Oral polio vaccine (OPV) 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. 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.

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 and areas with high incidence of polio should continue to use oral polio vaccine. So that IPV can be introduced sequentially across the country in order to reduce VAPP.

Inactivated Polio Vaccine (IPV): 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 & IVP 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: 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.

Contra Indications of IPV:

Side effects: No serious side effects are reported. Rarely anaphylaxis may occur with in few minutes to few hours after vaccination.

Advantages: Safe in immunodeficient hosts, patient on steroids & Radiation therapy, in elderly and even in pregnancy. Does not require stringent conditions during storage & transport and has a longer shelf life.

Disadvantages: There are few limitations of IPV as well.

WHO stand on Polio Vaccination (2002): IPV is not recommended for routine immunization in developing countries because of its high cost.

IAP stand on IPV: Indian Academy of Pediatrics Committee on Immunization (IAPCOI) recommends that Government of India should immediately license the use of IPV in India. As the number of wild polio virus cases in the country decreases, it is inevitable that one would have to gradually shift from OPV to IPV in the next few years. Therefore government should consider incorporating IPV gradually in the national immunization schedule in a phased manner starting from the states where polio has been eliminated.

References :

1. Dowdle WR, De Gourville E et al. Polio eradication: the OPV paradox. Rev Med Virol 2003;13(5):277-91.
2. Parent du Chatelet I, Merchant AT, Fisher-Hoch S. Serological response and poliovirus excretion following different combined oral and inactivated poliovirus vaccines immunization schedules. Vaccine 2003;21(15):1710-8.
3. Yeh SH, Ward JI, Partridge S et al. Safety and immunogenicity of a pentavalent diphtheria, tetanus, pertussis, hepatitis B and polio combination vaccine in infants. Pediatr Infect Dis J 2001;20(10):973-80.
4. Update on Immunization Policies, Guidelines and Recommendation. Indian Pediatrics2004; 41: 239-244.
5. Mittal SK, Mathew JL. Vaccine associated paralytic poliomyelitis. Indian J Pediatr 2003; 70: 573-577.
6. Thakker N, Shendurniker N. Controversies in Polio Immunization. Indian J Pediatr 2003; 70: 567-571.
7. Core information for the development of Immunization Policy. WHO- Vaccines and Biologicals. 2002 Update

1. What is polio vaccine given for?
Polio vaccine is given to protect against poliomyelitis, the dreaded childhood disease leading to paralysis of various groups of muscles. Polio is caused by 3 types of poliovirus type I, II & III.

2 What are the types of polio vaccine?
There are two types of polio vaccine, one is live oral polio vaccine (OPV) & the other is injectable killed polio vaccine (IPV). Both these vaccines contain all the 3 types of polioviruses. The IPV used currently is the enhanced potency IPV. In all subsequent discussions IPV will mean the current enhanced potency IPV.

3 Which of the 2 types of polio vaccine is used?
India & many other countries use only OPV. Infact IPV is not available in India. IPV was the first polio vaccine to be marketed in the world. Western countries like USA used IPV for mass vaccination and were immediately rewarded with significant drop in polio cases. Subsequently OPV was introduced in the market. Because of convenient oral route and cost effectiveness, the whole world started using predominantly OPV. Extensive coverage with OPV in the Western Hemisphere almost eradicated poliomyelitis. As the number of clinical cases of wild polio fell, the OPV induced VAP (vaccine induced paralysis) became a significant problem both on humanitarian as well as medicolegal point of view. Hence, countries like USA switched to a serial IPV followed by OPV schedule so that IPV induced immunity will prevent subsequent OPV induced VAP. Now that there are no cases of clinical polio for 10 years or more, USA has totally switched over to only IPV and does not use OPV anymore.

After polio is eradicated from a country, it may be prudent to switch over to IPV totally because of 2 reasons. One is that it will prevent all the cases of VAP which if occur will be unacceptable in the scenario of polio eradication. Second is that IPV will not interfere with virus isolation from stools of children during ongoing surveillance programme.

4 How are polio vaccines available?
OPV is available as vial containing multidoses, usually 25 doses in ready to use liquid forms. It is a live vaccine. The dose is 2 drops per dose. IPV is available as a single dose vial containing 0.5 ml of vaccine. It is also available as combination vaccines containing IPV+DPT or IPV+DPT +Hepatitis B or IPV+DPT+Hib or IPV+DPT+Hepatitis B+Hib or in combination with DPaT. IPV is not available in India at present.

