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Although the available evidence cannot exclude with certainty contaminated polio vaccines as the source of the AIDS pandemic, the balance of evidence argues against it—contaminated vaccines do not appear to have been responsible for what is shaping up to be one of the worst pandemics in human history. Does this conclusion mean that we do not need to worry about vaccines as a source of emerging disease? No. Vaccine contamination has already shown its ability to cause outbreaks of nasty viral diseases. As the United States was playing catch-up during its first year in World War II, a yellow fever vaccination program inadvertently infected over a quarter million soldiers with the hepatitis B virus. Fifty thousand became sick, but they got a lucky break. Apparently, when the hepatitis B virus infected people through this vaccine, it generated a fantastically low number of carriers and thus almost no chronic hepatitis and no liver cancer. Otherwise the vaccination might have caused around ten thousand deaths among the soldiers, and perhaps many more as the virus continued to spread through their contacts.Now, more than a half century later, vaccine researchers are still nervously watching a contaminant of polio vaccines: simian virus 40 (SV40), the virus that lent credibility to the polio vaccine hypothesis for the origin of AIDS. SV40 naturally infects the monkey cells in which the polio vaccine viruses were grown. Safety experts knew it was there but were not particularly concerned about it because it was not known to be harmful to humans. Their level of concern changed in the early 1960s when experimental studies showed that the virus could cause lung and brain cancer in hamsters. The virus was expunged from the vaccines by the mid-1960s, but that was too late for those of us who were vaccinated between 1955 and 1963. In the United States “those of us” make up about one third of the population. No one knows yet just how bad SV40 is. We are now the experimental hamsters being studied to obtain the answer. We do know that SV40 is found in most cases of a rare, dangerous lung cancer called mesothelioma, which has been increasing in the United States, Britain, and Europe since the use of SV40-contaminated vaccines. This is the same kind of lung cancer that SV40 causes in hamsters. The virus sabotages the same defenses against cancer that are sabotaged by papillomaviruses: p53 and a protein known as retinoblastoma suppressor. The emerging consensus is that SV40 is not just an innocent bystander but is causing the human mesotheliomas in concert with asbestos. SV40 is also found in human brain tumors. Whether it causes brain tumors is still being debated, but its activity in these cells is not comforting: as in the mesotheliomas, SV40 was found in human brain tumors bound to the cells’ tumor-suppressing proteins.The role of SV40 in human cancer will become more apparent as the experiment on the human hamsters continues. The most recent studies indicate that those who received the vaccine are about twice as likely to develop mesothelioma; the risk of the particular brain tumor most associated with SV40 appears to be increased by about one third in vaccine recipients.If SV40 does not get transmitted from person to person, then the cancer caused by SV40 contamination of polio vaccines can be viewed as another it-could-have-been-worse lesson imparted by the vaccine practices of the twentieth century. But the potential for transmission of SV40 between people is unclear. It has been recovered from children and HIV-infected patients who were born after SV40 had been purged from polio vaccines. The source of these SV40 infections is unknown, but if the virus is transmissible from person to person, our lesson may be just beginning. Concern is heightened because SV40 is a polyomavirus, a kind of virus that includes two other members, called JC and BK, which are full-fledged human pathogens. Most people are infected, for example, with JC polyomavirus, which resides in the brain and other organs. The scope of its effects are not yet clear, but it has recently been linked to colon cancer.Though vaccine contamination is frightening, in the long run it will probably turn out to be preventable, as soon as the people responsible for vaccine safety know what infectious contaminants to look for. This goal could be reached in time to benefit our grandchildren or great-grandchildren. In the meantime, we can expect that molecular techniques for identifying unwanted hitchhikers will provide incremental advances toward this goal.*41\225\2*

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