While one year has passed without polio caused by natural poliovirus, we can claim complete eradication only after we ensure the absence of wild and vaccine polioviruses in the population.

Today, India passes one whole year without polio caused by natural (wild) poliovirus — a major milestone towards polio eradication. This spells relief from an agonising decade of wild polioviruses refusing to surrender. Many experts believed that India posed the greatest challenge to polio eradication for epidemiological reasons; our success proves it can be achieved in other countries where the obstacles are more programmatic than biological. For the Global Polio Eradication Initiative (GPEI), this is a shot in the arm.

The decade of agony

The year 2000 was the target date for global eradication set by the World Health Assembly in 1988. Intense efforts by countries, guided by GPEI, resulted in success in most countries and partial success in all countries. Of the 3 types of polioviruses, type 2 was globally eradicated in 1999 — with the last case in Uttar Pradesh. But transmission of types 1 and 3 continued in six countries. Later, two more succeeded, leaving India, Pakistan, Afghanistan and Nigeria with continuing transmission beyond 2005. In India, during the last decade, over 95 per cent of cases occurred in U.P. and Bihar — arguably the world's most difficult spots for eradication. In 2000 and 2001, there were 265 and 268 cases but in 2002 an outbreak occurred with 1600 cases, mostly type 1. Then, the numbers dwindled to 225 and 134 in 2003 and 2004, and 66 in 2005. All hopes of success were shattered by another polio outbreak in 2006, with 648 cases of type 1 and 28 of type 3 polio.

Since type 1 showed a cyclical nature of outbreaks every fourth year — 1998, 2002 and 2006 — the next outbreak in 2010 had to be averted at all costs. Type 2 had taught us that sequential eradication of one type at a time was realistic. So type 1 was targeted and the tactics paid off — we had less than 100 cases each during 2007-2009, 18 in 2010 and just one case on January 13, 2011 — none since.

Type 3 cases were less than 10 in 2004 and 2005. Unfortunately, while type 1 was singled out for attack, type 3 outbreaks developed, first in Bihar (2007-08) and then in U.P. (2008-09). So in 2010, there was yet another change of tactic, now focussing on type 3 along with type 1. There were 24 cases of type 3 in 2010 and none in 2011. In U.P., the last wild virus polio was reported on April 21, 2010 and in Bihar on September 1, 2010 — both type 3. So we have now come 20 months without a case in U.P. and 15 months in Bihar. The case of January 13, 2011, was reported not from U.P. or Bihar, but from Howrah in West Bengal.

Problems & innovative solutions

By 1988, nearly 70 countries had achieved the elimination of wild poliovirus transmission through their routine national immunisation programmes, some using the inactivated poliovirus vaccine (IPV) but others using the oral poliovirus vaccine (OPV). For countries with polio, the World Health Organisation recommended the exclusive use of OPV for its low cost and ease of inoculation by mouth — as two drops. On the flip side, the very fact that many countries using OPV could not control polio with routine immunisation indicated that it was not as effective as in other countries. The difference was clear: tropical/ subtropical countries with low income, overcrowding, high birth rates, and high child mortality faced low effectiveness of OPV, whereas those with the opposites had high vaccine effectiveness.

In India, the disparities of such factors spelt differential effectiveness among States. Not only did some communities exhibit lower vaccine effectiveness, they also had more intense wild virus transmission. The conjunction of both problems made U.P. and Bihar stand out as the most difficult regions for polio eradication. Indian scientists had actually warned the GPEI of such pitfalls but global leaders from rich countries couldn't believe that such extreme variations could exist with wild virus epidemiology and vaccine effectiveness. Once that lesson was learned, the progress was rapid.

Wild polioviruses exist in 3 types, and OPV also contains attenuated strains of the 3 types. So it is called trivalent OPV (tOPV). Among the 3 types, type 2 is the most efficient; that was why type 2 wild virus disappeared in 1999, within a few years of national pulse vaccinations. But type 2 in the tOPV also interferes with the others, making them very inefficient. From 2000, the frequency of campaigns with tOPV was increased in U.P. and Bihar, but to no avail. Type 2 had to be removed from tOPV to get the best out of types 1 and 3. In 2005 and thereafter, a new monovalent type 1 OPV (mOPV-1) was used in U.P. and Bihar — it is three times more effective than tOPV. This was one factor of success. But gaps in immunity were created against type 3; consequently, type 3 outbreaks occurred in 2007-2009. Then, a bivalent vaccine (bOPV with 1 and 3) was developed. It was non-inferior to mOPV-1 or mOPV-3. From early 2010, bOPV has been widely used in U.P. and Bihar during campaigns, while tOPV is used everywhere for routine immunisation.

While the problem of “failure of vaccine” was being addressed, there was also the problem of “failure to vaccinate”. Seasonally, millions of families from U.P. and Bihar migrate for work — some to Maharashtra or Punjab, others within their States. Their children missed both routine and campaign doses. The tactic of vaccination in transit — in trains/buses and in stone quarries/brick kilns — became the norm from 2005. As all bottlenecks were cleared, success ensued.

Tribute to the nation

Many global experts marvel at the ability of Indians to work with diligence and sincerity, and at India's tenacity in spite of pessimistic prophecies of failure. So a tribute is due: to the families of children and all workers, district managers — medical and administrative — State leadership, the National Polio Surveillance Project personnel, the Government of India staff working alongside the global polio partners, WHO, UNICEF, Rotary International and the U.S. Centres for Disease Control, and the vaccine manufacturers who up-scaled production on demand, and filled the prescriptions for mOPV-1 and 3 and for bOPV. All of them deserve our applause and gratitude.

In many other programmes in India, poor implementation is the oft-repeated reason for failures and delays. The success of implementation depends on the design of the programme and proper supervision of activities. The government must learn and apply this lesson in all other faltering health projects — against TB, malaria, child mortality and under-nutrition.

What next?

For certification of eradication, two more years should pass without any case of wild virus polio. Poliovirus can remain silently in circulation for short periods; so, complacency must not set in. We must continue working as if we still have poliovirus lurking somewhere, only to show up when least expected. There is also the threat of importation of wild virus from Pakistan, Afghanistan and Nigeria.

Vaccine viruses by themselves can rarely cause polio; the balance is roughly one case of vaccine-associated paralytic polio (VAPP) replacing 200 cases of wild virus polio. Yet, in the absence of wild virus polio, VAPP is unacceptable. Moreover, vaccine viruses may gradually revert to wild-like properties if allowed to circulate. Such circulating vaccine-derived polioviruses (cVDPV) cropped up in many OPV-using countries recently, including India since 2009. If allowed to grow, they can capture the niche vacated by wild viruses. We have to stop OPV to stop VAPP, but some cVDPV may already be in silent circulation to show up in outbreaks one or more years later. The safest solution is to introduce IPV, reach 90 per cent or more coverage and only then stop OPV. That will pre-empt the evolution of cVDPVs. Only after we ensure the absence of wild and vaccine polioviruses in the population can we claim complete success of polio eradication. That is the challenge of the present decade.

(The author was professor of clinical virology in the Christian Medical College, Vellore until retirement, and has served on several Global and National Committees on Immunisation and Polio Eradication.)

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