At first it seemed impossible: A widely celebrated polio vaccine given to millions of people in the 1950s was contaminated with a monkey virus--a virus that causes cancer in animals. But a report released Tuesday by the Institute of Medicine (IOM) allays fears that this virus has caused a wave of cancer in the vaccinated population. It may be causing some rare cancers, but more research is needed to find out.
Some 40 years ago, the virus SV40 was discovered in the monkey kidney extracts used to make the Salk vaccine. Ever since, concern has risen that the vaccine may have triggered an epidemic of cancer (Science, 10 May, p. 1012). The virus kept turning cultured cells cancerous, and it kept causing tumors in animals. That debate heated up in the past decade, after researchers began finding SV40 DNA in four types of rare human cancers--the same kinds it causes in animals--and press reports emphasized that tens of millions of people could have been exposed.
The IOM committee examined all major epidemiological studies to see whether people exposed to SV40-contaminated vaccine have a higher risk of developing cancer. Although the studies were flawed, the panel decided they were good enough to rule out a cancer epidemic. But evidence suggests that millions of people may be infected with SV40, and the panel concluded that the virus could very well have come from the contaminated vaccine. Biological data also suggest, but do not prove, that the virus can sometimes cause human cancer. "We acknowledge that SV40 at least could have a carcinogenic effect but epidemiological evidence does not suggest that it actually did," says IOM committee member Steven Goodman, a biostatistician at Johns Hopkins School of Medicine in Baltimore. Even so, he adds, "there's a body of evidence [on SV40 carcinogenicity] that has to be taken quite seriously."
Overall, the IOM report "really closes the book on the discussion" of past epidemiological work, says pediatric oncologist Bob Garcea of the University of Colorado Health Sciences Center in Denver. Although SV40 may yet turn out to cause cancer in humans, the risk, if any, is "not remotely in the ballpark" of well-known carcinogens such as tobacco smoke or asbestos, adds Goodman.