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Cancer Link to Polio Vaccine Called Unlikely

But U.S. panel calls for further research

TUESDAY, Oct. 22, 2002 (HealthDayNews) -- The U.S. government doesn't need to review its polio vaccine program in light of concerns that shots contaminated with a monkey virus may be linked to certain cancers, an expert panel said today.

Although simian virus-40 (SV40) does cause cancer in laboratory animals, the Institute of Medicine panel said there's no solid evidence connecting the tainted vaccine with human cancers. Moreover, SV40 hasn't been present in the polio vaccine since the early 1960s, so the shots have long been free of the virus, the panel noted.

Still, the panel said that it couldn't definitively rule out the possibility that SV40 might increase the risk of certain cancers in those who received the vaccine, and that more research is needed to settle the question.

Salk polio vaccine was originally made using tissue from the kidneys of rhesus and cynomolgus monkeys. In 1960, researchers discovered that some of those cells could be infected with SV40, which is widespread in rhesus monkeys but causes them no symptoms. Potentially tainted vaccines were administered to people in the United States between 1955 and 1963, but the inoculations have been clean of the virus since then.

The panel estimated that between 10 percent and 30 percent of polio shots may have been tainted with SV40 before it was eliminated from the inoculations. That could have left 10 million to 30 million Americans exposed to the microbe through the injections. The oral polio vaccine, which is no longer recommended in this country, was never contaminated with the virus.

Previous research has shown that SV40 is present in certain cancers, including mesothelioma, osteosarcoma, ependymoma and non-Hodgkin's lymphoma. However, no research has demonstrated that the virus can trigger these diseases by entering the body through a vaccine, the panel said. If it could, scientists have assumed, people who received the tainted shots would have higher than normal rates of these cancers.

But they don't.

"We've gone through that age range and have not seen an increase in these tumors," said Dr. Marie McCormick, a Harvard University public health expert and chair of the IOM panel. "The epidemiological studies to date don't really show that kind of experience."

The first suggestion that SV40 might be linked to human cancers arose in the 1970s, when researchers found signs of the virus in some childhood brain tumors. In 1994, the organism was identified in lung cancers called mesotheliomas, which until then had been associated with asbestos exposure.

Janet Butel, a virologist at Baylor College of Medicine in Houston, and an expert in SV40, said as many as 50 percent of mesotheliomas in the United States have evidence of infection with SV40. However, in other countries, such as Finland and Turkey, the virus isn't present in the tumors. These nations didn't use contaminated polio vaccine, she noted.

Butel, who called the IOM report "very thoughtful and thorough," said it will be extremely difficult to demonstrate a connection between tainted inoculations and cancer. But she believes the shots may have introduced the virus into people, who since have been spreading it to each other. How SV40 is transmitted among humans isn't understood, she said, and should be a focus of future research.

Butel said she hoped the IOM report would prompt the National Cancer Institute to fund studies of SV40's role in people. So far, it has not, she said.

The panel called for more studies, especially those to detect SV40 in people. One problem with existing tests is that the virus is nearly identical to at least two ubiquitous and harmless microbes that infect humans. The committee discouraged researchers from conducting more population studies in people exposed to contaminated polio vaccine until a better method for identifying SV40 is available.

The Institute of Medicine falls under the umbrella of the National Academies of Science, an independent research organization in Washington, D.C., that advises the federal government on health and science issues.

What To Do

For more on SV40 and polio vaccine, visit the U.S. Centers for Disease Control and Prevention's National Immunization Program or the Institute of Medicine.

SOURCES: Marie McCormick, M.D., Sc.D., professor and chair, department of maternal and child health, Harvard School of Public Health, Boston; Janet Butel, professor and chair, department of molecular virology and microbiology, Baylor College of Medicine, Houston; Oct. 22, 2002, Institute of Medicine report
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