by Kuntal Prasad
(New Delhi, India)
How necessary is an injectable polio vaccine?
My doctor says, injectable polio vaccine is essential, even though my child is getting OPV including pulse polio vaccine.
The Expert, Ren Chats, Answers -
IPV, Inactivated Polio-virus Vaccine is an injectable vaccine. It is highly potent vaccine invented by Jonas Salk in 1952.
Whereas, OPV contains live attenuated form of the 3 polio viruses.
1. IPV contains 3 viruses in an inactivated form.
2. IPV has excellent & highly predictable immunogenicity, efficacy and safety.
3. Maternal antibodies dampen IPV’s response in first 8-12 weeks of life.
4. Seroconversion rate is 90-100% after
2 doses of IPV, given at 2 & 4 months of age. Third dose boosts antibody levels & ensures perpetuation of immunity.
5. Herd effect of OPV & IPV.
OPV mimics natural infection that spreads in the community. It immunizes all the children of the community by the effect of gut immunity which wanes with time. This can lead to wild polio outbreaks in children immunized with only OPV.
IPV produces excellent IgG mediated humoral immunity, which prevents polio infection from progressing in to the blood stream & cause muscle paralysis.
The milder local mucosal immunity (IgA) is compensated by the abundant IgG, which has an excellent inhibitory influence on local infection - The herd effect of IPV.
6. Higher the OPV efficacy & coverage, greater the herd effect, contact immunization of non-immunized children, due to spread of the live virus from OPV.
OPV , unlike IPV, shows marked variation in its immunogenicity in different countries. In India, per dose efficacy of the trivalent OPV is only 13–9%. Thus the herd effect seen in developed countries is not seen in India.
High population densities, malnutrition & poor sanitation increase the risk of other enterovirus infection jeopardizing the efficacy of OPV making frequent OPV doses essential for optimal response.
7. Vaccine Derived Polio Viruses (VDPVs) arise from mutation & recombination of OPV viruses in the human gut. These viruses cause a major risk. Therefore there is a shift from all OPV to sequential OPV-IPV schedules.
8. OPV's rapid onset of action as compared to IPV makes it the vaccine of choice for control of outbreaks. India, Pakistan, Afghanistan & Nigeria are yet endemic for Polio.
In spite of 100% coverage with OPV, median age of contracting polio in India has remained 12-18 months & 2010 statistics show 41 wild polio cases per year.
All IPV schedule is only for the countries who have eradicated wild polio.
9. Combination schedule of OPV & IPV presents
i. Better mucosal immunity
ii. Much higher seropositivity
iii. Concurrent OPV compensates IPV's subdued response when it is used at 6, 10 and 14 weeks of age
iv. Risk of vaccine associated paralytic poliomyelitis (VAPP) with OPV+IPV is extremely low, because newborn infants receive first 3 doses of OPV under the protection of maternal antibodies against VAPP
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