Polio Vaccine, and Cancer

by CDC

What is SV40?

Simian virus 40, or SV40, was discovered in 1960. It occurs naturally in some species of monkeys, though it does not typically cause symptoms or illness except in cases where the animal has chronic problems with its immune system. In those cases, the animals develop lesions associated with SV40 in their kidneys and brains.

SV40 is not related to HIV, the virus that causes AIDS in humans, or to simian immunodeficiency virus (SIV), the virus that causes an AIDS-like disease in some monkey species.

Why is there so much interest in SV40?

Soon after its discovery in 1960, SV40 was identified in polio vaccine. It was found in the injected 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 required that new stocks of polio vaccine be free of SV40. However, existing polio vaccine stocks were not recalled and were 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 the virus was found 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.

Does polio vaccine being given in the U.S. today contain SV40?

No, polio vaccines being used today do not contain SV40. SV40 was completely removed from the seed strains of the vaccine viruses in the early 1960s. The polio vaccine currently used in the U.S. (inactivated polio vaccine, or IPV) is no longer prepared in primary rhesus monkey kidney cells. It is produced in human or African green monkey cell lines that have been extensively tested for contaminants, including SV40.

The poliovirus used in IPV is killed with formaldehyde. This procedure also kills viral contaminants, such as SV40. Formaldehyde was also used in the SV40-contaminated vaccine, but in 1961 researchers found that the process killed 99.99% of SV40 and 1 in 10,000 SV40 particles survived. Today's testing methods are better. Any live SV40 would be detected by these methods.

What about concerns that the testing methods used to screen oral polio vaccines could have missed certain strains of SV40?

Oral polio vaccine (OPV, which is no longer recommended for use in the U.S. but is used elsewhere in the world) differs from IPV because it contains weakened, rather than killed, poliovirus. Because it is a live vaccine, formaldehyde or other inactivation agents were not used in producing OPV. The poliovirus that was used to produce OPV was grown on monkey kidney cells. Screening for SV40 in the monkey kidney cells used to produce OPV was implemented in the early 1960s after the virus was first discovered. Manufacturers also treated the stocks of weakened poliovirus in order to remove any SV40 that might have been present in them.

A study raised concern that some lots of OPV may have been contaminated with a slow-growing SV40 strain that would not have been detected with the methods used to test it. However, this study did not follow the actual testing protocol used to ensure that vaccine is free of SV40. Subsequent studies confirmed studies from the early 1960s showing that the testing methods used were sufficient to detect even slower-growing strains of SV40. In addition, researchers from the FDA used the very sensitive polymerase chain reaction (PCR) methodology to search for SV40 DNA in OPV manufactured in the U.S. between 1972 and 1996 (the FDA only tested vaccines produced as far back as 1972, because there were no existing lots of OPV at FDA that were produced between 1962 and 1972). SV40 DNA sequences were not found in any of the vaccine lots tested. OPV is no longer produced in the U.S.; if production were to be resumed, it would continue to be under extremely strict conditions that eliminate the possibility of any contamination with SV40.

Who received SV40 contaminated polio vaccine in the U.S.?

Over 98 million Americans received one or more doses of IPV (the injected form of the polio vaccine) during the period (1955-1963) when some of the vaccine was contaminated with SV40. However, not all doses of IPV were contaminated. It has been estimated that 10-30 million of the 98 million people who received a polio shot actually received a vaccine that contained SV40. In addition, about 10,000 volunteers who received an experimental oral polio vaccine (OPV) between 1959-1961 may have been exposed to SV40 (the vaccine was later licensed in 1963, subsequent to SV40 removal from the seed stock). All of the evidence to date indicates that after 1963, all vaccines on the U.S. market were free of SV40.

About the Author

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The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.