The last polio case was reported in India in 2011. India was certified polio-free on 27 March 2014, but the immunization programme continues since two of its neighbors ( Pakistan and Afghanistan) remain polio-endemic and due to the threat of vaccine-derived polio. Since 2009, India has witnessed 41 cases of Vaccine-derived poliovirus (VDPV), including two such cases in 2014
Switch to bivalent is necessary as in settings with low immunization coverage, live vaccine virus used in Oral polio vaccines (OPV) can multiply for long and undergo mutations to gain neuro – virulence. This VDPV can cause paralysis and circulate in the community to cause outbreaks. Vaccine-associated paralytic polio (VAPP) is a rare but serious adverse event following OPV administration. VAPP tends to occur in both OPV recipients and their unimmunized contacts.
Currently, two types of polio vaccines are mainly used in National health programs in India. The trivalent OPV (tOPV) contains live attenuated polioviruses of all three serotypes ( 1,2,3). Bivalent OPV (bOPV) contains two serotypes of live attenuated poliovirus (type 1 and 3). More than 90% VDPV cases in India are caused by type 2 virus.
OPV was preferred over IPV for public health programs during pre-eradication period, mainly due to its lower costs and ease of implementation. Now, VAPP and VDPV overwhelmingly outnumber polio due to WPVs, and therefore OPV has to be discontinued as early as feasible, for ethical reasons. It is imperative to eliminate the iatrogenic risk of VAPP at any cost, (in line with the principle of first do no harm).
There is an increased risk of emergence of cVDPVs during the withdrawal of trivalent OPV as the immunity level against type 2 poliovirus will decrease. To prevent such an emergence of VDPV, it is recommended that before this switch population immunity against type 2 polio virus be boosted by introduction of at least one dose of Inactivated Polio Vaccine (IPV) in the UIP. Global OPV2 withdrawal requires the absence of ‘persistent’ cVDPV2 for at least 6 months.
If one dose of IPV is used, it should be given from 14 weeks of age (when maternal antibodies have diminished and immunogenicity is significantly higher), and can be co-administered with an OPV dose. New schedule (three OPV doses + one IPV dose)