Polio Vaccine’s Contamination Described
Oral polio vaccine contaminated with a monkey virus was administered to hundreds of millions more people than previously thought, new research shows. Some scientists think the virus, called SV40, might be linked to cancer.
Scientists had assumed that polio vaccines were virus-free after 1962. The virus was discovered in 1960 and is known to have contaminated some vaccines made in the kidney cells of rhesus monkeys from 1954 to 1961. But Soviet vaccines -- also used in many Eastern European, Asian and African nations -- were contaminated until the early 1980s, vastly multiplying known exposure to the virus, according to a study in the current issue of the journal Cancer Research. The National Cancer Institute sponsored the study.
Consequences of that exposure remain unknown. U.S. health officials say the virus poses no problem, citing studies that found no increase in cancer among those who received the polio vaccine before the early ‘60s.
“SV40 can cause cancer in laboratory animals. There is limited evidence that SV40 can infect humans, but there is no evidence that it can cause human health problems,” the Centers for Disease Control and Prevention said in a statement.
But some scientists say the new study may call that research into question, because extensive migration of Eastern Europeans into the West could have skewed comparisons of SV40 infections and cancer incidence in people born before and after 1961.
“Our findings have practical implications for all the epidemiological studies that looked for increases in cancer among those vaccinated, because they all were based on the assumption that everybody born after 1961 could not have been exposed to the virus,” said Loyola University pathologist Michele Carbone, who headed the study.
The World Health Organization recommended in 2000 that all seed stocks used to manufacture polio vaccine be tested for SV40. Carbone headed a multi-laboratory study with the Food and Drug Administration and its British equivalent, the National Institute for Biological Standards and Control.
Carbone and colleagues tested vaccine samples from 13 countries and the WHO seed. With the exception of samples from the former Soviet Union, all vaccines were free of SV40.
“The contamination was traced to the original Sabin vaccine seed produced in the 1950s,” which was distributed globally and used for Soviet vaccines until the 1980s, Carbone said.
“When SV40 was discovered, the U.S., United Kingdom and WHO used an anti-SV40 serum to inactivate the virus. In contrast, the U.S.S.R. used a different methodology, adding a chemical compound that was supposed to inactivate SV40 but did not work.”
In their paper, Carbone and colleagues demonstrated that the technique was ineffective. The Soviet Union switched in 1981 to a polio vaccine seed provided by the WHO that was free of SV40 contamination.
Carbone is credited with revitalizing interest in SV40, which he thinks predisposes some people to develop rare tumors called mesotheliomas. The tumors affect the cells lining the chest and lung, usually in those exposed to asbestos.
Recent studies have detected DNA sequences from SV40 in samples of tissues from mesotheliomas and other rare human cancers including brain tumors, lymphomas and osteosarcomas, or bone cancers.
Critics maintain that most SV40 cancer reports are false positives and that the link to human cancers remains unproved.
“The big question is, what do we make of SV40 in humans? This is a big question that we still can’t answer,” said Dr. Hilary Koprowski of Thomas Jefferson University in Philadelphia, who created one of the first polio vaccines and developed the rabies vaccine. “The hypothesis is that the original vaccine was contaminated with SV40 and now, many years later, may be causing problems. But there’s no evidence for that.”
According to Dr. Robin A. Weiss of University College London: “This latest paper shows that contamination of certain polio vaccines went on for longer than we suspected. However, whether or not this type of contamination is genuinely linked in any cancers in humans remains controversial and subject to litigation. I, for one, remain rather skeptical.”
Both scientists agreed with Carbone that vaccine manufacturers should use cell lines free of infectious agents, not kidney tissue from live monkeys, to produce vaccines.
The U.S. stopped using fresh monkey kidneys for polio vaccine in 2000, but China and several other countries still make vaccine with fresh tissue, Carbone said. The new study did not examine Chinese vaccines.
Koprowski has fought the use of animal cells for more than 50 years. He has been trying to grow vaccines and human antibodies in plant cells.
“We’re trying to grow flu vaccine in barley. Whenever you go into animal products, there is a chance you’ll face contamination. With plants, it’s much safer and much less expensive.”