To achieve the global eradication of polio, firstly, all wild polioviruses, and, secondly, all vaccine viruses – which may also cause disease – must be completely eliminated. Oral poliovirus (OPV) vaccines will enable elimination of the wild viruses, but elimination of all vaccine viruses can only be achieved through either the discontinuation of OPV or the exclusive use of inactivated poliovirus (IPV) vaccines.
The ‘final stages’ of global eradication of polio refers to the period between achieving the global absence of wild viruses and reaching the point where we can confidently withdraw all polio immunisation. The former could be achieved by 2002 and certified by 2005, if no wild virus is found in spite of diligent search. Only after an interval of unpredictable duration could we be bold enough to discontinue immunisation globally. We do not want successive annual birth cohorts of children to remain susceptible to infection by polioviruses, and must, therefore, be sure that absolutely no risk of poliovirus infection remains. Such risk may be due to silent wild virus transmission in some corner of the world, importation from such foci, accidental or intentional introduction of virus from laboratory stock, or reversion of vaccine viruses to neurovirulence.
If OPV were abruptly discontinued, then any new cohort of children would be unprotected, while the revertant vaccine viruses may still be prevalent among children or in the environment. Phased withdrawal of OPV may raise ethical questions, as it leaves children at risk from infection by vaccine-derived viruses. However, use of IPV during the ‘final stages’ of global eradication would avoid this problem. As IPV can be combined with the diphtheria, tetanus and pertussis (DTP) vaccine, no additional injections or clinic visits would be required.
Ultimately, cessation of polio vaccination altogether would result in savings of more than US$1 billion annually.