For India, which has successfully kept naturally-occurring ‘wild’ polioviruses at bay for three whole years, a new challenge looms.

India is among 140 countries that rely on the oral polio vaccine (OPV). These countries have now been asked to introduce an injectable inactivated polio vaccine (IPV) into their routine childhood immunisation programme by the end of next year.

The oral vaccine, which is cheap and easily administered, uses live but weakened forms of the poliovirus. But the live vaccine viruses can occasionally revert to virulence.

Vaccine-derived viruses can gain the ability to transmit within communities and even pass from one country to another. Such ‘circulating vaccine-derived polioviruses’ (cVDPV) have struck over 700 children since the year 2000, producing outbreaks in several countries, including Pakistan, Afghanistan, Nigeria and Somalia.

More than 95 per cent of the cVPDV cases in recent years have been of the type 2 strain (the poliovirus has three strains, types 1, 2 and 3). Polio caused by a wild type 2 virus was, on the other hand, last seen 15 years back.

The Global Polio Eradication Initiative (GPEI), which coordinates the global fight again polio, therefore wants to stop all use of OPV that contains the type 2 vaccine strain. Trivalent OPV, with all three types of vaccine strains, is to be replaced by bivalent OPV with only type 1 and type 3 vaccine strains.

But before making that switch, the GPEI has asked all countries using trivalent OPV to introduce at least one dose of the injectable IPV vaccine into their routine immunisation programme. IPV, which is more expensive, uses killed forms of the three types of wild viruses and carries no risk of reversion to virulence. This vaccine would provide protection against any type 2 vaccine-derived viruses lingering in the environment.

Trials carried out with IPV in Tamil Nadu around the 1980s by veteran virologist T. Jacob John and his colleagues at the Christian Medical College at Vellore had found that the injectable vaccine was very effective against the poliovirus in an Indian context. An independent effort that tested IPV in Mumbai slums came to the same conclusion.

If IPV had been introduced in India’s routine immunisation programme and OPV used in mass campaigns, “we could have eliminated polio decades ago,” writes Dr. Jacob John in an article appearing in the Economic & Political Weekly next month.

A multi-centre study under way in the country is examining the immunity that would be provided when bivalent OPV and IPV are employed in routine childhood immunisation.

But for IPV to be effectively deployed, routine immunisation coverage has to be improved. According to the National Family Health Survey of 2005-06, while over 80 per cent of children in Tamil Nadu had received all basic vaccinations, only 23 per cent of those in Uttar Pradesh and 33 per cent in Bihar were similarly covered.

Lessons from India's success in polio eradication are being used to enhance routine immunisation and reach under-served communities. This includes drawing up comprehensive micro-plans for routine immunisation, intensively training frontline health workers who will carry out vaccinations and putting in place monitoring systems so that corrective measures can be taken when needed.

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