Immunization, Vaccines and Biologicals


Polio is an infectious viral disease, which mainly affects young children. There are 3 types of virus (1, 2 and 3). The virus is transmitted through contaminated food and water, and multiplies in the intestine, from where it can invade the nervous system. One in 200 infections leads to irreversible paralysis, usually in the legs. Among those paralysed, 5-10% die when their breathing muscles become immobilized.

There are two types of polio vaccine in use: the oral polio vaccine (OPV) and the inactivated polio vaccine (IPV). Advantages of OPV compared with IPV are ease of administration (does not need a trained health worker) and the lower cost. The advantage of IPV is that it is not a "live" vaccine (i.e. it is inactivated) and thus carries no risk of vaccine-associated paralysis. Different OPV vaccines (trivalent, bivalent and monovalent) are available, to protect against one or more virus types. The choice of vaccine depends primarily on the prevalence of the three types of virus circulating in the target population. IPV protects against types 1, 2 and 3.

Polio cases have decreased by over 99% since 1988, from an estimated 350 000 cases to just over 1600 in 2009. Only four countries, Afghanistan, India, Nigeria and Pakistan, have never interrupted circulation of the wild poliovirus, although transmission is now limited to small areas within these countries. A further 19 countries, previously polio-free, reported cases and outbreaks due to imported wild poliovirus in 2009.

WHO position papers


Page last updated: 4 June 2010