(photo credit: Thinkstock)
It is a mistake to believe that the deadly and incapacitating polio virus has
been eradicated by vaccination, according to a Tel Aviv University
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
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
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.
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.
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.
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
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.
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
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.
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
But for now, Shulman recommends that health authorities
continue immunization using inactivated vaccines to keep their populations safe.