5 When is OPV given?
OPV is given as 2 drops per orally. The first dose is given at birth -15 days as zero dose OPV. Next 3 primary doses are given at 4 weeks interval starting at 6-8 weeks of age. It is given along with 3 primary doses of DPT. Indian Academy of Pediatrics & most of the pediatricians recommended a 5th primary dose at 9 months along with measles vaccine.

6 When is the booster dose of OPV given?
A booster dose of OPV is recommended by Govt. of India under EPI at 15-18 months of age along 1st booster dose of DPT. Indian Academy of Pediatrics & most of the Pediatricians recommend one more booster i.e. 2nd booster of OPV at 4-6 years of age along with 2nd booster dose of DPT. Govt. of India under EPI does not recommended 2nd OPV booster & recommends only DT & not DPT as 2nd booster at 4-6 years of age.

7 Why is there a difference in recommendations for OPV?
Initially EPI recommended only 3 primary doses & one booster dose of OPV. The seroconversion & protection with only 3 primary doses was shown to be as less as 60% in some studies. This was also proved by the fact that poliomyelitis did occur even in a fully immunized child. Hence, Indian Academy of Pediatrics & most of the pediatricians recommend 5 primary doses to have better efficacy (80 - 90% protection) & lesser chances of mishap. Now the EPI recommends an extra OPV during primary schedule as zero dose at birth. As the immunity can wear off & lead to risk of polio by 5 years, IAP also recommends another booster at 4 - 6 years of age.

8 Why is a dose of OPV given at birth & why is it called as zero dose?
Initially OPV & DPT primary dose were started after three months of age thinking that maternal antibodies in the baby will interfere with the vaccines. But it was proved that it is not true & hence the schedule is now started at 6 - 8 weeks of age. Yet one is totally protected only after completion of all three doses i.e. by 16 - 18 weeks. There can be some breakthrough cases of polio, which can occur as early as 3 - 4 months. Hence, to protect against that, 1 dose of OPV is given at birth -15 days of life. Secondly it is believed that OPV given at birth will lead to better seroconversion following 3 more primary doses & chances of OPV induced paralytic polio (VAP) will be reduced. The extra dose given at birth is called zero dose so that next doses of OPV could be still called as 1st, 2nd &3rd dose to make it congruent with the 1st, 2nd & 3rd dose of DPT.

9 How are the polio vaccines stored?
Both OPV & IPV are to be stored in a refrigerator. OPV stocks can be stored frozen. Working vials should be stored at 2-8⁰ C in the chiller tray. Once thawed it should not be frozen again as repeated freezing & thawing decreases the potency. Being a live & thermolabile vaccine, strict cold chain should be maintained while transporting the vaccine. Even in the field the OPV vial in use should be kept on ice. Once a vial is opened the balance should be discarded & not used again. Nowadays vaccine vial monitors are available which change color when subjected to a fixed hours of cumulative exposure to more than 8⁰ C. Hence they help monitor the potency of vaccine. OPV is a pink colored solution. It may turn yellow due to changes in pH of vaccine. Such discoloured vaccine or presence of visible turbidity in the vaccine is suggestive of growth of bacteria or fungi. Such vaccines should be discarded & not used.

IPV should never be frozen & should be stored at 2-8⁰C. If frozen by mistake, it should be discarded.

10 What is a vaccine vial monitor?
Vaccine vial monitor is a thermo - sensitive device applied on any vaccine mainly OPV vial. At an unsafe cumulative exposure to> 8⁰C, it changes its color. It has a circle within a square. If the circle is lighter than square it means it is a good vaccine. When the circle becomes of the same color shade or darker than the square, it means it's unsafe to use the vaccine & it should be discarded.

11 What is the efficacy of polio vaccine?
The efficacy of 3 doses of OPV in West is around 80-90% while that in temperate countries like India is only 60-70%. The reasons behind poor efficacy in temperate climate are multifocal. One is the poor cold chain maintenance. Second is the interference by other GI viral infections, which are so common in countries like India. Lastly, there could be some genetic reasons for the poor response. The efficacy goes upto 90% with 5 doses of OPV in India. Theoretically, one may require 10-15 doses of OPV to reach near 100% efficacy. That is what is achieved by pulsing for 3-5 years with 2 pulse polio doses every year as has been proved in some countries. The protective efficacy is> 95% with 3 doses of IPV.

