But finding the virus in a tumor does not prove that it caused the tumor. Most large population studies have not found a higher cancer risk in people exposed to the vaccine. There is no blood test that can tell whether a person is infected with SV40, and there are no special steps to take to prevent the rare cancers being studied, researchers said.

The report released yesterday was prepared by a 14-member committee on vaccine safety convened by the Institute of Medicine, which advises the government on public health. The report is the fifth in a series of eight that the government commissioned on vaccination. Fear of vaccine side effects has led some parents to shun vaccination, and outbreaks of diseases like whooping cough and measles have resulted, the report said.

After analyzing more than 100 studies, the committee said that most evidence argued against a connection between the polio vaccine and cancers, but that those weaknesses in the studies made it impossible to dismiss the potential link.

There is no doubt that SV40 causes tumors in rodents. Four cancers have been linked to it in animals, the bone tumors called osteosarcomas; ependymoma, a type of brain tumor; mesothelioma, a cancer in the membranes around lungs and other organs; and non-Hodgkin's lymphoma, a cancer that affects the immune system. But it is not clear whether the virus causes tumors in people.

The virus has been detected in tumor cells from people with the four cancers that develop in rodents. Whether it caused the tumors is not known, nor is it known whether people with the virus contracted it from polio vaccine or whether the virus can spread between people. The virus may have been in other vaccines. SV40 may also spread from animals to people who work with them and to laboratory workers.