The oral polio vaccine (OPV) AIDS hypothesis is a now-discredited hypothesis that the AIDS pandemic originated from live polio vaccines prepared in chimpanzee tissue cultures, accidentally contaminated with simian immunodeficiency virus and then administered to up to one million Africans between 1957 and 1960 in experimental mass vaccination campaigns. Data analyses in molecular biology and phylogenetic studies contradict the OPV AIDS hypothesis; consequently, scientific consensus regards the hypothesis as disproven. A 2004 Nature article has described the hypothesis as "refuted". Background: polio vaccines Two vaccines are used throughout the world to combat poliomyelitis. The first, a polio vaccine developed by Jonas Salk, is an inactivated poliovirus vaccine (IPV), consisting of a mixture of three wild, virulent strains of poliovirus, grown in a type of monkey kidney tissue culture (Vero cell line), and made noninfectious by formaldehyde treatment. The second vaccine, an oral polio vaccine (OPV), is a live-attenuated vaccine, produced by the passage of the virus through non-human cells at a sub-physiological temperature. The passage of virus produces mutations within the viral genome, and hinders the virus's ability to infect nervous tissue. Both vaccines have been used for decades to induce immunity to polio, and to stop the spread of the infection. However, OPV has several advantages; because the vaccine is introduced in the gastrointestinal tract, the primary site of poliovirus infection and replication, it closely mimics a natural infection. OPV also provides long lasting immunity, and stimulates the production of polio neutralizing antibodies in the pharynx and gut. Hence, OPV not only prevents paralytic poliomyelitis, but also, when given in sufficient doses, can stop a threatening epidemic. Other benefits of OPV include ease of administration, low cost and suitability for mass vaccination campaigns. Oral polio vaccine Oral polio vaccines were developed in the late 1950s by several groups, including those led by Albert Sabin, Hilary Koprowski and H. R. Cox. A poliovirus type 1 strain called SM was reported in 1954. A less virulent version of the SM strain was reported by Koprowski in 1957. The name of the vaccine strain was "CHAT" after "Charlton", the name of the child who was the donor of the precursor virus. The Sabin, Koprowski and Cox vaccines were clinically tested in millions of individuals and found to be safe and effective. Because monkey trials found fewer side effects with the Sabin vaccine, in the early 1960s, the Sabin vaccine was licensed in