12. what is herd immunity?
The protection extended to even the unvaccinated by mass vaccination is called as herd immunity. For a disease to occur, one needs the infecting organism, the susceptible host and the cycle of transmission. In an endemic area, the cycle of transmission continues from one infected person to another susceptible and so on. Herd immunity in polio is possible in 2 ways. One is the excretion of vaccine poliovirus, which can infect the unvaccinated susceptible contacts and leads to indirect 'vaccination'. However, as the vaccine virus is an attenuated strain, this may not be very efficacious. The second way of herd immunity is blocking the transmission of polio virus. When many hosts are vaccinated, especially simultaneously, the transmission of wild virus is blocked, as it can not find a suitable susceptible. The virus cannot survive in environment for more than 48-72 hours. This appears to be a major way of herd immunity and the mechanism behind the pulse strategy. When 80-90% of susceptible hosts are vaccinated, the herd effect will protect remaining 10-15% unvaccinated hosts.

13 What are the side effects of polio vaccines?
OPV has minimum side effects. It can lead to GI upset like diarrhea, vomiting. It does not lead to fever. Most of the side effects seen with OPV +DPT are due to the DPT vaccine. The most important but extremely rare side effect with OPV is vaccine associated paralysis (VAP). It occurs in 1 in million doses. Massive benefits of OPV far outweigh the rare risk of paralysis. In west, though, it can become a medico -legal problem. Hence in west, people use first IPV to induce systemic antibodies & then use OPV, which will ead to local gut immunity without the risk of vaccine induced paralysis due to the partial immunity conferred by prior IPV.

IPV can lead to local side effects like pain, swelling, redness and tiredness in 10 -20 % of vaccinees. It can lead to fever, which is mild & lasts for 24-48 hours. If not given at proper site it can damage important nerves like sciatic nerves if given in gluteal region. Like any other IM injection it can precipitate paralysis in a patient who is already in incubation period of polio, as can occur during polio epidemics. Hence IPV or any other IM injection should be avoided in an unimmunized sick child with fever especially during season of polio epidemics. Lastly, like any other injection, an abscess may form if aseptic precautions are not followed while giving IM injection.

14. What are the contraindications of polio vaccine?
OPV is contraindicated in immunocompromised patients especially if IPV is available. In HIV patients, OPV can be given during asymptomatic phases especially in India where IPV is not available. OPV can be given to a patient with diarrhea, but that dose should not be counted & hould be followed by an extra dose. IPV is safe even in immunocompromised host. It is to be avoided in an unimmunized child during polio epidemics especially if the child is sick with fever, as it can induced paralysis in such a child who can be in incubation period of wild polio infection. IPV is contraindicated if patient has developed severe adverse effect to its use in past.

15. What if a child comes late for polio vaccine?
Paralytic poliomyelitis can occur till 15 years of age. Most of the children are immune by 5 years naturally due to subclinical infections. Hence, OPV is given to any child who presents till 5 years of age. Such a child should receive 3 primary doses followed by 1st booster 1 year after 3rd primary dose & a second booster 3 years -4 years after the 1st booster dose provided the child is still less than 5 years old then.

16. What if a child comes late for subsequent doses?
As OPV + IPV induce T cell memory, there is no need to restart the schedule if the patient comes late. Just finish the remaining doses & that will be enough. However it is important to realize that the patient is not protected till completion of the schedule. Hence it is not advisable to postpone the doses.

17.Can polio vaccine be given along with other vaccines?
Both OPV /IPV can be given along with other vaccines. In fact, they are given along with DPT most of the time. One can even give Hib & Hepatitis B along with OPV/IPV. Abroad, combination vaccine containing IPV + DPT, IPV + DPT + Hib, IPV + DPT + Hepatitis b , IPV + DPT + Hep B + Hib are available to minimize the number of pricks to the child.

18 Why is OPV preferred over IPV? What is the role of IPV?
OPV is cheaper and being given orally is more acceptable. It is safe with virtually no side effects. It induces both local gut as well as systemic immunity. It leads to excellent herd immunity. Mass vaccination is easier with oral vaccine. IPV is available in injectable form & is costlier. It can precipitate paralytic polio in a child who is in incubation period of wild polio especially during epidemics. Hence OPV is preferred our IPV for routine and mass vaccination.

