Two types of vaccine are available against poliomyelitis, inactivated vaccine (IPV, Salk) and a live attenuated oral vaccine (OPV, Sabin). Both vaccine formulations contain all three polio types.
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Oral Polio Vaccine (OPV) /Sabin
The oral polio vaccine was the most widely used vaccine for the prevention of poliomyelitis. It is composed of attenuated strains of the three poliovirus types and is administered orally. It has been instrumental in nearly eradicating the virus from the planet. Lately, WHO has recommended the replacement of OPV with inactivated poliovirus vaccine (IPV) as scientist finds evidence of reversion to virulence in OPV strains.
At least two or three doses are considered necessary to ensure adequate immunity. In some countries, even five to six or more doses are given in the primary course. Revaccination is used to varying degrees. A full primary course induces an antibody response against all three types in more than 90% of vaccinated individuals and gives a high degree of protection against disease.
OPV also induces intestinal immunity due to the production of secretory IgA antibodies. This is important for inhibiting virus replication in the gut, diminishing the virus’s possible spread to susceptible contacts. OPV is almost non-reactogenic and is very safe. However, an attenuated vaccine strain may induce paralytic disease in a few cases. This occurs in about one case per 1–10 million vaccine doses.
Inactivated Poliovirus Vaccine (IPV)
Inactivated poliovirus vaccine (IPV) was the first vaccine used against poliomyelitis. It contains all three types of poliovirus inactivated by formaldehyde and is administered parenterally. The use of IPV in the late 1950s was followed by a 90% reduction of poliomyelitis cases when it was replaced in many countries by the more easily administered OPV around 1960. Newer IPVs have higher immunogenic potency, leading to a reintroduction of IPV in many developed and developing countries.
The primary vaccination course with IPV consists of two or three doses, usually followed by revaccination after intervals of about 5–10 years during childhood and adolescence. Some countries are using a combination of OPV and IPV.
Passive immunization is of little value.
Salk killed vaccine
- Viral pools of adequate titer were filtered and inactivated with formalin (1:4000) at 37°C / 12-15 days and given by injection.
- Three doses are given 4-6 weeks apart, plus a booster six months later.
- 1st dose to babies after the age of 6 months.
Sabin live polio vaccine ( OPV)
- developed by plaque selection in monkey kidney tissue culture on HDCC.
- Stringent precautions must be taken to ensure freedom from extraneous agents like SV40 and B- virus.
- Issued in monovalent or trivalent form; 3 doses at 4-8 weeks intervals.
- Vaccine stabilized by MgCl2; shelf life at 4-80°C is four months and at -200C is two years.
- Improper storage conditions and cold chain failure are partly responsible for OPV’s apparent failure.
Oral polio virus strains in the live vaccines infect the host, multiply in the host cells, and disseminate in the community. It produces not only IgM and IgG but also IgA in the intestine and becomes resistant to re-infection, but OPV is not safe in immunocompromised individuals.
If the alimentary tract of a child is infected with another enterovirus at the time vaccine is given, the establishment of polio infection and immunity may be blocked. Frequent diarrhoeal diseases prevent colonization by vaccine viruses. Breastfeeding immediately before or after the vaccine may neutralize the vaccine virus.
Both killed and live vaccines protect the CNS, but the gut develops a far increased degree of resistance after administering a live-virus vaccine.