Vaccines Against Poliomyelitis

Dr.  Parang Mehta, MD.

Polio is an ancient disease, and has been feared through centuries.  Before the vaccines became available, it caused thousands of deaths every years, and left an even larger number crippled for life.  Polio vaccine was introduced in 1955, and since then, this terrible disease has become rarer and rarer.  Currently, most countries are free from this illness, and we may soon be able to eradicate it from the world.  In 2020, endemic poliomyelitis is found only in two countries - Afghanistan and Pakistan.  Outbreaks happen in neighbouring countries and other places where travellers carry it.  That is why global eradication of the disease is such a priority.

Though polio is endemic in only two countries now, vaccination against it is needed because of the threat posed by travel, migration, and bioterrorism

Poliomyelitis (to give it its full name) is a viral disease.  It is caused by three types of viruses, named simply types 1, 2, and 3.  Polio vaccines have traditionally been active against all three types.  Type 2 virus has been eradicated around the world now, and many countries are using bivalent (combination of type 1 and 3) or monovalent vacicnes.

Two types of vaccine are available -- the injectable (killed vaccine) and the oral (live vaccine).  Both the vaccines are effective, and different countries use one or the other, or sometimes a combination of the two types.

Inactivated Polio Vaccine (IPV)

The inactivated polio vaccine (called the Salk vaccine after its developer, Dr. Jonas Salk) was introduced in 1955.  It is made from killed polioviruses of all three types.  It is given by injection, and is a very effective vaccine.  99% of children that receive three doses of this vaccine develop immunity to polio.

The primary series of two doses of IPV are given at the age of 2 and 4 months, and the third between age 6-18 months.  A fourth dose after age 4 years is sometimes recommended.  The primary series can be given as three doses with the DPT and other vaccines, for operational reasons.

IPV is available as a combined vaccine with other childhood vaccines, to reduce the number of injections a child needs to take.  

Oral Polio Vaccine (OPV)

The oral poliomyelitis vaccine is a live vaccine, made by weakening the three types of polio viruses.  It is generally very effective in producing immunity to polio.  However, in developing countries, its efficacy has been found to be low.

In most countries, this vaccine was given in a primary series of three doses in the first year, with one or two boosters.  Some countries use a larger number of doses.  Once polio has been eradicated from the country, this vaccine is usually replaced by IPV, because of the problem of VAPP (see sidebar).

What is Pulse Polio?

We have now realised that no number of doses of the oral polio vaccine (OPV) to an individual child can guarantee protection against poliomyelitis.  Safety against this crippling disease requires eradication of the infection from the world.

The Pulse Polio Campaigns being conducted for the past few years are aimed at eradication of Polio.  It is very important that all children in the target age (birth to five years) recieve the oral polio vaccine on the appointed day, regardless of earlier vaccination.  This will deny the wild (disease causing) virus any place to survive, and it will be wiped out.

What is VAPP?

The oral polio vaccine (OPV) is live polio viruses, which have been weakened.  These weakened viruses, when introduced into the body, stimulate the body's immunity, but do not cause paralytic disease.  That, at least, is the theory.  Sometimes, the vaccine virus reverts to a virulent form, and causes paralytic disease.  This type of disease is known as vaccine associated paralytic poliomyelitis (VAPP).

VAPP is very rarely seen, occurring once in 2 to 3 million doses of OPV.  The first dose of OPV is much more likely to cause VAPP than later doses.  Recipients older than 18 years of age are also at a higher risk.  VAPP is especially common in children with immunodeficiency disorders.  Once a country has eliminated wild polio viruses, this side effect of oral polio vaccine becomes unacceptable.

IPV consists of killed bacteria, and so can never cause VAPP.

 

Last Revision: September 19, 2020