Polio Vaccine
Ira Shah
Polio Vaccine - Introduction
Polio (poliomyelitis) mainly affects children under five years of age. Polio virus invades the nervous system and can cause total paralysis in a matter of hours. Polio is mainly passed through person-to-person (i.e., fecal-oral) contact, and infects persons who do not have immunity against the disease.

In 1988, the forty-first World Health Assembly, consisting then of delegates from 166 Member States, adopted a resolution for the worldwide eradication of polio. It marked the launch of the Global Polio Eradication Initiative, spearheaded by the World Health Organization (WHO), Rotary International, the US Centers for Disease Control and Prevention (CDC) and UNICEF. The primary strategies for achieving this goal are:

- high infant immunization coverage with four doses of oral poliovirus vaccine (OPV) in the first year of life;
- supplementary doses of OPV to all children under five years of age during national immunization days (NIDs)
- surveillance for wild poliovirus through reporting and laboratory testing of all acute flaccid paralysis (AFP) cases among children under fifteen years of age;
- targeted "mop-up" campaigns once wild poliovirus transmission is limited to a specific focal area.

Polio cases have decreased by over 99% since 1988, from an estimated 350,000 cases then, to 1604 reported cases in 2009. However, polio eradication campaign is way behind where its planners hoped it would be at this stage. There are still four countries (Nigeria, India, Pakistan and Afghanistan) in which transmission has never been interrupted. Furthermore, 14 countries in which poliomyelitis had been eradicated have reported re-introduction, admittedly small numbers of cases, but showing that the threat is still quite real. Persistent pockets of polio transmission in northern India, northern Nigeria and the border between Afghanistan and Pakistan are the current focus of the polio eradication initiative.

Pulse Polio Immunization (PPI) involves providing additional OPV doses to every child aged <5 years at intervals of 4-6 weeks. The aim of PPI is to "flood" the community with OPV within a very short period of time, thereby interrupting transmission of virus throughout the community. In 2005, new monovalent polio vaccines (mOPV type 1 and type 3) were licensed and used to enhance the impact of supplementary immunization activities (SIAs) in the key remaining reservoirs of wild polio. Despite the use of mOPVs and the intensification of the global eradication efforts in 2007, indigenous wild poliovirus type 1 (WPV1) and 3 (WPV3) transmission has continued.

IPV is only contraindicated in children who have experienced a severe allergic reaction to a previous dose of IPV or to streptomycin, polymyxin B, or neomycin. OPV is contraindicated in children who have immunosuppressive conditions due to congenital immunodeficiency (agammaglobulinemia or hypogammaglobulinemia), cancer (leukemia or lymphoma), immunosuppressive chemotherapy, or acquired immunodeficiency syndrome (AIDS). OPV also is contraindicated for family and other close contacts of immunocompromised people because of the risk of spread of OPV to the affected person.

Child who has not received any polio vaccination so far: OPV at birth, IPV at 6, 10 and 14 weeks, OPV at 6 months and 9 months, IPV at 15-18 mths and OPV at 5 years. OPV on all pulse polio immunization days.

Child who has completed primary series of OPV: IPV may be offered as catch up vaccination for children less than 5 years of age who have completed primary immunization with OPV. IPV can be given two doses at 2 month interval. OPV need not be given with these IPV doses.

OPV is a very heat sensitive vaccine having a shelf life of 2 years at a temperature of -20oC, 6 months at 2 to 8oC and 1-3 days at room temperature. OPV should be stored at -20ooo
Oral Polio vaccine (OPV): Oral polio vaccine has been the choice for routine immunization in over 120 countries that have eliminated poliomyelitis. Following the same experience, India is also using OPV as routine immunization and pulse polio immunization as well in order to become polio free. OPV has few advantages like low cost, event to administer, inducing gut immunity and herd effect, which interrupt wild poliovirus circulation. OPV is constituted from three Sabin strains of polioviruses, types 1, 2, and 3, in concentrations of 10,00,000, 1,00, 000, and 6,00,000 infective units (TCID50) per dose, respectively.

However by using OPV India was able to control the disease but failed to achieve target of polio eradication by year 2000. So few questions were raised regarding its efficacy, administration strategy, safety concerns, choice of vaccine for individual protection, eradication & vaccine schedule etc.

