Polio vaccine is a vaccine used to prevent poliomyelitis (polio). Two types are used: an inactivated poliovirus given by injection (IPV) and a weakened poliovirus given by mouth (OPV). The World Health Organization (WHO) recommends all children be fully vaccinated against polio. The two vaccines have eliminated polio from most of the world, and reduced the number of cases reported each year from an estimated 350,000 in 1988 to 33 in 2018. The inactivated polio vaccines are very safe. Mild redness or pain may occur at the site of injection. Oral polio vaccines cause about three cases of vaccine-associated paralytic poliomyelitis per million doses given. This compares with 5,000 cases per million who are paralysed following a polio infection. Both types of vaccine are generally safe to give during pregnancy and in those who have HIV/AIDS, but are otherwise well. However, the emergence of circulating vaccine-derived poliovirus (cVDPV), a form of the vaccine virus that has reverted to causing poliomyelitis, has led to the development of novel oral polio vaccine type 2 (nOPV2), which aims to make the vaccine safer and thus stop further outbreaks of cVDPV. The first successful demonstration of a polio vaccine was by Hilary Koprowski in 1950, with a live attenuated virus that people drank. The vaccine was not approved for use in the United States, but was used successfully elsewhere. The success of an inactivated (killed) polio vaccine, developed by Jonas Salk, was announced in 1955. Another attenuated live oral polio vaccine, developed by Albert Sabin, came into commercial use in 1961. The polio vaccine is on the World Health Organization's List of Essential Medicines.
Medical uses
Interruption of person-to-person transmission of the virus by vaccination is important in global polio eradication. This is because no long-term carrier state exists for poliovirus in individuals with normal immune function, polio viruses have no non-primate reservoir in nature, and the survival of the virus in the environment for an extended period appears to be remote. The two types of vaccine are inactivated polio vaccine (IPV) and oral polio vaccine (OPV).
Inactivated When the IPV (injection) is used, 90% or more of individuals develop protective antibodies to all three serotypes of poliovirus after two doses, and at least 99% are immune following three doses. The duration of immunity induced by IPV is not known with certainty, although a complete series is thought to protect for many years. IPV replaced the oral vaccine in many developed countries in the 1990s mainly due to the (small) risk of vaccine-derived polio in the oral vaccine.
Attenuated Oral polio vaccines are easier to administer than IPV, as they eliminate the need for sterile syringes, so are more suitable for mass vaccination campaigns. OPV also provided longer-lasting immunity than the Salk vaccine, as it provides both humoral immunity and cell-mediated immunity.
One dose of trivalent OPV produces immunity to all three poliovirus serotypes in roughly 50% of recipients. Three doses of live-attenuated OPV produce protective antibodies to all three poliovirus types in more than 95% of recipients. As with other live-virus vaccines, immunity initiated by OPV is probably lifelong. OPV produces excellent immunity in the intestine, the primary site of wild poliovirus entry, which helps prevent infection with wild virus in areas where the virus is endemic. OPV does not require special medical equipment or extensive training. Attenuated poliovirus derived from the OPV is excreted for a few days after vaccination, potentially infecting and thus indirectly inducing immunity in unvaccinated individuals, thus amplifying the effects of the doses delivered. Taken together, these advantages have made it the favored vaccine of many countries, and it has long been preferred by the global eradication initiative. The primary disadvantage of OPV derives from its inherent, if small, risks. As an attenuated but active virus, it can induce vaccine-associated paralytic poliomyelitis (VAPP) in roughly one individual per every 2.7 million doses administered. The live virus can circulate in under-vaccinated populations (termed either variant poliovirus or circulating vaccine-derived poliovirus, cVDPV), and over time can revert to a neurovirulent form causing paralytic polio. This genetic reversal of the pathogen to a virulent form takes a considerable time and does not affect the person who was originally vaccinated. With wild polio cases at record lows, 2017 was the first year where more cases of cVDPV were recorded than the wild poliovirus. Until recent times, a trivalent OPV containing all three viral strains was used, and had nearly eradicated polio infection worldwide. With the complete eradication of wild poliovirus type 2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, supplemented with monovalent type 2 OPV in regions where cVDPV type 2 was known to circulate. The switch to the bivalent vaccine and associated missing immunity against type 2 strains, among other factors, led to outbreaks of circulating vaccine-derived poliovirus type 2 (cVDPV2), which increased from two cases in 2016 to 1037 cases in 2020. A novel OPV2 vaccine (nOPV2), which has been genetically modified to reduce the likelihood of disease-causing activating mutations, was granted emergency licencing in 2021, and subsequently full licensure in December 2023. This has greater genetic stability than the traditional oral vaccine and is less likely to revert to a virulent form. Genetically stabilised vaccines targeting poliovirus types 1 and 3 are in development, with the intention that these will eventually completely replace the Sabin vaccines.
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