What is IPV?
Inactivated polio vaccine (IPV) consists of inactivated (killed) strains of all three poliovirus types. IPV is given by intramuscular injection and must be administered by a trained health worker. IPV produces antibodies in the blood to all three types of poliovirus. In the event of infection, these antibodies prevent the spread of the virus to the central nervous system and protect against paralysis. In this way, IPV prevents infection, but it does not stop transmission of the virus.
Why use OPV?
The oral polio vaccine is simple to administer. It can be given by volunteers and does not require trained health workers or sterile injection equipment. The vaccine is relatively inexpensive. For several weeks after vaccination, the vaccine virus replicates in the intestine, is excreted in the faeces and can be spread to people in close contact. This means that, in areas where hygiene and sanitation are poor, vaccination with OPV can result in "passive" immunization of people who have not been directly vaccinated. After three doses of OPV, a person becomes immune for life and can no longer transmit the virus to others if exposed again. Thanks to this "gut immunity", OPV is the only effective weapon to stop transmission of the poliovirus when an outbreak is detected.
Why use IPV?
An increasing number of polio-free countries use IPV as the vaccine of choice. This is because the risk of emergence of a circulating vaccine-derived poliovirus (cVDPV) with continued routine use of OPV (see below) is deemed greater than the risk of a wild virus importation. However, as IPV prevents infection but does not stop transmission of the virus, until OPV is completely phased out, it will continue to be used wherever a polio outbreak has to be contained, even in countries that use exclusively IPV in their routine immunization programme. Three doses of IPV confer life-long protection against the disease. National immunization programmes will continue to administer IPV for some years after global polio eradication has been declared.
Why is OPV being phased out?
The Polio Eradication & Endgame Strategic Plan 2013-2018 (the Endgame Strategy) set the goal of a polio-free world by 2018. Achieving this goal requires a number of steps, including cessation of the use of OPV. Although OPV is safe and effective, in extremely rare cases (about 1 in every 2.7 million first doses of the vaccine), the live attenuated vaccine virus in OPV can cause paralysis. It is believed that in some cases vaccine-associated paralytic polio (VAPP) can be triggered by immune deficiency. The extremely low risk of VAPP is well known and accepted by most public health programmes in the world because, without OPV, hundreds of thousands of children would be crippled every year. A second disadvantage of OPV is that, very rarely, the virus in the vaccine changes genetically and starts to circulate in a population. Such viruses are known as circulating vaccine-derived polioviruses (cVDPV). To remove any risk of polio due to vaccine-derived polioviruses, OPV will be phased out globally from April 2016.