WASHINGTON (Reuters Health) Jul 11 - Though there is still no clear consensus, a majority of researchers told a quasi-governmental health panel Thursday that simian virus 40 (SV40) has become established in humans, and that it plays a role in causing cancer-- including in people who had virus-contaminated polio vaccines in the 1960s and 1960s.

The experts addressed the Institute of Medicine's (IOM) Immunization Safety Committee, which met to hear the latest epidemiologic and lab data on SV40.

SV40's role in cancer has been debated since the early 1960s, when it was discovered that the naturally-occurring monkey virus was in inactivated polio vaccine. Monkey kidney cells used to produce the vaccine were fingered as the source. In 1961, scientists discovered SV40 caused cancer in rodents. The government required all future polio vaccine to be SV40-free.

Even so, some contaminated vaccine still on shelves may have been used, and as many as 98 million children had already been exposed during the government's mass immunization program from 1955 until early 1963.

Scientists have tried to unravel whether SV40 is prevalent in humans, and if the polio vaccine caused infection, or if SV40 had been in humans previously. In the 1970s, SV40 was isolated in human tumors, especially brain and bone cancers, and, more recently, in mesotheliomas.

Michele Carbone of Loyola University in Chicago has linked SV40 to mesothelioma. He called SV40 is a "potent human carcinogen," capable of transforming cells and inhibiting the p53 tumor suppressor gene. But he added that the virus is not likely to act alone, and that co-factors, like asbestos, lead to cancer.

In 2002, several researchers found SV40 in non-Hodgkin's lymphoma. Janet Butel, a Baylor College of Medicine virologist, isolated SV40 in NHL and has sequenced SV40 genomes. In three NHL samples, the SV40 strain was the same as one found in samples of a polio vaccine used in the 1950s, she said. SV40 "seems to be established in humans," and "it is causing infection," said Butel.

"Perhaps it was there before the polio vaccine, I don't know," she said, adding that widespread vaccine use may have broadly distributed SV40 in humans. The virus has been found in many nations.

Another researcher, Jeffrey Kopp of the National Institutes of Diabetic, Digestive and Kidney Diseases, said a small study he and colleagues recently completed found SV40 in blood and urine of both healthy people and kidney disease patients, and that "argues for relatively common infection in the general population." Kopp's study will be in September's Journal of the American Society of Nephrology.

There were several nay-sayers, including long-time SV40 researcher Keerti Shah of Johns Hopkins University Bloomberg School of Public Health. Shah has been unable to isolate SV40 in human urine, and said that lab studies have been so inconsistent that they do not prove causality.

Howard Strickler, an epidemiologist at the Albert Einstein College of Medicine, presented several studies he's conducted, all showing no association between SV40 and cancer incidence, he said.

In a 1998 study in the Journal of the American Medical Association, Strickler and colleagues "found no relation between SV40 exposure and ependymoma incidence," he said. Strickler also analyzed mesothelioma rates and said that incidence has remained at a steady 3% a year. It is very rare and not rising in women, said Strickler, saying that argued against the possibility that the cancer is being driven by polio vaccine exposure.

Also, rates have only increased among people over 50, who were least likely to have been vaccinated, he said.

That was disputed by Carbone, who said a large number of people aged 20 to 45 were vaccinated in the 1960s, and that age group is having more mesothelioma.

Susan Fisher, chief of the division of epidemiology at the University of Rochester, conducted a cohort study, comparing people born between 1955-59 (likely exposed to vaccine) to those born between 1961-65, and documented cancer incidence in that group from 1973 to 1993. In the exposed cohort, there was a 178% increase in mesothelioma, and an increased incidence of ependymoma and osteosarcoma. But, because the cancers are rare, the differences between the exposed an unexposed group were not statistically significant, said Fisher.

In two to three months, the IOM Committee will issue a report, along with recommendations based on their assessment of the data.