Many experts feel that injectable vaccine should be introduced during the transition phase

With oral vaccines, India appears to have achieved what was once thought a Herculean task — decisively breaking the circulation of wild polio viruses that paralysed countless children.

But the use of oral vaccines, which contain live but weakened strains of the virus, can be a bit like riding a tiger. Discontinuing them, without risking a 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 inactivated polio vaccine (IPV), which must be administered as an injection, into the immunisation programme.

Massive coverage

With the oral vaccine, two drops help raise the immunity of children who receive it. The ease of administration meant that gigantic mass immunisation campaigns could be carried out. In India, each round of national immunisation had, according to the Union Health Ministry, 24 lakh vaccinators under 1.5 lakh supervisors visiting over 20 crore households to ensure that the nearly 17.2 crore children under 5 received the vaccine. Other teams immunised children at railway stations, bus stands, construction sites and in market areas.

Apart from blocking the polio virus from invading nerve cells and causing paralysis, the oral vaccine, 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.

Drawbacks

But the oral vaccine 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, resulting in what are known as vaccine-derived polioviruses (VDPV). A high proportion of those immunised with OPV excretes revertant viruses, noted Neal Nathanson of the University of Pennsylvania and Paul Fine of the London School of Hygiene and Tropical Medicine in a commentary published in the journal Science in 2002. Significantly, their commentary was titled ‘Poliomyelitis Eradication — A Dangerous Endgame.'

While no child in India has been crippled by a wild polio virus during the past year, the country saw seven cases of paralysis caused by VDPV during 2011. Such VDPV can readily circulate and spread.

At present, the intention is to stop immunisation with the oral vaccine globally once the wild polio viruses have been eradicated.

“The challenge will be to synchronise global cessation of OPV immunisation and then manage the transition, potentially lasting several years, to the point where residual VDPVs have been eliminated,” observed Bruce Aylward, the World Health Organisation's Assistant Director-General for Polio, Emergencies and Country Collaboration, and Tadataka Yamada of the Gates Foundation in a paper published last year in The New England Journal of Medicine.

Many experts believe that the transition will have to be handled by introducing IPV, injectable vaccine that wealthy nations already use to immunise 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-come countries, an IPV administration schedule that will cost no more than the existing OPV regime, said Dr. Aylward and Dr. Yamada. For India, a sensible option would be introduce IPV in its immunisation programme alongside the oral vaccine, said Dr. Jacob John, a distinguished virologist who was with the Christian Medical College at Vellore till his retirement and worked extensively on polio immunisation. OPV could be withdrawn as IPV immunisation levels picked up.