BETHESDA, Md. - Millions of Americans were exposed to a monkey virus by taking a contaminated polio vaccine in the 1950s. Scientists now are probing whether the exposure could increase their risk of certain rare cancers.
Government officials play down such fears, because the types of cancer involved do not appear to be increasing among people old enough to have received the tainted vaccine. Today's polio vaccine is tested to ensure it is free of this monkey virus, called SV40.
But scientists recently found genetic pieces of SV40 lurking inside tumors removed from cancer victims, and injecting the virus into laboratory animals gives them cancer.
That's far from proof that SV40 actually harmed a person. But international scientists who spent two days furiously debating the issue contend the virus might predispose some people to certain cancers of the brain, bone and lung.
"It may act independently or as a co-factor" with known cancer-causers like asbestos, said Dr. Michele Carbone of Loyola University Medical Center, among the first to discover SV40 material inside human tumors.
And some question whether continuing to use monkey tissue to make vaccines might let as-yet-unknown viruses sneak in.
"Make it in anything but animals," said Barbara Loe Fisher of the National Vaccine Information Center, which advocates better vaccine safety.
"We have the technology to make vaccines in human cell lines that are clean," Carbone said. He added that Americans shouldn't fear polio vaccine and noted he recently had his child immunized.
The government held the SV40 meeting at the National Institutes of Health to see how serious the issue is. Officials concluded Tuesday that more research is needed, but they wouldn't commit to funding it.
Still, government doctors worried that even debating SV40 publicly would frighten people. "We do a grave disservice to the public if we were now to question the safety of the current polio vaccines on the basis of SV40," warned Dr. Arthur Levine of the NIH.
Making polio vaccines using human cells - the kind sold in Canada but not here - isn't risk-free either, Levine said, because they must be tested for human infections.
Mass vaccination with the then-new polio vaccine began in 1955. But in 1960, doctors discovered SV40 in monkeys and in the vaccine. It caused cancer when injected into hamsters, so the Food and Drug Administration ordered companies to manufacture virus-free versions.
By the time they reached the market in 1963, as many as 98 million people may have been exposed to SV40.
Whether that early contamination posed any harm was debated in the 1960s and 1970s. The controversy faded away until 1992 - when Carbone found SV40's genetic fingerprint inside human cancer.
Carbone tested preserved samples of rare child brain tumors called ependymomas, bone tumors and a particularly deadly lung cancer called mesothelioma that mostly strikes people exposed to asbestos.
He found pieces of SV40's genetic material inside 60 percent of the brain and lung cancers and a third of the bone cancers he tested.
Other scientists got mixed results. British researchers spotted SV40 signs in 44 percent of the mesotheliomas they tested, and a Baylor University researcher even culled the actual virus from a tumor. Not all the tumors were from people exposed to the tainted vaccine - and some researchers found SV40 in noncancerous tissue, suggesting the virus either had spread or was in humans before the tainted vaccine.
Other scientists couldn't find the virus in human tissue at all and questioned whether their colleagues were fooled by laboratory contamination.
Carbone and other researchers now have preliminary evidence that SV40 may do damage by tying up proteins vital to keeping cells from turning cancerous.
Also, SV40 appears related to two human viruses that harmlessly infect nearly all Americans but which cause rare cancers in people with severely damaged immune systems, such as AIDS patients. That suggests SV40 could similarly be a risk only to immune-suppressed patients.
NIH's Dr. Howard Strickler says rates of these cancers are not escalating among people old enough to have received the tainted vaccine. Swedish researchers also found no jump when comparing 700,000 Swedes who took tainted American vaccine against the general population that received virus-free vaccine.
Those studies wouldn't detect small rises in very rare tumors, Strickler acknowledged.
He called the data "intriguing" but emphasized it does not "point us in a clear direction of whether this virus is a cause of cancer."