Why is polio vaccine prepared in injection form?

Answer 1

There are still two types of polio vaccines available. One of those is administered PO ("per os" ~ Latin for "by mouth") known as OPV (oral polio vaccine) and the other is the injected form IPV (inactivated polio vaccine). Oral Polio Vaccine (OPV) is a live attenuated (weakened) vaccine. As the name implies, IPV is a vaccine made with inactive/"dead" virus particles.

The polio virus is still in the environment in some places and usually gets into the body by way of the intestines, so the oral vaccine is especially effective against the "wild" viruses through gastrointestinal immunity. The wild viruses are not as prevalent as in the past due to the long term use of polio vaccines, and are currently seen in only some areas of the world without that history of vaccinations.

There are no preservatives in the live attenuated oral polio vaccine, but there are some traces of some antibiotics used in the purification of contaminates that may get in the vaccine during manufacture. Those people with antibiotic Allergies need to talk to their health care professionals about any allergic reaction risks, as you would for any prescriptions in allergic individuals.

The live virus also presents some extra difficulty with handling and storage, and is more difficult to preserve in hot areas or over long transport to out of the way locations. This is compounded by strong regulations about the storage and preservation of the OPV that adds costs to maintain compliance and so can make IPV more preferred.

The OPV produces immunity to three poliovirus types. Three doses can produce immunity in 95% of those who receive oral polio vaccine. One dose is effective in 50%.

There can be some "secondary immunization" provided to others by those who receive the oral vaccine and pass the virus through their feces. This gives, in a sense, a vaccination to the others exposed to their feces and the minute doses of the vaccine it contains. This is a plus in the more remote and economically deprived locations.

There is also the injected IPV (inactivated polio vaccine) that mostly prevents the virus from getting into the body through the nervous system. This is what is seen used most often in areas of the world that do not still have wild polio in the environment (like in the US). However, the live vaccine is still used in areas of the world where there is more incidence of the disease in the "wild". This is because any concerns about an increase in the virulence of the weakened live virus that could potentially cause infection are offset by the increased effectiveness, need for fewer doses, and often life long protection that a live vaccine produces.