The last case of wild type-2 poliovirus was seen in West Bengal way back in 1999. Since then no child in the world has been stuck by wild type-2 poliovirus nor has the virus been seen in the environment anywhere in the world. This is the reason why the Global Polio Eradication Initiative says that wild poliovirus type-2 has been “eradicated”. The official confirmation came in September 2015 when the Global Commission for the Certification of Poliomyelitis Eradication declared wild polio virus type 2 as eradicated globally.
Yet, many newspapers (The Times of India, Deccan Chronicle, DNA, The Indian Express and The Hindu) reported that a “wild” type-2 poliovirus was detected in a water sample collected recently from a sewage treatment plant in Amberpet, one of the high-risk pockets in Hyderabad. The sample was collected on May 16 and was reported as positive for VDPV on June 7. So from where did the “wild” type-2 poliovirus suddenly emerge after not been found in the environment for the last 17 years (since 1999)? The culprit was a Press Trust of India (PTI) report that was carried by all newspapers except The Hindu. This is nothing but a classic example of lack of application of mind.
The BBC went a step further to say, “vaccinations against the “wild” polio virus — which occurs naturally — were stopped in 2011 after no new cases of the disease were reported.”
The global polio eradication programme would have suffered a major setback if the type-2 virus found in Hyderabad was indeed a wild strain and not a vaccine derived poliovirus (VDVP). It would have been biggest news of the day and every newspaper in the world would featured it prominently on the front page.
In is essential to remember that all polio cases caused by type-2 virus since 1999 have been caused only by vaccine-derived poliovirus (VDPV). The type 2 strain in the trivalent OPV (containing all three strains of polio virus) has caused over 90 per cent of VDPV cases in the world in the last 10 years.
In the case of wild type-3 strain, the last time it was found in the environment was in November 2012. In all probability wild type-3 has been eradicated, although it is too early to confirm it.
The oral polio vaccine contains live, weakened viruses. In rare instances, the live, weakened strains can continue to multiply for long and mutations may arise. If there are six or more nucleotide changes in the type-2 weakened virus then it is called vaccine-derived poliovirus (VDVP) type-2. Where hygiene is poor and children are not sufficiently protected through polio vaccination, the vaccine-derived poliovirus can cause acute flaccid paralysis (AFP).
According to the AFP surveillance data up to May 21, 2016, in the year 2016, there have been two instances when type 2 VDPV was found in the environment in Delhi. And there has been one case of AFP caused by type-2 VDPV in Bihar in 2016.
According to a PIB release, between Januaru 2015 and May 2016, 14 sewage samples collected from across the country tested positive for VDPV. “All of these have been responded to urgently and appropriately with polio vaccination campaigns. None of the VDPVs detected in the sewage infected any children,” it says.
It is only because type-2 virus strain is no longer present in the environment and all polio cases caused by type-2 strain since 1999 have been due to vaccine derived poliovirus that a global switch from a trivalent (containing all three strains of the virus) oral polio vaccine to a bivalent (only type-1 and type-3) OPV was made across 155 countries between April 17 and May 1, 2016.
Polio — whether caused by wild strain or vaccine-derived poliovirus — can be eradicated only when oral polio vaccine is eventually withdrawn after wild polio transmission has been stopped. The just concluded withdrawal of type 2 strain from OPV by switching over to a bivalent OPV is the “first major step” of this withdrawal process.