Injectable polio vaccineR. PrasadViruses in OPV can become neuro-virulent and spread, causing polio in under-immunised,susceptible children. Photo: S.JamesInstead of shielding children from polio, oral polio vaccinecauses diseaseA 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 poliofree more than two years ago, children are getting infected with the polio virus. The onlydifference 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 fordeciding 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-riskchildren.“True polio eradiation is zero incidence of polio virus infection, by both wild and vaccineviruses. 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 arestill 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 thedisease.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 andother east European countries.Vaccine-associated paralytic polio (VAPP) is an adverse reaction to OPV and occurs when thelive, attenuated viruses used in the vaccine, which are genetically prone to reversal, causeneurovirulence. The virus itself may not transmit to other children.“There are only 15 per cent genetic differences between wild polio viruses and the weakenedviruses used in oral polio vaccine,” said Dr. John. The weakened viruses in OPV are geneticallyhighly 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 causingpolio in under-immunised, susceptible children, as in the current case.“Therefore, vaccine-derived poliovirus is epidemiologically riskier [than VAPP] for thecommunity,” he said. “These genetically reverted vaccine-derived viruses can silently spread andcause 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 bymore OPV campaigns, VDPV can be stopped; thus controlling VDPV is easier than in the caseof the wild virus.“Everybody knew about OPV’s neuro-virulence,” Dr. John said. “I picked up the transmissibilityissue early, but nobody listened. In general, live, weakened viruses used in vaccines are supposedto 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 beforeshowing 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 beexpected when OPV is discontinued.The polio endgame is to introduce IPV and continue using oral vaccine and stop using OPV onceIndia attains high levels of injectable vaccine coverage using IPV.