Millions May Have Received Contaminated Polio Vaccine

Millions May Have Received Contaminated Polio Vaccine

http://sciencenow.sciencemag.org/cgi/content/full/2005/1115/1

A polio vaccine produced by at least one eastern European manufacturer was contaminated with a potentially cancer-causing virus as late as 1978, according to a new study. Although most early batches of the vaccine contained this virus, modern manufacturing procedures were thought to have removed it by 1962. The findings indicate that millions more people were exposed to the risky virus than previously thought.


[foto] Contaminant. The SV40 virus has shown up in the polio vaccine stocks of an eastern European company.
CREDIT: CDC

Introduced in 1954, the polio vaccine was a public health breakthrough that brought the spread of the paralytic virus to a screeching halt. To produce enough poliovirus to make vaccine, manufacturers grew the virus in the kidney cells of rhesus monkeys. Unbeknownst to researchers at the time, rhesus monkeys are often infected with SV40, a virus that may cause cancer in humans. Poliovirus stocks made using monkey cells were soon nearly universally contaminated with SV40. Once the problem was discovered
in 1959, most manufacturers eliminated SV40 from their virus stocks with an anti-SV40 antiserum. One eastern European vaccine manufacturer (EEVM) and perhaps others, however, instead heated the virus mixture to inactivate SV40. To prevent future contamination, vaccine producers switched to growing poliovirus in SV40-resistant African green monkeys instead of rhesus monkeys.

Although these measures were widely believed to have been successful, no one had done a comprehensive analysis. So pathologist Michele Carbone of Loyola University in Chicago and colleagues tested stored samples from the EEVM and manufacturers in 12 other countries for SV40 contamination. As reported in the 15 November issue of Cancer Research, all stocks were negative for the virus except for several from the EEVM. The problems with the EEVM's vaccines likely persisted for more than a decade--an EEVM virus stock produced in 1966 and used through 1978 was positive for infectious SV40 virus. The researchers also demonstrated that the EEVM's heat inactivation technique does not adequately destroy SV40 and was the likely reason behind the continued contamination.

The effect of the contaminated vaccine on cancer rates is still largely unknown. While SV40 has been found in some human tumors, it is difficult to flag it as the cause of any particular cancer, says virologist Janet Butel of Baylor College of Medicine in Houston, Texas. However, she says the Loyola study is well done and believes it will make scientists rethink the assumption that the polio vaccine was no longer contaminated with SV40 after 1962.

--KAREN ROSS

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