Monday, July 11, 2011

Polio Vaccine

There are two types of polio vaccine, both of which were developed in the 1950s. The first, developed by Jonas Salk, is a formalin-killed preparation of normal wild type polio virus. This is grown in monkey kidney cells and the vaccine is given by injection. It elicits good humoral (IgG) immunity and prevents transport of the virus to the neurons where it would otherwise cause paralytic polio. This vaccine is the only one used in some Scandinavian countries where it completely wiped out the disease.

A second vaccine was developed by Albert Sabin. This is a live attenuated vaccine that was produced empirically by serial passage of the virus in cell culture. This resulted in the selection of a mutated virus that grew well in culture and, indeed, in the human gut where the wild type virus grows. It cannot, however, migrate to the neurones. It replicates a normal infection since the virus actually grows in the vaccinee and it elicits both humoral and cell-mediated immunity. It is given orally, a route that is taken by the virus in a normal infection since the virus is passed from human to human by the oral-fecal route. This became the preferred vaccine in the United States, United Kingdom and many other countries because of it ease of administration (often on a sugar lump), the fact that the vaccine virus replicates in the gut and only one administration is needed to get good immunity (though repeated administration was usually used). In addition, the immunity that results from the Sabin vaccine lasts much longer that that by the Salk vaccine, making fewer boosters necessary. Since it elicits mucosal immunity (IgA) in the gut, the Sabin vaccine has the potential to wipe out wild type virus whereas the Salk vaccine only stops the wild type virus getting to the neurons.

The attenuated Sabin vaccine, however, came with a problem: back mutation. This may result from recombination between wild type virus and the vaccine strain. Virulent virus is frequently isolated from recipients of the Sabin vaccine. The residual cases in countries that use the attenuated live virus vaccine (about 8 per year in the US until recently) resulted from mutation of the vaccine strain to virulence. About half of these cases were in vaccinees and half in contacts of vaccinees. Paralytic polio arises in 1 in 100 cases of infection by wild type virus and 1 in 4 million vaccinations as a result of back reversion of the vaccine to virulence. This was deemed acceptable as the use of the attenuated virus means that the vaccine strain of the virus still replicates in the body and gives gut immunity via IgA.

The vaccinee who has received killed Salk vaccine still allows wild type virus to replicate in his/her gastro-intestinal tract, since the major immune response to the injected killed vaccine is circulatory IgG. As noted above, this vaccine is protective against paralytic polio since, although the wild type virus can still replicate in the vaccinee's gut, it cannot move to the nervous system where the symptoms of polio are manifested.
By the way, today Polio vaccines have been further improved – and Polio hopefully eradicated soon.

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