Global Vaccine Switch
From April 25, India will make a switch from trivalent oral polio vaccine (OPV) to bivalent OPV. The switch will take place globally as it has been recommended by the Strategic Advisory Group of Experts (SAGE) on Immunization.
What exactly is the switch?
The switch refers to the replacement of trivalent OPV with bivalent OPV in routine immunization and supplemental immunization activities in all the countries around the world within a 2-week timeframe (April 17 to May 1). Trivalent OPV would no longer be used anywhere in the world. This switch has been labelled as the largest globally coordinated withdrawal of one vaccine and the roll-out of another into a routine immunization programme in history. 155 countries are expected to make the switch.
What is the difference between trivalent OPV and bivalent OPV?
Trivalent OPV contains all three strains of the polio virus (type 1, type 2 and type 3) while bivalent OPV contains only two type of strains (type 1 and 3 only). Both vaccines are administered orally.
Why the switch is being made?
Wild poliovirus type 2 was eradicated in 1999. However, under rare instances, the trivalent OPV is known to have caused polio in unprotected children. All the type 2 polio cases at present have been caused only by vaccine-derived polioviruses. This is because the OPV has live but attenuated (weakened) virus, which on rare instances become virulent and continues to circulate in the community and causes vaccine derived poliovirus. It is estimated that the type 2 strain in the trivalent OPV has resulted in over 90% of the vaccine derived poliovirus in the world in the past 10 years. So, the withdrawal of type 2 strain and switching over to a bivalent OPV is a major step to ensure polio eradication. But the move has to be globally synchronized else there are chances of importing the Type 2 vaccine derived polio viruses from other countries.
Why OPV cannot be eliminated entirely rather than switch to bivalent OPV?
Inactivated Polio Virus (IPV) is an inactivated vaccine and is not a live attenuated vaccine. So, it has no risk of spreading vaccine derived polio virus. But, unlike OPV, it does not replicate in the gut and induces lower levels of intestinal immunity. IPV is also less effective in reducing fecal-oral transmission than the OPV.
What steps have been taken in India?
India has introduced Inactivated Polio Virus (IPV) vaccine into the routine immunization programme in a phased manner from November 30, 2015. IPV is considered safer than OPV because it contains all three polio strains in a killed form. According to SAGE, it is observed that the better protection is ensured when two doses of 0.1ml each of IPV are given intradermally rather than giving a single full dose of 0.5ml intramuscularly at 14 weeks. This is because of the better seroconversion and higher antibody level when 2 doses are administered intradermally. This is due to the fact that the antigen presenting cells are more in number in the intradermal region of the skin than the subcutaneous or muscle tissue. So even if a tiny dose is administered, more antigen presenting cells pick up the antigen and transport them to the nearby lymph node where it is presented to the T cells. Ultimately, the effect gets enhanced since it is given in two doses.
Two pronged vaccination strategy has been adopted in India, wherein children of Andhra Pradesh, Karnataka, Kerala, Maharashtra, Odisha, Tamil Nadu, Telangana and Puducherry will get two doses of 0.1ml each of IPV administered intradermally at 6 and 14 weeks while the children in the rest of states/UTs will get a single dose of 0.5ml of IPV administered intramuscularly at 14 weeks. The reason for this difference is due to the existence of extensive immunization coverage, well developed infrastructure and human resources to administer IPV in two doses in those seven states and puducherry while the rest of the states/UTs lack them.