But on the other hand, OPV is less efficacious in temperate climate. It has the potential of vaccine associated paralysis (VAP) & worldwide experience has proved that polio eradication is not possible just by high coverage of universal immunization with OPV. It also needs strict cold chain maintenance. Where as IPV is more efficacious, less thermolabile & has no chances of vaccine induced paralysis. Of late even IPV is shown to lead to local gut immunity as well as herd immunity. Good routine coverage of 5 primary doses of IPV can eradicate polio from a country. However, pulsing is easier with OPV than with IPV.

Abroad schedules of immunization take advantages of both OPV + IPV. Like in USA, they used to give 3 primary doses of IPV that leads to good systemic immunity without fear of vaccine associated paralysis. This was followed by a dose of OPV at 6 months to induce local gut immunity.

Again after having achieved polio elimination it is important to demonstrate that no wild poliovirus is circulating in environment. At such time OPV can interfere with virus identification. Instead if only IPV is used, any poliovirus isolated form stools of child will be presumed to be wild poliovirus making it easy to identify, without the need of intratypic facilities. Hence IPV has its own role in immunization. But IPV is still not available in India.

19. What is pulse polio immunization?
It is a strategy of mass immunization by which one can eradicate poliomyelitis. Extra doses (pulses) of OPV are given to all children below 5 years of age in an area (like country, state, city) at a time on a given day. Such pulses are repeated every year. The aim is to achieve 100% coverage.

20. How does pulse immunization work?
Wild poliovirus can survive either in the intestines of a susceptible host (usually children <5 years of age) or in the sewage water. In the sewage water it can survive only for 48 - 72 hrs & hence it has to find shelter into another susceptible host for it to survive & to continue cycle of transmission. When you give pulse OPV to all the children of <5 years of age, the intestines of these children are flooded by vaccine virus and hence wild poliovirus cannot get entry into it. As wild poliovirus cannot survive for more than 48-72 hrs in environment, its circulation will drastically fall. When such pulse is repeated after 6 weeks it still further reduces transmission. Such pulses done every year will ultimately eradicate the wild polio virus from nature.

21. How many pulses are usually required?
2-pulses/year strategy took 10 years of such pulsing in some developing countries & hence is unlikely to succeed in recent future in India. Ideal would be to give 3 pulses at 6 weeks interval in a year.

Hence from 1999 onwards, 4 pulsing/year has been tried (In some states of India 6 pulses/year were given). We will have to wait for the results to see its impact. There are 3 criteria to be fulfilled before eradication is continued. One is that there are no clinical cases of wild polio occurring in an area. Secondly that there is ability to detect a case of wild polio should it occur i.e. show effective surveillance programme whereby you report at least 1 case of non-polio AFP (acute flaccid paralysis) per 100,000 children <15 years old. (Which is the expected incidence of non-polio AFP). Lastly you have to show absence of wild poliovirus circulating in the environment. When these criteria are met with for 3 consecutive years that area is declared as polio eliminated area. When whole world will be polio eliminated true polio eradication can be dreamt & certified.

22. Who should be given pulse polio doses?
The aim is to flood the intestines of susceptible hosts by OPV. Hence all children <5 years of age should receive pulse OPV and this is given even if they have completed their routine OPV, even if they have taken a vaccine in recent past, even if they are going to receive a vaccine in the recent future, even if they are hospitalized, even if they may have taken or not taken prior pulse doses of OPV. Only very sick children especially in hospital intensive care units, preterm babies, leukemic children or severely immunocompromised children should not be given pulse OPV. This, mind you well, is an extremely rare situation. Even those who are travelling should receive the pulse dose wherever they are on the day of pulse immunization. Even if a baby is born after the last date of registration or even if the name is not found at a center, pulse OPV must be given at any near by centre.

23. Do you need to give regular OPV doses when pulse OPV is taken?
The answer is an emphatic yes. No vaccination programme can be successful without continuing routine programme. In fact due to this wrong notion, routine vaccination does suffer a set back including not giving DPT & even other EPI vaccines. This is neither advisable nor desirable. Hence it is our primary duty to ensure good coverage of routine immunization too, besides the success of pulse polio campaign.