India finally becomes Polio free

India moved a long way from 741 cases in 2009 to 1 two years later. And this makes us epidemically speaking free of the scourge of polio. In the context of a nation like India, this can be truly called as a major milestone as far as public health policy is concern. 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.

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 (Global Polio Eradication Initiative), resulted in success in most countries and partial success in all countries. Of the 3 types of polioviruses, type 2 (PV2) was globally eradicated in year 1999 – with the last case in Uttar Pradesh. But transmission of type 1 & 3 continued in 6 countries.

In India, during the last decade, over 95% of cases occurred in UP & Bihar – arguably the world’s most difficult spots for eradication.

Since type 1 showed a cyclical nature of outbreaks every 4 year – 1998, 2002 & 2006 – the next outbreak in 2010 had to be averted(to avoid) at all cost. 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 1 case on January 13, 2011 – none since.

Type 3 cases were less than 10 in 2004 & 2005. Unfortunately, while type 1 was singled out for attack, type 3 outbreaks developed, first in Bihar (2007-08) and then in UP (2008-09). So, in 2010 there was yet another change of tactic, now focusing on type 3 along with type 1.

Problems & Innovative Solutions

For countries with polio, the WHO recommended the exclusive use of OPV(Oral Poliovirus Vaccine) for its low cost and ease of inoculation(to inject vaccine) by mouth - as two drops. On the flip side, the very fact that many countries using OPV could not control polio with routine immunization indicated that it was not as effective as in other countries. The difference was clear: tropical / subtropical countries with low income, overcrowding, high birth rate, and high child mortality faced low effectiveness of OPV, whereas those with opposites had high vaccine effectiveness.

Wild polioviruses exist in 3 types, and OPV contains attenuated strains of all 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. 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 UP & Bihar, but to no avail. Type 2 had to be removed from tOPV to get the best out of type 1 & 3. In 2005 and therefore, a new monovalent type 1 OPV (mOPV-1) was used in UP & Bihar – it is 3 times more effective than tOPV. This was one factor of success. But the gaps in immunity were created against type 3; consequently, type 3 outbreaks occurred in 2007-2009. Then, bivalent vaccine (bOPV with 1 & 3) was developed. It was no inferior to mOPV-1 or mOPV-3. From early 2010, bOPV has been widely used in UP & Bihar during campaigns, while tOPV is used everywhere else for routine immunization.

Challenges for stopping OPV

The use of OPV, which contains live but weakened strains of virus, can be a bit like riding a tiger. Discontinuing them, without risking the resurgence of polio that would undo all that has been achieved, is going to be a tricky exercise. The endgame for complete polio eradication could well involve incorporating the IPV (Inactivated Polio Vaccine), which must be administered as an injection, into the immunization program.

Apart from blocking the polio virus from invading nerve cells and causing paralysis, the OPV, by duplicating a natural infection, is said to raise immunity in the mucosal lining of the gut. That makes it more difficult for the virus to replicate there and spread through faeces to others. But the OPV has its drawbacks also. In rare cases, the live but attenuated viral strains in the vaccine can themselves cause polio. Moreover, these viruses can revert to virulence(the relative ability of a micro-organism to cause disease) , resulting in what is known as Vaccine-Derived Polio Virus (VDPV).

While no child in India has been crippled by a wild polio virus during the past year, the country saw 7 cases of paralysis caused by VDPV during 2011. At present, the intention is to stop immunization with the oral vaccine globally once the wild polio viruses have been eradicated. Many experts believe that the transition will be handled by introducing IPV, injectable vaccine that wealthy nations already use to immunize children. However, the cost of IPV, considerably more than that of OPV, has been an issue. It now appears increasingly feasible to create, for low income countries, an IPV administration schedule that will cost no more than the existing OPV regime.