Polio vaccine
Polio vaccine is a vaccine used to prevent poliomyelitis. Two types are used: an inactivated poliovirus given by injection and a weakened poliovirus given by mouth. The World Health Organization 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, a form of the vaccine virus that has reverted to causing poliomyelitis, has led to the development of novel oral polio vaccine type 2, 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 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.
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 and oral polio vaccine.Inactivated
When the IPV 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 risk of vaccine-derived polio in the oral vaccine.Attenuated
Oral polio vaccines were easier to administer than IPV, as they eliminated the need for sterile syringes, so were 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 nature. As an attenuated but active virus, it can induce vaccine-associated paralytic poliomyelitis in roughly one individual per every 2.7million doses administered. The live virus can circulate in under-vaccinated populations, 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 type2 this was phased out in 2016 and replaced with bivalent vaccine containing just types 1 and 3, supplemented with monovalent type2 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, which increased from two cases in 2016 to 1037 cases in 2020.
A novel OPV2 vaccine, 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.
Schedule
In countries with endemic polio or where the risk of imported cases is high, the WHO recommends OPV vaccine at birth followed by a primary series of three OPV doses and at least one IPV dose starting at 6 weeks of age, with a minimum of 4 weeks between OPV doses. In countries with more than 90% immunization coverage and low risk of importation, the WHO recommends one or two IPV doses starting at two months of age followed by at least two OPV doses, with the doses separated by 4–8 weeks depending on the risk of exposure. In countries with the highest levels of coverage and the lowest risks of importation and transmission, the WHO recommends a primary series of three IPV injections, with a booster dose after an interval of six months or more if the first dose was administered before two months of age.Side effects
The inactivated polio vaccines are very safe. Mild redness or pain may occur at the site of injection. They are generally safe to be given to pregnant women and those who have HIV/AIDS, but are otherwise well.Allergic reaction to the vaccine
Inactivated polio vaccine can cause an allergic reaction in a few people, since the vaccine contains trace amounts of antibiotics, streptomycin, polymyxin B, and neomycin. It should not be given to anyone who has an allergic reaction to these medicines. Signs and symptoms of an allergic reaction, which usually appear within minutes or a few hours after receiving the injected vaccine, include breathing difficulties, weakness, hoarseness or wheezing, heart-rate fluctuations, skin rash, and dizziness.Vaccine-associated paralytic polio
A potential adverse effect of the Sabin OPV is caused by its known potential to recombine to a form that causes neurological infection and paralysis. The Sabin OPV results in vaccine-associated paralytic poliomyelitis in around one individual per every 2.7million doses administered, with symptoms identical to wild polio. Due to its improved genetic stability, the novel OPV has a reduced risk of this occurring.Contamination concerns
In 1960, the rhesus monkey kidney cells used to prepare the poliovirus vaccines were determined to be infected with the simian virus-40, which was also discovered in 1960 and is a naturally occurring virus that infects monkeys. In 1961, SV40 was found to cause tumors in rodents. More recently, the virus was found in certain forms of cancer in humans, for instance brain and bone tumors, pleural and peritoneal mesothelioma, and some types of non-Hodgkin lymphoma. However, SV40 has not been determined to cause these cancers.SV40 was found to be present in stocks of the injected form of the IPV in use between 1955 and 1963; it is not found in the OPV form. Over 98 million Americans received one or more doses of polio vaccine between 1955 and 1963, when a proportion of vaccine was contaminated with SV40; an estimated 10–30 million Americans may have received a dose of vaccine contaminated with SV40. Later analysis suggested that vaccines produced by the former Soviet bloc countries until 1980, and used in the USSR, China, Japan, and several African countries, may have been contaminated, meaning hundreds of millions more may have been exposed to SV40.
In 1998, the National Cancer Institute undertook a large study, using cancer case information from the institute's SEER database. The published findings from the study revealed no increased incidence of cancer in persons who may have received vaccine containing SV40. Another large study in Sweden examined cancer rates of 700,000 individuals who had received potentially contaminated polio vaccine as late as 1957; the study again revealed no increased cancer incidence between persons who received polio vaccines containing SV40 and those who did not. The question of whether SV40 causes cancer in humans remains controversial, however, and the development of improved assays for detection of SV40 in human tissues will be needed to resolve the controversy.
File:Bundesarchiv B 145 Bild-F025952-0015, Bonn, Gesundheitsamt, Schutzimpfung.jpg|thumb|Doses of oral polio vaccine are added to sugar cubes for use in a 1967 vaccination campaign in Bonn, West Germany
During the race to develop an oral polio vaccine, several large-scale human trials were undertaken. By 1958, the National Institutes of Health had determined that OPV produced using the Sabin strains was the safest. Between 1957 and 1960, however, Hilary Koprowski continued to administer his vaccine around the world. In Africa, the vaccines were administered to roughly one million people in the Belgian territories. The results of these human trials have been controversial, and unfounded accusations in the 1990s arose that the vaccine had created the conditions necessary for transmission of simian immunodeficiency virus from chimpanzees to humans, causing HIV/AIDS. These hypotheses, however, have been conclusively refuted. By 2004, cases of poliomyelitis in Africa had been reduced to just a small number of isolated regions in the western portion of the continent, with sporadic cases elsewhere. Recent local opposition to vaccination campaigns has evolved due to lack of adequate information, often relating to fears that the vaccine might induce sterility. The disease has since resurged in Nigeria and several other African nations without necessary information, which epidemiologists believe is due to refusals by certain local populations to allow their children to receive the polio vaccine.