Poliomyelitis Vaccination

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Chapter: Pharmaceutical Microbiology : Vaccination And Immunization

Poliovirus, a picornavirus, has three immunologically distinct serotypes (I, II and III) and there may be three phases of the disease.





Poliovirus, a picornavirus, has three immunologically distinct serotypes (I, II and III) and there may be three phases of the disease. The first is an acute infection of lymphoid tissues associated with the gastrointestinal tract (Peyer’s patches), during which time the virus can be found in the throat and in faeces. The second phase is characterized by an invasion of the bloodstream, and in the third phase the virus migrates from the bloodstream into the meninges. Infections range in severity from asymptomatic (the majority of cases) to paralytic poliomyelitis which may cause permanent neurological damage and muscle paralysis. Paralytic poliomyelitis is a major illness, but occurs in only 0.1-2% of cases. It is characterized by the destruction of large nerve cells in the anterior horn of the brain, resulting in varying degrees of paralysis, and unvaccinated adults are at greater risk of paralytic infection than children. The infection is transmitted by the faecal-oral route.

Polio is the only disease, at present, for which both live and killed vaccines compete. Since the introduction of the killed virus (Salk) in 1956 and the live, attenuated virus (Sabin) in 1962 there has been a remarkable decline in the incidence of poliomyelitis. The inactivated polio (Salk) vaccine (IPV) contains formalin-killed poliovirus of all three serotypes. On injection, the vaccine stimulates the production of antibodies of the IgM and IgG class that neutralize the virus in the second stage of infection. A course of three injections at monthly intervals produces long-lasting immunity to all three poliovirus types. The live, oral polio (Sabin) vaccine (OPV) is now less commonly used. Advantages over the IPV vaccine include lower costs and easier administration. OPV contains attenuated poliovirus of each of the three types and is administered, as a liquid, onto the tongue. The vaccine strains infect the gastrointestinal mucosa and oropharynx, promoting the common immune response, and involving both humoral and secretory antibodies. IgA, secreted within the gut epithelium, provides local resistance to the first stages of poliomyelitis infection. OPV therefore provides protection at an earlier stage of the infection than does IPV. Infection of epithelial cells with one strain of entero-virus, however, may inhibit simultaneous infection by related strains. At least three administrations of OPV are therefore given, with each dose conferring immunity to one of the vaccine serotypes. These doses must be separated by a period of at least 1 month in order to allow the previous infection to lapse. Booster vaccinations are also provided to cover the eventuality that some other enterovirus infection, present at the time of vaccination, had reduced the response to the vaccine strains. Faecal excretion of vaccine virus will occur and may last for up to 6 weeks after administration. Such released virus may spread to close contacts and infect or (re)immunize them. Vaccine-associated poliomyelitis may occur through reversion of the attenuated strains to the virulent wild type, particularly with types II and III and has been estimated to occur once per 4 million doses. Since the introduction of OPV, notifications of paralytic poliomyelitis in the UK have markedly dropped. However, from 1985 to 1995, 19 of the 28 notified cases of paralytic poliomyelitis were associated with reverted vaccine strains (14 recipients, 5 contacts). As the risk of natural infections with poliomyelitis within developed countries has now diminished markedly, the live vaccine strains present the greatest risk. Consequently, OPV has now been replaced with IPV as the polio vaccine of choice in the UK and USA.


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