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Report downplays early polio vaccine, cancer linkStill, researchers can't rule out possible tie
By Saundra Young
WASHINGTON (CNN) -- Researchers don't have sufficient evidence to prove that exposure to polio vaccine contaminated with a monkey virus has led to cancer in humans, according to a new report released by the Institute of Medicine on Tuesday. While scientists looking at population studies found no increased rates of cancer in people who received the vaccine contaminated with simian virus-40 (SV40) between 1955 and 1963, they say a possible link cannot be completely ruled out. According to the report, there is strong evidence that SV40 is a cancer-causing virus. However, it's not clear whether exposure to the virus results in a number of rare cancers including non-Hodgkin's lymphoma; mesothelioma, a rare form of cancer found in the sac lining the chest or abdomen; osteosarcoma, a type of bone cancer most often occuring in children; and ependymoma, a rare form of brain tumor. "Biological data can help shape research directions, but cannot prove causality on their own," said Marie McCormick, chairwoman of the Institute's Immunization Safety Review Committee. "Given the uncertainties raised by all the studies, our report offers a research strategy and suggests a process for handling contamination of vaccines should it ever occur again." . During early production of the polio vaccine, tissue cultures used to grow the vaccine came from the kidneys of rhesus and cynomolgus monkeys. It wasn't until 1960 that researchers discovered that the tissue could be infected with SV40, a previously unknown virus that caused harmless infections in certain monkeys. Most of the contamination was in the inactivated polio vaccine. The vaccine has been free of SV40 since 1963. An estimated 10 to 30 percent of the vaccine given between 1955 and 1963 was contaminated with the virus, exposing between 10 to 30 million Americans. But researchers have no way of knowing who actually received the contaminated vaccine. Because there was no specific blood test for SV40, there is no way to tell how many people were actually infected with the virus. According to the report, studies that examined the potential link between the vaccine and cancer in humans showed no increased risk of cancer in those who received the vaccine. But studies also show that SV40 DNA has been found in some human tumors. The report calls for additional research and for the development of sensitive blood tests and standardized techniques that can detect the virus in people that may have been infected. The first polio vaccine was introduced in 1955. By 1994, the disease had been eliminated from the Western Hemisphere. The Institute of Medicine is part of the National Academy of Sciences and brings together experts to provide health policy advice to government and the public.
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