There are 2 kinds of Polio vaccines available worldwide.
- Polio injection : Inactivated Poliovirus Vaccine, IPV (Salk vaccine), licensed in 1955
- Polio drops : Attenuated Oral Poliovirus Vaccine, OPV (Sabin vaccine), licensed as monovalent OPV in 1961 and as trivalent OPV in 1963. Bivalent Oral Polio drops have been introduced in April 2016.
Inactivated Poliovirus Vaccine (IPV)
- IPV contains inactivated (killed) poliovirus strains of all three poliovirus types 1, 2 and 3
- IPV acts by producing antibodies against all three types of poliovirus in the blood. When Polio infection occurs, these antibodies prevent the spread of the virus to the central nervous system and protect the recipient against paralysis
- IPV is a shot given as an intramuscular injection (preferred) or subcutaneous injection in the leg or arm depending on the age of the recipient. IPV is available as a single antigen shot or combined with other routine vaccine antigens like Diphtheria, Tetanus, whole-cell or acellular Pertussis, Hepatitis B, or Haemophilus influenzae type b
- IPV triggers an excellent protective immune response in most people
Oral Poliovirus Vaccine (OPV)
- Oral Poliovirus Vaccine is composed of live, attenuated polioviruses derived from their parent wild poliovirus strains by growing them in non human cells to give 3 vaccine strains known as Sabin 1, 2 and 3
- The attenuated viral strains have negligent neurovirulence and transmissibility
- OPV replaced IPV in mass vaccination campaigns soon after the introduction due to its ease of administration, no shots just drops, being cheaper because of low production costs and being able to induce superior mucosal immunity of the intestines along with humoral immunity
- Bivalent OPV drops consisting of attenuated strains of type 1 and 3 Polioviruses have been introduced in india as per WHO guideline to eradicate Polio from world. Attenuated Type 2 Poliovirus sometimes regains its virulence and has been found to be associated with vaccine derived paralysis.
Routine Immunization Schedule for Polio
As per WHO guidelines, both OPV and IPV are recommended for immunization of children in India.
- Birth dose of OPV is given soon after birth to increase the immune response for subsequent polio vaccine doses
- 3 doses of IPV alone or as a part of combination vaccines should be given at 6 weeks, 10 weeks and 14 weeks of age. Minimum age of IPV administration is 6 weeks
- Alternatively, 2 doses of IPV at 2 months and 4 months of age is also valid
- If IPV is not feasible for all the primary doses, give IPV shot along with OPV drops at 14 weeks of age. Other doses can be OPV alone.
- At least 2 doses of OPV at 6 months and 9 months should be given after the IPV schedule
- Booster dose of IPV should be given at 12-18 months of age if primary doses of IPV are given
- Third dose of OPV should be given at 4-6 years of age
- Additional doses of OPV on all pulse polio days for children till 5 years of age are mandatory
Safety concerns related to IPV
IPV alone or in combination vaccine is not associated with adverse events other than pain, redness and swelling at the site of injection. It carries no risk of Vaccine-Associated Paralytic Poliomyelitis (VAPP) since it is not a live vaccine.
Safety concerns related to OPV drops
Vaccine-Associated Paralytic Poliomyelitis (VAPP) is a rare (1 case per 1000000 first doses) adverse event associated with OPV. Sabin viruses (mainly sabin2) used in attenuated oral Polio drops can get back the neurovirulence and transmissibility properties of wild poliovirus in the events of low immunization coverage of the population. These vaccine strains can actually cause Paralysis which is indistinguishable from Poliomyelitis caused by wild strains and can only be diagnosed with laboratory analysis. Such outbreaks have been reported in some countries including a large outbreak in Nigeria and characterised as being caused by circulating vaccine-derived poliovirus. It is very important to immunize all the children timely with primary doses of polio vaccine as timely administration of vaccines maintains herd immunity against vaccine preventable diseases. Birth dose of OPV builds mucosal immunity in the intestines before any enteric pathogen interfere with the immune response of further doses. VAPP is also not a threat to the newborns due to presence of maternally derived antibodies. OPV birth dose is not contraindicated in infants born to HIV infected mothers. It is essential to maintain high levels of immunization coverage in India even today when the country has been taken out from the WHO list of Polio endemic countries in 2014.
Why is OPV still used in India along with IPV?
OPV has been discontinued for use in USA and some other parts of developed world since 2000 after the outbreaks of Vaccine-Associated Paralytic Poliomyelitis (VAPP) were reported in 3 continents. According to WHO, OPV drops are still recommended for use in Polio-endemic countries or where the risk of importation and transmission of wild type Poliovirus is high. India is now Polio free but still under the risk of spread of Polio from neighbouring Polio-endemic countries like Pakistan and Afghanistan. Pulse Polio campaigns utilizing OPV drops should not cease even now to maintain herd immunity and Polio free status of the country. So, the schedule following the administration of both IPV and OPV doses which work complementary to each other in Polio eradication drive, is now recommended in India . Bivalent OPV drops introduction in the schedule replacing trivalent OPV drops is the next step in the drive to eradicate Polio from world.
How do Oral Polio drops provide superior immunity than Polio injection?
Attenuated Oral Poliovirus Vaccine (OPV) provides high levels of intestinal mucosal immunity along with hum oral immunity. It stops multiplication of wild Poliovirus inside the intestines and shedding of viruses through the faeces. It also provides long-term immunity, boost immunity and indirectly immunizes others through the spread of attenuated vaccine viruses (herd immunity).
Although Polio injection (IPV) provides an excellent immunity against progression of Poliomyelitis and that includes controlling rare events of VAPP, IPV has a limitation of being able to induce very low levels of mucosal immunity in the intestine. IPV mainly works on developing antibodies in the blood against poliovirus (humoral immunity). So, a person immunized with IPV when infected with wild poliovirus can still have virus multiplying in the intestines and is able to shed the viruses in faeces. This leads to continued circulation of the wild type poliovirus in the community and the whole idea of Polio eradication can go haywire if only IPV is used in the country. As polio drops provide superior immunity against Polio, they should be used along with polio injection to provide full and effective immunity to a child.
Marketed Polio Products
Here is the list of WHO pre-qualified vaccine products available in market. Click on name of the product to know more.