As stated in the consensus recommendations also, this
schedule is an interim arrangement to take care of VAPP
cases and also to pave the way to ultimately all-IPV
schedule. The OPV is retained mainly for two reasons, first,
its propensity to induce superior intestinal mucosal
immunity to decrease the spread of WPV, and secondly, to
avoid confusion regarding OPV at community level that would
have resulted had we gone for complete cessation of OPV use
since the vaccine is exclusively employed in ongoing SIAs
and RI in India. Though it’s true that ‘effective’ mucosal
immunity is not visible at ground level, especially in the
two endemic hotspots, yet there is no trial that
demonstrates superior or even comparable intestinal immunity
of IPV in India. The ongoing trials may have some answers
and may ultimately settle the issue.
There is limited experience of using IPV
in routine immunization schedules in developing countries.
Where IPV has or is being used (for example, in Egypt,
states in the Gulf Cooperation Council, Malaysia, South
Africa, and Yogyakarta Province, Indonesia), it is usually
administered in a sequential schedule with OPV. This
schedule is also in accordance to WHO policy which states
that "IPV alone may be considered an alternative to
sequential schedule only in countries that have the lowest
risk of both WPV importation and WPV transmission [1].
The last two doses of polio vaccines i.e.
IPV at 15-18 months and OPV at 5 years are retained
primarily to accord long-lasting protection to individual
vaccine. We may be erring on ‘over-immunizing’ an
individual, but in the absence of any indigenous trial and
experience, this was the safest path to choose.
The main reason why industrialized
countries have switched over to ‘all IPV’ schedule and
deprived their children the ‘critical benefit of gut
immunity’ is safety concerns of OPV. As stated earlier, we
are providing the best of both the vaccines till the
‘services’ of OPV are still available while minimizing the
damage inflicted by it.