How many, if any, of the tens of millions of people worldwide given doses of polio vaccine contaminated by SV40 were infected by the virus? If people were infected, have some developed rare cancers as a result? And is SV40 now spreading from person to person?
More than 40 years after SV40 was first discovered, in polio vaccine, these crucial questions remain fiercely controversial. Those on both sides of the debate can point to seemingly strong evidence to support their views.
Several early studies suggest that SV40 did infect some of the people given contaminated vaccines. For instance, a 1964 study found the virus in the faeces of 10 out of 35 children about a week after they were given contaminated oral polio vaccine, though the children showed no obvious symptoms.
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There is also no doubt that SV40, which has been extensively studied in many labs worldwide, can cause tumours in animals. The virus also turns human cells grown in culture cancerous. One of the viral proteins has been shown to block key anti-cancer genes.
In recent years, numerous studies in many different labs worldwide have found SV40 DNA in certain types of rare lung, brain and bone tumours, and even in non-Hodgkin鈥檚 lymphoma, a more common cancer. The evidence is strongest for the deadly lung cancer mesothelioma. The rise in the number of cases of mesothelioma in recent decades is thought to be due to asbestos exposure, but some researchers think SV40 increases the risk of cancer in people exposed to asbestos and also explains cases of mesothelioma where there is no evidence of asbestos exposure.
However, levels of the virus in tumours are much lower than those of other viruses known to trigger cancer. Even if the virus really is present in some tumours, say critics, this does not prove it caused them. Those who think SV40 does trigger cancer point to evidence of a 鈥渉it and run鈥� mechanism, whereby SV40 might disappear from cancerous cells after triggering the cancer.
Most epidemiological studies comparing those who may have been given contaminated vaccines prior to 1963 with those vaccinated later have found no difference in cancer rates. But there are flaws in these studies. For instance, it is not clear exactly who received contaminated doses. And if SV40 is contagious, or if some batches of the vaccine used in western countries were contaminated after 1963 (as some lawyers in the US allege), the control group would not be a proper control group. Because of all these problems, a 2002 report by a US Institute of Medicine committee concluded that a link between SV40-contamination of polio vaccines and cancer can neither be accepted or rejected.
Another vexed issue is infectivity. In monkeys, SV40 spreads via faeces and urine. Could the virus now be spreading among humans in the same way? Could this explain why SV40 has been found in tumours in people too young to have received contaminated vaccines? Antibodies to SV40 have been found in up to 12 per cent of people in some countries. But SV40 is very similar to two related human viruses, so these tests may actually be detecting antibodies to these two viruses as well.
Two groups now claim to have developed tests that will detect only antibodies specific to SV40. One, led by Denise Galloway of the Fred Hutchinson Cancer Research Center in Seattle, has failed to find SV40-specific antibodies either in the general population in the US or in people with non-Hodgkin鈥檚 lymphoma. Galloway says the results suggest it is not a widespread virus. But both her method and her conclusions are disputed by others in the field.
Another test is being developed by Bharat Jasani at the University of Wales College of Medicine in Cardiff. It is based on detecting immune cells that are primed to respond to SV40, rather than antibodies. He says preliminary results suggest that a small proportion of people with mesothelioma have been exposed to SV40, though he stresses that further work is needed.
鈥淚 haven鈥檛 come off the fence yet,鈥� says Jasani, who describes himself as a constructive sceptic. 鈥淚 don鈥檛 think we have done enough work. There鈥檚 a long way to go yet.鈥