like wild poliovirus and continue the threat of contagion.
Medical professionals widely believe that after the wild virus is eradicated, resources dedicated to polio immunization can be redirected. But this isn't so, he says. He recommends that public health agencies take a three-pronged approach: Vaccination policies to maintain "herd immunity" (a 95 percent immunization rate for polio) 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.
Dr. Shulman's research was recently published in PLoS ONE. He has also been invited as an informal expert to the World Health Organization's annual meeting on polio this fall.
A decade-long chase
While the eradication of polio is seemingly within reach, this is not the time to relax, Dr. Shulman warns. Most countries only investigate 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 percent genomic change in the virus permits the virus to behave like a wild poliovirus. If a population isn't sufficiently immunized, this spells trouble.
Israel is among the few countries that practice environmental surveillance for polio, beginning in 1989. Checking designated sites along sewage systems every month for evidence of the virus allows for early detection before there are paralytic cases. For the past decade, 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.
Dr. Shulman says 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 explains. For example, he and his colleagues traced a wild poliovirus discovered in sewage from the Gaza District to a village in Egypt.
New hope for curing persistent infections
Convinced by the efficacy of Israel's environmental surveillance program, 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, Dr. Shulman sees 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.
The fellow 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 mutants strains have not become resistant to the drugs under investigation. But for now, Dr. Shulman recommends that health authorities continue immunization using inactivated vaccines (IPV) to keep their populations safe.
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