WASHINGTON (Reuters Health) - There is no way to be certain if a polio vaccine used decades ago, which was contaminated with a potentially cancer-causing monkey virus, actually led to an increased cancer rate in humans, a scientific panel from the Institute of Medicine concluded Tuesday.

As many as 30 million Americans are thought to have been exposed to a virus known as SV40 through polio vaccine injections between 1955 and 1963. SV40 stands for simian virus 40, a pathogen that found its way into the vaccine through a production method using cultured kidney cells from monkeys to grow poliovirus.

That method is no longer used in production, and polio vaccines have been free of the virus since 1963, according to a report released by the IOM's immunization safety review committee.

Still, nearly 100 million children and adults received polio vaccine between 1955 and 1963. Studies have estimated that between 10% and 30% of them were exposed to SV40 through contaminated lots of vaccine.

The vaccine raised safety concerns because SV40 is known to cause cancer in rodents and to promote abnormal human cell growth in laboratory experiments. DNA studies in the1990s found the virus in some human tumors, leading researchers to suspect that the virus could be also be a cause of human cancers.

In a report released Tuesday, experts found "moderate" evidence that exposure to the contaminated vaccine may have led to full blown SV40 infection in people who received it. They also cited moderate evidence that SV40 may cause cancers in humans under natural conditions since the virus can be present in some tumors.

But they also found persistent flaws in studies suggesting higher cancer rates in people potentially exposed to contaminated vaccine, casting some doubt on a hard link between vaccine use and later tumors in humans.

The committee ultimately found that human evidence "was inadequate to accept or reject" a causal relationship between the contaminated vaccine and human cancers, said Dr. Marie C. McCormick, a professor of maternal and child health at the Harvard School of Public Health and the chair of the IOM committee.

Some studies have identified higher rates of non-Hodgkin's lymphoma, mesothelioma (a rare cancer most often associated with asbestos exposure), and other cancers in people vaccinated in the late 1950s or early 1960s. But the IOM experts refused to draw a connection between vaccinations and later cancer cases because the studies did not specifically identify people infected with SV40.

"At this point, researchers have no way to know with certainty which individuals received the contaminated polio vaccine decades ago," McCormick said.

"For every study that shows an increase, there is another study that shows a decrease" in cancer rates, added Dr. Steven Goodman, an associate professor of oncology at the Johns Hopkins School of Medicine and a member of the IOM committee.

McCormick said that she saw little need for increased vigilance in persons who received the vaccine between 1955 and 1963 since the cancers in question remain rare and there is no good test for either SV40 or cancers like mesothelioma and non-Hodgkin's lymphoma.

"I don't think increased vigilance is really the issue," she said.

Still, Goodman acknowledged that a lack of reliable data on exposure rates and the behavior of SV40 bring "an element of uncertainty to our conclusions."

The committee recommended that researchers develop reliable tests for detecting exposure to SV40 before they undertake any more large-scale studies looking for elevated cancer rates in vaccinated people.

Scientists are still unsure whether the vaccine is the only source of SV40, or whether the virus can be transmitted via human-to-human contact. Researchers would have to go back and determine whether the virus was present in humans before the vaccine came into use to form a solid link between the vaccine and later cancers, McCormick said.