Efficacy concerns: Various studies from developing countries suggest that after 3 doses of OPV, the mean proportion of infants with detectable serum neutralizing antibodies level was only 73% (36-99%) for type-1, 90% (71-100%) for type-2 & 70% (40-99%) for type-3 polio virus. Data suggests that there is wide variation among OPV vaccines in developing countries. These findings were later confirmed by randomized trials in Brazil & Gambia. Study from India found that with 5 doses of OPV at an interval of 4 weeks, seroconversion rates are 88.7%, 93.5% & 96.5% for type 1, 2 & 3 respectively. This sub optimal seroconversion was related to many factors including - interference with other enteroviruses, inhibition of type 1 & 3 virus from type 2 virus, diarrheal illnesses & presence of maternal antibodies. Inspite of routine use of OPV supplemented with mass immunization achieving, immunization coverage of 85-90%, Gaza & Wert banks continued to experience polio outbreaks indicating its inefficiency in controlling the disease.

Vaccine Associated Paralytic Poliomyelitis (VAPP): VAPP though rare is a serious complication of OPV. Vaccine virus has the ability to mutate to become neurovirulent & cause paralytic illness in the subject or its contacts. From 1980 to 1998, 152 cases of paralytic polio were reported from USA of which 95% (144) were related with oral polio vaccine administration. Risk of VAPP was estimated as 1 case per 1.5-2.2 million doses administered in 1989 to 1991. Data from UK suggested VAPP risk as 1 case per 1.4 million doses of OPV administered from 1985-1991. There is higher risk of VAPP following first dose of OPV and in children with B-cell immunodeficiency. Study in India by Kehler KA et al concluded that the risk of VAPP was 1 per 4.1-4.6 million dose administered. Recipient risk was 1 per 12.2 million doses. 1st dose recipient risk was 1 per 2.8 million doses & subsequent dose recipient risk was 1 case per 13.9 million dose administration which are much lower as compared to other countries.

National polio surveillance project defined VAPP as AFP cases in whom -
- Onset of paralysis
- History of vaccine administration before the onset of paralytic illness
- Residual weakness 60 days after onset of paralysis and
- Single vaccine related poliovirus was isolated in stool samples without isolation of wild poliovirus.

Solutions put forward: India has gained remarkable success in controlling poliomyelitis through pulse polio immunization. Recent surveillance reports indicate that most of the wild virus is in circulation in Northern India (esp. UP, Bihar) causing paralytic polio. Kerala is polio free from some time so epidemiology has changed but the policy & strategy to eradicate polio are still the same across the country. So few strategies have been put forward in order to tackle this situation. It is proposed that, in the areas where polio virus (wild) is not circulating. IPV (Injectable Polio Vaccine) should be commenced in order to reduce VAPP and areas with high incidence of polio should continue to use oral polio vaccine.

Inactivated Polio Vaccine (IPV): IPV is derived from three wild strains of poliovirus, types 1, 2, and 3, which are inactivated by formalin. IPV is highly immunogenic with > 90% of vaccine recipients developing protecting antibodies following 2 doses of vaccine and > 99% after 3 doses. These antibodies persist for several years after primary immunization with IPV. Study in Tamil Nadu (India) comparing efficacy of OPV & IPV has shown higher efficacy with latter (66% Vs 92%). There were initial concerns that IPV induces lower level of mucosal immunity than OPV & it doesn't get excreted in the stools of vaccines so it may be less efficacious in preventing wild virus circulation. But now studies have shown that enhanced IPV (eIPV) induces adequate amount of IgA formation in nasopharyngeal and intestinal secretions almost equivalent to that induced by oral polio vaccine.

Schedule of IPV: Many schedules have been tested e.g. eIPV only, sequential eIPV - OPV in combination with DPT and as part of a number of investigational combination including acellular pertussis, Hib vaccine, Hepatitis B vaccine etc. Combined schedule of OPV & IPV may be useful to accelerate the eradication of polio in developing countries. Combined schedule with simultaneous delivery of OPV & IPV at 6,10 & 14 weeks had an excellent serologic response in 3 different parts of world with mucosal immunity equivalent to that provided by OPV alone without the risk of VAPP. Sequential IPV/OPV vaccination trials confirmed its efficacy in polio eradication and safer than OPV by reducing VAPP among recipients. Denmark, Israel have become polio free only after introducing OPV-IPV combination. In one study it was suggested that in countries where wild type virus no longer circulates and where general hygiene is good eIPV alone might be the vaccine of choice.

Side effects of IPV: No serious side effects are reported. Rarely anaphylaxis may occur within few minutes to few hours after vaccination.

Advantages of IPV: It is safe in immunodeficient hosts, patient on steroids & radiation therapy, in elderly and even in pregnancy. It does not require stringent conditions during storage & transport and has a longer shelf life.

Disadvantages of IPV: There are few limitations of IPV as well. Virus content is 10,000 times more than OPV hence it is costlier. It requires trained personnel to administer. There is no excretion of virus, hence contacts are not benefited. It is not useful in controlling epidemics.

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