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Updated: June 13, 2013 01:12 IST

Injectable polio vaccine

R. Prasad
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Viruses in OPV can become neuro-virulent and spread, causing polio in under-immunised, susceptible children. Photo: S.James
Viruses in OPV can become neuro-virulent and spread, causing polio in under-immunised, susceptible children. Photo: S.James

Instead of shielding children from polio, oral polio vaccine causes disease

A 10-month-old baby in Beed district, Maharashtra who recently got infected by a vaccine-derived poliovirus (VDPV) type 2 is yet another reminder that despite India being declared polio free more than two years ago, children are getting infected with the polio virus. The only difference is that the virus in question is not the wild type but a vaccine-derived one.

As per the WHO definition, since only wild-type infections are taken into consideration for deciding the polio-free status of a country, the latest infection will not alter India’s polio status. But that technical difference offers little consolation to the victim and many other at-risk children.

“True polio eradiation is zero incidence of polio virus infection, by both wild and vaccine viruses. This new definition was universally accepted only last year,” said Dr. T. Jacob John, member of the WHO committee on global polio eradication.

The root cause of the problem is the use of live, weakened polio viruses in the oral polio vaccine (OPV) for inducing immunity against the wild-type virus. Although weakened, the viruses are still alive and can “quite often” undergo genetic changes (back-mutate) to cause neuro-virulence (polio). Hence the very vaccine that is supposed to protect children against polio causes the disease.

According to a 2000 paper in The Lancet, unlike in the case of the inactivated polio vaccine (IPV) given as injections, no trial was ever conducted on OPV before it was used in USSR and other east European countries.

Vaccine-associated paralytic polio (VAPP) is an adverse reaction to OPV and occurs when the live, attenuated viruses used in the vaccine, which are genetically prone to reversal, cause neurovirulence. The virus itself may not transmit to other children.

“There are only 15 per cent genetic differences between wild polio viruses and the weakened viruses used in oral polio vaccine,” said Dr. John. The weakened viruses in OPV are genetically highly unstable and have a tendency to drift back to the wild (back-mutate) and become neuro-virulent.

Aside from becoming neuro-virulent, the viruses can attain the capability of spreading causing polio in under-immunised, susceptible children, as in the current case.

“Therefore, vaccine-derived poliovirus is epidemiologically riskier [than VAPP] for the community,” he said. “These genetically reverted vaccine-derived viruses can silently spread and cause polio in children who are not sufficiently vaccinated.”

India had 21 VDPV polio cases in 2009, five in 2010 and seven in 2011 and one in 2012. But by more OPV campaigns, VDPV can be stopped; thus controlling VDPV is easier than in the case of the wild virus.

“Everybody knew about OPV’s neuro-virulence,” Dr. John said. “I picked up the transmissibility issue early, but nobody listened. In general, live, weakened viruses used in vaccines are supposed to be highly stable and non-transmissible. But OPV breaks both these rules.”

Worse, VDPV strains can silently circulate (cVDPV) for many months, even 1-2 years before showing up with polio cases,” notes a 2013 paper in the Indian Journal of Medical Research.

Hence the chances of the silently spreading cVDPV causing polio cases in children can be expected when OPV is discontinued.

The polio endgame is to introduce IPV and continue using oral vaccine and stop using OPV once India attains high levels of injectable vaccine coverage using IPV.

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