Studies do not seem to suggest that people who got the vaccine have experienced a higher rate of cancer, but the virus, called SV40, does have the potential to damage cells and turn them cancerous, the Institute of Medicine panel said.

When the injected polio vaccine was first developed, it was grown on tissue taken from the kidneys of monkeys. In 1960, researchers found that these tissues could be infected with SV40, a previously unknown virus that causes a common and harmless infection in some monkeys.

Scientists moved to take it out, and the polio vaccine has been free of SV40 since 1963.

Polio is a disease that kills or paralyzes. It once left thousands of children living in "iron lungs," unable to breathe. At its peak in the United States in 1952, polio caused more than 20,000 cases of paralysis.

Thanks to the vaccine, polio was eliminated from the Western Hemisphere by 1994. Work is underway to eradicate it from pockets in parts of Africa and Asia.

"Researchers estimate that 10 percent to 30 percent of the polio vaccine given to adults and children in the United States between 1955 and 1963 was contaminated with SV40, potentially exposing between 10 million and 30 million Americans to the virus," the Institute of Medicine, which put together the panel that wrote the report, said in a statement.

"However, as with all viruses, not everyone who comes into contact with SV40 will become infected," it added.

The institute, an independent study group that reports to Congress and the federal government on medical issues, said there is no need to review polio vaccine policy as the vaccine has been free of the virus for decades.

At issue is whether people who claim to have developed cancer because of the vaccine have a legitimate complaint. There is not enough evidence to tell, the panel concluded.

"The vast majority of population studies, which carry the most weight in establishing causal relationships, have found no increased rates of cancer in people who received the vaccine contaminated with simian virus-40," it said in a statement.

"However, a possible link cannot be completely ruled out because of limitations in the available data and in the way the studies were conducted."

The group also noted that the virus does seem to be capable of changing cells, perhaps starting the uncontrolled growth that is the hallmark of cancer.

"While there is a strong body of biological evidence that SV40 is capable of causing cancer, it is not clear that exposure to the virus through the tainted polio vaccine could cause certain cancers suspected of being associated with SV40 -- mesothelioma, osteosarcoma, ependymoma and non-Hodgkin's lymphoma," it added.