Study finds no link between polio vaccine, cancer

Last Updated: 2003-01-01 10:00:14 -0400 (Reuters Health)

NEW YORK (Reuters Health) - A new US study casts some doubt on the theory that a polio vaccine used 40 years ago might have increased the risk of a rare cancer among the millions who received the vaccine.

Researchers found no evidence of an increased rate of pleural mesothelioma among those Americans most likely to have received polio vaccine contaminated with a potentially cancer-causing monkey virus. Between 10 and 30 million Americans may have been exposed to the virus through polio vaccine injections given between 1955 and 1963.

The monkey virus, called simian virus 40 (SV40), got into the vaccine through a production method using cultured kidney cells from monkeys to grow poliovirus. That method is no longer used, and polio vaccines have been free of SV40 since 1963, according to US health officials.

Concerns about the potential health effects of the contaminated vaccine center on the fact that SV40 causes cancer in rodents and on evidence of SV40 DNA in samples of certain human tumors--particularly pleural mesothelioma.

Pleural mesothelioma is a rare cancer of the mesothelial tissue that surrounds the lungs and lines the chest. It is most often found among people who were exposed to high levels of asbestos on the job.

However, evidence of SV40 DNA in pleural mesothelioma has raised concerns that exposure to the virus through polio vaccination might be causing or contributing to some cases of the disease.

But not all studies have found such DNA evidence in mesothelioma tumor samples, point out the authors of the new study, led by Dr. Howard D. Strickler of Albert Einstein College of Medicine in New York City.

And a report released two months ago by the US Institute of Medicine (IOM) concluded that so far, research has failed to find a hard link between the contaminated polio vaccine and later cancer.

In this latest study, reported in the January 1st issue of the Journal of the National Cancer Institute, Strickler's team looked at national data on pleural mesothelioma rates between 1975 and 1997.

Throughout that period, they found, pleural mesothelioma remained rare. Overall, fewer than one person per 100,000 was diagnosed with the cancer each year.

And although there were increases in the cancer over time, the biggest jumps were seen in the oldest age groups, who were unlikely to have received the contaminated vaccine, according to the researchers.

In addition, cancer rates among those between the ages of 25 to 54--who had the greatest exposure to the polio vaccine between 1955 and 1963--remained stable or decreased over time, the researchers report.

Finally, they note, the rate of pleural mesothelioma remained far lower among women than men, a long-standing difference related to men's greater occupational exposure to asbestos.

The "persistent rarity" of the cancer in women is "noteworthy," the study authors write, because both sexes had equal exposure to the SV40-tainted polio vaccine.

Despite their findings, however, Strickler and his colleagues point out that, given the detection of the monkey virus in mesothelioma samples, researchers must continue to follow people who were potentially exposed to the contaminated vaccine.

A major problem in studying the issue, however, is that there is no way to tell for certain which people were exposed to SV40 through polio vaccination years ago. The recent IOM report called on researchers to develop reliable tests for detecting SV40 exposure.

SOURCE: Journal of the National Cancer Institute 2002;95:38-45.

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