Polio Vaccine - Contamination Concerns

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). 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. 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's lymphoma. However, it has not been determined that SV40 causes these cancers.

SV40 was found to be present in stocks of the injected form of the polio vaccine (IPV) in use between 1955 to 1963. It is not found in the OPV form. 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. 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 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. 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.

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. 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. The results of these human trials have been controversial, 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. 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, often relating to fears that the vaccine might induce sterility. 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.