Oral Polio Vaccine

While the NHS’ childhood immunisation programme makes use of the inactivated polio vaccine (IPV) as part of combination injections that immunise against polio and other diseases, there is another polio vaccine in widespread use which has some important applications. In this article we look at the oral vaccine and how it was and is used as part of immunisation schemes in the world today.

The oral polio vaccine or OPV

Unlike its contemporary, the IPV, the oral polio vaccine is a live attenuated vaccination made up of living poliovirus. The poliovirus has been specially grown in such a manner that is in a weakened (attenuated) state. This means that even though the virus is live, it is safe for human applications.

The OPV was developed after the IPV, and one of the groups leading its production was headed by Dr Albert Sabin. These vaccines were eventually associated with Dr Sabin, and the strains of virus used are sometimes referred to as Sabin strains.

The oral vaccine is extremely effective because it is a live, attenuated strain. Live viruses confer more lasting immunity because they act like an actual infection, stimulating more of the immune system than an inactivated alternative would. Our immune systems are extremely complex and involve different elements that work in concert or isolation. While the IPV stimulates a part of the system called IgG –mediated immunity (based on the production of the IgG antibody), the OPV stimulates the entire immune response. This is why a single dose of OPV can provide lasting immunity, while multiple booster shots of IPV are needed to maintain immunity against the condition.

The OPV works by providing its immunity in the intestines, which is where poliovirus makes its way into the body. In contrast IPV works by largely preventing the effects of the virus on the nervous system.

Is the OPV still used?

The OPV is still used in parts of the world where poliovirus is endemic (still regularly affecting the population). In these locations the protection offered by the OPV is needed to reduce the transmission of the disease and protect communities as a whole.

In the UK the OPV is no longer part of routine childhood immunisation. This is because live viruses carry with them some small risks, which while acceptable where the disease is widespread and a major concern, are more of a concern where transmission of polio is almost negligible. Here in the UK IPV is used routinely, and has proven highly effective.

The major concern with live vaccines like OPV is the risk of reversion. This is a phenomenon that occurs when an attenuated virus undergoes a change in its genetic material which makes it virulent. While the chances of this are very small, it is still a concern, which is why IPV is widely used in countries where polio infection has been dramatically reduced in the wider population.

OPVs are still sometimes provided as travel vaccines for adults travelling to parts of the world where the transmission of the disease is still a concern.

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