Although simian virus-40 (SV40) does cause cancer in laboratoryanimals, the Institute of Medicine panel said there's no solid evidenceconnecting the tainted vaccine with human cancers. Moreover, SV40 hasn'tbeen present in the polio vaccine since the early 1960s, so the shotshave long been free of the virus, the panel noted.
Still, the panel said that it couldn't definitively rule out thepossibility that SV40 might increase the risk of certain cancers inthose who received the vaccine, and that more research is needed tosettle the question.
Salk polio vaccine was originally made using tissue from the kidneysof rhesus and cynomolgus monkeys. In 1960, researchers discovered thatsome of those cells could be infected with SV40, which is widespread inrhesus monkeys but causes them no symptoms. Potentially tainted vaccineswere 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 polioshots may have been tainted with SV40 before it was eliminated from theinoculations. That could have left 10 million to 30 million Americansexposed to the microbe through the injections. The oral polio vaccine,which is no longer recommended in this country, was never contaminatedwith the virus.
Previous research has shown that SV40 is present in certain cancers,including mesothelioma, osteosarcoma, ependymoma and non-Hodgkin'slymphoma. However, no research has demonstrated that the virus cantrigger these diseases by entering the body through a vaccine, the panelsaid. If it could, scientists have assumed, people who received thetainted 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 inthese tumors," said Dr. Marie McCormick, a Harvard University publichealth expert and chair of the IOM panel. "The epidemiological studiesto date don't really show that kind of experience."
The first suggestion that SV40 might be linked to human cancers arosein the 1970s, when researchers found signs of the virus in somechildhood brain tumors. In 1994, the organism was identified in lungcancers called mesotheliomas, which until then had been associated withasbestos 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 inthe United States have evidence of infection with SV40. However, inother countries, such as Finland and Turkey, the virus isn't present inthe tumors. These nations didn't use contaminated polio vaccine, shenoted.
Butel, who called the IOM report "very thoughtful and thorough," saidit will be extremely difficult to demonstrate a connection betweentainted inoculations and cancer. But she believes the shots may haveintroduced the virus into people, who since have been spreading it toeach other. How SV40 is transmitted among humans isn't understood, shesaid, and should be a focus of future research.
Butel said she hoped the IOM report would prompt the National CancerInstitute 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 inpeople. One problem with existing tests is that the virus is nearlyidentical to at least two ubiquitous and harmless microbes that infecthumans. The committee discouraged researchers from conducting morepopulation studies in people exposed to contaminated polio vaccine untila better method for identifying SV40 is available.
The Institute of Medicine falls under the umbrella of the NationalAcademies of Science, an independent research organization inWashington, D.C., that advises the federal government on health andscience issues.
What To Do
For more on SV40 and polio vaccine, visit the U.S. Centers forDisease Control and Prevention's