Iatrogenic (vaccine-induced) polio
A major concern about the oral polio vaccine (OPV) is its known ability to revert to a form that can achieve neurological infection and cause paralysis.[37] Clinical disease, including paralysis, caused by vaccine-derived poliovirus (VDPV) is indistinguishable from that caused by wild polioviruses.[38] This is believed to be a rare event, but outbreaks of vaccine-associated paralytic poliomyelitis (VAPP) have been reported, and tend to occur in areas of low coverage by OPV, presumably because the OPV is itself protective against the related outbreak strain.[39][40]
As the incidence of wild polio diminishes, nations transition from use of the oral vaccine back to the injected vaccine because the direct risk of iatrogenic polio (VAPP) due to OPV outweighs the indirect benefit of immunization via subclinical transmission of OPV. When IPV is used, reversion is not possible but there remains a small risk of clinical infection upon exposure to reverted OPV or wild polio virus. Following the widespread use of polio vaccines in the mid-1950s, the incidence of poliomyelitis declined rapidly in many industrialized countries. The use of OPV was discontinued in the United States in 2000 and in 2004 in the UK, but it continues to be used around the globe.[16][29]
The rate of vaccine-associated paralytic poliomyelitis (VAPP) varies by region but is generally about 1 case per 750,000 vaccine recipients.[41] VAPP is more likely to occur in adults than in children. In immunodeficient children, the risk of VAPP is almost 7,000 times higher, particularly for persons with B-lymphocyte disorders (e.g., agammaglobulinemia and hypogammaglobulinemia), which reduce the synthesis of protective antibodies.[38] The World Health Organization considers the benefits of vaccination to far outweigh the risk of vaccine derived polio. Outbreaks of vaccine derived polio have been stopped by multiple rounds of high-quality vaccination, in order to immunize the entire population.[42]
Outbreaks of VAPP occurred independently in Belarus (1965–66), Canada (1966–68), Egypt (1983–1993), Hispaniola (2000–2001), Philippines (2001), Madagascar (2001–2002),[43] and in Haiti (2002), where political strife and poverty have interfered with vaccination efforts.[44] In 2006 an outbreak of vaccine-derived poliovirus occurred in China.[45] Cases have been reported from Cambodia (2005–2006), Myanmar (2006–2007), Iran (1995, 2005–2007), Syria, Kuwait and Egypt.[46] Since 2005, The World Health Organization has been tracking vaccine-caused polio in northern Nigeria caused by a mutation in live oral polio vaccines.[47]
Contamination concerns
In 1960, it was determined that the rhesus monkey kidney cells used to prepare the poliovirus vaccines were infected with the SV40 virus (Simian Virus-40).[48] SV40 was also discovered in 1960 and is a naturally occurring virus that infects monkeys. In 1961, SV40 was found to cause tumors in rodents.[49] More recently, the virus was found in certain forms of cancer in humans, for instance brain and bone tumors, mesotheliomas, and some types of non-Hodgkin's lymphoma.[50][51] However, it has not been determined that SV40 causes these cancers.[52]
SV40 was found to be present in stocks of the injected form of the polio vaccine (IPV) in use between 1955 to 1963.[48] It is not found in the OPV form.[48] Over 98 million Americans received one or more doses of polio vaccine between 1955 to 1963 when a proportion of vaccine was contaminated with SV40; it has been estimated that 10–30 million Americans may have received a dose of vaccine contaminated with SV40.[48] 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.[53]
In 1998, the National Cancer Institute undertook a large study, using cancer case information from the Institutes SEER database. The published findings from the study revealed that there was no increased incidence of cancer in persons who may have received vaccine containing SV40.[54] 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.[55] 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.[52]
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 were the safest.[13] 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, now the Democratic Republic of the Congo, Rwanda and Burundi.[56][57] The results of these human trials have been controversial,[58] and accusations in the 1990s arose that the vaccine had created the conditions necessary for transmission of SIV from chimpanzees to humans, causing HIV/AIDS. These hypotheses have, however, been refuted.[56] 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. However, recent opposition to vaccination campaigns has evolved,[59][60] often relating to fears that the vaccine might induce sterility.[61] The disease has since resurged in Nigeria and in several other African nations, which epidemiologists believe is due to refusals by certain local populations to allow their children to receive the polio vaccine.[62]
(Source from en.Wikipedia.org/wiki/Polio_vaccine)
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