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Vaccine Safety > Issues of Interest > Cancer 
New Institute of Medicine (IOM) Report
Immunization Safety Review: 
SV40 Contamination of Polio Vaccine and Cancer
Questions & Answers

Questions answered on this page:
  1. Why was the review done?
  2. How does the committee examine a hypothesis?
  3. What vaccine safety concern did the IOM committee examine in the October 2002 report?
  4. What were the committee's conclusions regarding SV40-contaminated polio vaccine and cancer?
  5. What recommendations did the committee make regarding policy analysis, communication, and research?

Related links/pages:

  1. Why was the review done?

Public confidence in our immunization programs is essential to our nation’s health. In recent years, increasing public attention has focused on issues regarding vaccine safety. Vaccine safety concerns may decrease public acceptance of immunizations and result in resurgence of vaccine-preventable diseases. Issues involving the safety of vaccines, particularly childhood vaccines, may concern certain members of the public, health care professionals, the public health community, the media, Congress, vaccine manufacturers, and federal agencies.
In response to these concerns, the Centers for Disease Control and Prevention and the National Institutes of Health have asked the National Academy of Sciences, Institute of Medicine (IOM) to establish an independent expert committee to review hypotheses about existing and emerging immunization safety concerns. These reviews include an assessment of factors such as the biologic mechanisms of the hypothesis, competing alternative hypotheses, as well as the available scientific evidence to date.

This is the fifth report completed by the IOM committee. Previous reviews examined the following hypotheses:

  1. How does the IOM committee examine a hypothesis?

For each hypothesis to be examined, the committee assesses both the scientific evidence and the issue’s significance in a broader societal context. For this review, the scientific assessment has two parts:

  1. An examination of evidence of any biological mechanisms relevant to the hypothesis (these are classified as weak, moderate, or strong). Evidence for biological mechanism is not, by itself, sufficient to prove causality.
  2. An examination of the evidence regarding a possible causal relation between the vaccine and the adverse event. 

The significance assessment considers the nature of the health risks associated with the vaccine-preventable disease and with the adverse event in question and the level of public concern about the safety issue. The findings of the scientific and significance assessments provide the basis for the committee’s recommendations.

The Immunization Safety Review Committee is composed of 15 members with expertise in pediatrics, neurology, immunology, internal medicine, infectious diseases, genetics, epidemiology, biostatistics, risk perception and communications, decision analysis, public health, nursing, and ethics. The committee members were selected on the basis of strict criteria to eliminate any potential or perceived conflict of interest. 

  1. What vaccine safety concern did the IOM committee examine in the October 2002 report?

The committee reviewed the concern that SV40-contaminated polio vaccine could contribute to human cancers.

Some of the polio vaccine given from 1955-1963 was contaminated with a virus called simian virus 40 (SV40). The virus occurs in some species of monkeys, though it does not typically cause illness in the animals. SV40 was discovered in 1960 and, soon after, it was identified in polio vaccine. It was found mostly in the injected, inactivated form of the vaccine (IPV), not the kind given by mouth (OPV). At that time, rhesus monkey kidney cells, which contain SV40 if the animal is infected, were used in preparing viral vaccines. Because SV40 was not discovered until 1960, no one was aware that polio vaccine made in the 1950s could be contaminated. In 1961, the virus was found to cause tumors in rodents. That same year, the federal government established testing requirements to verify that all new lots of polio vaccine are free of SV40. However, existing polio vaccine stocks were not recalled and might have been used until 1963. When SV40 was discovered, researchers did not know if the virus could negatively affect people's health. Many viruses that harm animals have no effect on people because of the biological differences between animals and humans.

Interest in SV40 has increased in the last several years because some studies have found the virus in certain forms of cancer in humans, for instance mesotheliomas (rare tumors located in the lungs), brain, and bone tumors. More recently, SV40 has also been found to be associated with some types of non-Hodgkin's lymphoma. However, the data is inconsistent as other studies have not found the virus in such cancer tumors, or report inconsistencies in the presence of the virus.

Polio vaccines being used today do not contain SV40.

For more information on polio vaccine, SV40 and cancer go to

  1. What were the committee's conclusions regarding SV40-contaminated polio vaccine and cancer?

Causality. The committee reviewed a number of studies involving groups of people who received polio vaccine during 1955-1963. These studies show no increased risk of cancer. However, the committee identified a number of limitations in the studies and therefore concluded that there is not enough evidence to accept or reject the hypothesis that SV40-containing polio vaccine causes cancer.

Biological Mechanisms. The committee also reviewed research regarding the effects of SV40 on cultured cells and concluded that the biological evidence is strong that SV40 is a “transforming virus” (able to transform normal cells into abnormal, malignant cells). Similarly, the committee reviewed the existing data on the presence of SV40 in human cancer tumors. The committee pointed out that the detection of SV40 in tumors does not, by itself, demonstrate that the virus causes the tumors. They concluded that the biological evidence is of moderate strength that SV40 exposure could lead to cancer in humans. Finally, the committee considered the question of whether contamination of polio vaccine with SV40 is responsible for the presence of SV40 in humans, noting that other sources of exposure to the virus may exist. Evidence of SV40 has been found in blood samples obtained from people before 1955. The virus has also been found in people who were too young to have received contaminated polio vaccine. This suggests the possibility that the virus can be spread person-to-person, animal-to-person, or through laboratory exposure. The committee concluded that the biological evidence is of moderate strength that SV40 exposure from polio vaccine is related to SV40 infection in humans.

Significance. The committee concluded that concerns about exposure to SV40 through contaminated polio vaccines are significant because of the seriousness of cancers and because of the continuing need to ensure and protect public trust in the nation’s immunization program, even though today’s polio vaccine supply is free of all SV40.     

  1. What recommendations did the committee make regarding policy analysis, communication, and research?

Policy review. The committee did not recommend review of the current polio vaccine recommendations on the basis of concerns about cancer risks, because the vaccine in current use is free of SV40.

Policy Analysis and Communication. The committee recommended that the appropriate federal agencies develop a vaccine contamination and prevention plan and be given the resources to implement the plan. This would be a general plan and is not specific for polio vaccine. Such a plan should be communicated to the public and medical practitioners to assure trust in the vaccine supply.

Research. The committee recommended development of sensitive and specific blood tests for SV40 and techniques for SV40 detection. When this has been done, the committee recommends that pre-1955 samples of human tissue be tested for SV40. They also recommended further study into how SV40 may spread among humans, and argued that additional epidemiological studies of people who may have received contaminated vaccine should not be done until technical (laboratory) issues are resolved.

More Information. You can view the entire IOM report on SV40 Contamination of Polio Vaccine and Cancer at (exit site)

For more information on polio vaccine, SV40 and cancer go to  

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This page last modified on October 22, 2002


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