Types of polio vaccines :

Polio vaccines : Two types of vaccines are used throughout the world; they are

1. Inactivated (Salk) polio vaccine (IPV)
2. Oral (Sabin) polio vaccine (OPV)

Mechanism of oral polio vaccine (OPV) :

On administration, the live vaccine strains infect intestinal epithelial cells. After replication, the virus is transported to the Payer’s patches where a secondary multiplication with subsequent viraemia occurs. The virus spreads to other areas of the body resulting in the production of circulating antibodies which prevent dissemination of the virus to the nervous system and prevent paralytic polio.

Intestinal infection stimulates the production of IgA secretory antibodies which prevent subsequent infection of the alimentary tract with wild strains of polio virus, and thus is effective in limiting virus transmission in the community. Thus OPV induces both local and systemic immunity.

The vaccine progeny is excreted in the faces and secondary spread occurs to household contacts and susceptible contacts in the community. Non-immunized persons may therefore, be immunized. Thus widespread “herd immunity” results, even if only approximately 66% of the community is immunized

Advantages of of oral polio vaccine (OPV) :

1. Since given orally, it is easy ‘o administer and does not require the use of highly trained personnel.
2. Induces both humeral and intestinal immunity (PV induces only humoral immunity but not local or intestinal immunity).
3. Antibody is quickly produced, even a single dose elicits substantial immunity.
4. The vaccine excretes the virus and so infects others who are also immunized thereby (herd immunity).
5. Useful in controlling epidemics.
6. Relatively inexpensive.

Complication of oral polio vaccine (OPV) :

OPV is remarkably free from complications However, being living viruses, the vaccine viruses particularly type 3 do mutate in the course of their multiplication in vaccinated children, and rare cases of vaccine associated paralytic polio have occurred in recipients of the vaccine, & their contacts.

Contraindications of oral polio vaccine (OPV) : 

1. Acute infectious diseases
2. Fever
3. Diarrhoea & dysentery
4. Patients suffering from leukaemias and malignancy.
5. Those receiving corticosteroids.

Use of inactivated poliovirus vaccine (IPV) in developed countries :

 Since the efficacy of OPV is not 100%, even with potent vaccines properly administered, some cases may occur Universally, it is being considered that the enhanced inactivated poliovirus vaccine (IPV) for the first one or two doses in a continuation regimen with OPV may prevent post-vaccine poliomyelitis, So. although it is costly inactivated poliovirus vaccine (IPV) is used in developed countries.

Provocative reaction :

oral polio vaccine (OPV) can cause provocative reaction, Several provocative or risk factors have been found to precipitate an attack of paralytic polio in individuals already infected with polio viruses. They include fatigue, trauma intramuscular injections, operative procedures, such as tonsillectomy undertaken especially during epidemics of polio and administration of immunizing agents particularly alum-containing DPT.

Favouring points for eradication of polio :

1. Man is the only host.
2. No long term carrier state
3. OPV is easy to administer
4. OPV is cheap
5. Half-life of excreted virus 48 hours & spread can only occur during his period
6. Live attenuated viruses in OPV multiply in intestine & interrupt the transmission of the wild viruses.
7. OPV induces both humoral & intestinal immunity.
8. Antibody is quickly produced.
9. Viruses can be transmitted from a recently vaccinate child to close contacts who have not immunized.
10. No insect, vector or animal is involved in the transmission of the disease.
11. Having international cooperation (NID).

Disfavouring point for eradication of polio :

1. The poliovirus has 3 serotypes 1, 2 & 3 Immunization against one type does not protect completely against the other 2 types of viruses.
2. Poliovirus can survive in water for 4 months & in faeces for 6 months.
3. The virus is well adapted for the faeco-oral transmission.
4. Most infections are subclinical.
5. Overcrowding and poor sanitation favour the spread of infection in our country.
6. There is no specific treatment for polio.
7. OPV strict preservation by cold chain maintenance is required which is very difficult
8. Children developing polio in spite of dose of OPV is a common observation
9. Immunization of all children should be done simultaneously, which is actually difficult.