BETHESDA, MARYLAND-- Recent reports that the DNA of a monkey virus called SV40 lurks in some rare types of human cancers has reignited a nearly 40-year-old controversy over the safety of a polio vaccine that was widely administered in the late 1950s. But at the end of a symposium held yesterday and today at the National Institutes of Health (NIH) to vet the new evidence, many experts emerged confident that the virus does not pose a threat to people.

The controversy originally erupted in 1960 when researchers found that some batches of the polio vaccine, which was made from killed, whole virus grown in monkey cells, was contaminated with SV40. The discovery rang alarm bells a year later, when it was found that SV40 injected into hamsters can trigger cancerous tumors. The controversy died down after human population studies failed to turn up evidence of increased cancer rates in vaccinated individuals, and comparable contamination problems are not likely to happen now because cells used to grow viruses for vaccines are screened to eliminate any carrying SV40.

In 1994, however, researchers found SV40 DNA in mesotheliomas, a rare tumor linked to asbestos exposure, and a pair of studies published in the last several months reports having found the viral DNA in several types of brain tumors and osteosarcomas, a type of bone tumor. This suggests that infection by the virus might have led to the cancers, as it does in animals. Other labs have not been able to replicate some of the tumor studies, however, and NIH called the meeting to analyze all data and try to come to some consensus about what the viral sightings might mean.

So far, the researchers agree, there's no reason to doubt the original reports. "It appears that there's probably something there," says virologist Michael Fried of the Imperial Cancer Research Fund in London. But as before, the prevailing tone at the conference was that SV40 does not appear to pose a threat. In a just-completed study, Swedish and American epidemiologists announced today that they failed to document any cancer rise in the generation of people who received the contaminated vaccines. Concludes Arthur Levine, a symposium co-organizer and virologist from the National Institute of Child Health and Human Development, "There's no evidence that apparent harm occurred as a result of this massive exposure to SV40." The next step, experts say, will be to explore further why SV40 is showing up in these tumors in the first place.