Recent news reports that polio vaccines administered in the 1950s and 60s may have exposed many Americans to a potentially cancer-causing monkey virus have alarmed the public.
But most cancer experts say today’s vaccines are safe – and there is no reason to avoid vaccinations that can protect children and adults against infectious diseases, such as polio.
Polio was epidemic in the US during the 1950s, killing or crippling thousands of people. Almost overnight, Salk and Sabin vaccines saved generations from the threat of polio. Years later, however, some batches of the vaccines used from 1955 to 1963 were discovered to have been contaminated with a virus, called SV40, originally carried by Rhesus monkeys whose kidney cells had been used to grow the polio virus from which the vaccines were made.
The virus has since been found to cause a rare but almost invariably fatal cancer called malignant mesothelioma in hamsters. This cancer develops from the lining of the cavity in which the lungs reside. Prior to that, the only known risk factor for malignant mesothelioma was exposure to asbestos.
In October, 1998, Joseph Testa, MD and Michele Carbone, MD, of Fox Chase Cancer Center and Cardinal Bernardin Cancer Center of Loyola University Chicago, respectively, and colleagues published a study in the medical journal Cancer Research showing the monkey virus was present in human malignant mesothelioma tumors. Some of the tumors occurred in people who had no apparent history of exposure to asbestos.
Previous studies in animals infected with SV40 were more sensitive to the cancer-causing effects of asbestos. If future studies show the virus adds to the cancer-causing effect of asbestos in humans, researchers would advise construction workers and others in the general population, who test positive for the virus, to be particularly careful to limit their exposure to asbestos.
Most scientists are confident contemporary vaccines – carefully screened for any viruses – are completely safe. Dr. Carbone points out his own daughters were recently vaccinated against polio. However, some scientists recommend using human cells rather than monkey cells for manufacturing the vaccine.
Although the laboratory studies indicate need for additional research and careful monitoring of vaccine safety, epidemiological studies have not found evidence that the polio vaccine increases the risk of developing cancer. These studies compared children who might have received a contaminated vaccine between 1955 and 1963, to children vaccinated during the next five years. They found no significant difference in cancer risk.
Because some cancers, such as mesothelioma, typically effect adults over 50, some scientists note additional follow-up is needed before it is safe to conclude that people vaccinated between 1955 and 1963 will not be at greater cancer risk as they continue to age.
In an article published in the January 20, 1999 issue of the Journal of the National Cancer Institute, Janet S. Butel, PhD and John a Lednicky, MD of Baylor University said, “It appears the association of SV40 with human tumors must be taken seriously, but further studies are needed to prove causality.”
While some controversy still surrounds the SV40 story, and researchers are conducting more studies, cancer and vaccine experts agree on three things. First, there is no SV40 virus in today’s vaccines. Production methods were changed in the 1970s, and manufacturers who now use monkey cells only use kidney cells from animals bred in captivity and never exposed to SV40. The vaccines are also screened for any virus.
Second, it’s important people get the vaccinations they need to protect against disease. And third, there is no evidence of human cancer being caused by the virus that was in the vaccines of earlier decades.
The concerns of most cancer and vaccine experts were recently summed up by Dr. Carbone, “It would be a disaster if people were to turn away from vaccinations today because of what happened in an earlier time.”