CHICAGO (Reuters) - The risk of polio vaccine causing the disease has been eliminated in the United States because of a switch to a vaccine containing an inactive form of the virus, a government study said on Tuesday.

"The only threats from polio in the United States are from laboratories and the few remaining polio-endemic areas in Africa and Asia," said the U.S. Centers for Disease Control and Prevention in Atlanta.

Introduced in 1961, polio vaccine has contained the paralyzing illness. But rare cases triggered by the vaccine have only recently been wiped out, according to the report published in the Journal of the American Medical Association.

An average of nine U.S. cases of poliomyelitis a year were blamed on the oral vaccine that contained weakened strains of the virus.

But beginning in 1997, a gradual shift to an injectable vaccine that contains inactivated strains of the virus has eliminated vaccine-associated cases since 2000, the report said.

The older vaccine triggered the illness in one in 2.9 million cases, and there were 59 vaccine-associated U.S. cases between 1990 and 1999, the study said.

There have been nearly 800 cases of naturally occurring polio worldwide this year, most of them in Nigeria, according to the World Health Organization. An extensive vaccination effort underway could stem the threat next year.

The illness, which attacks the nervous system and brain, can cause lasting disabilities and occasionally kills its paralyzed victims.

Children in developed countries are routinely vaccinated against polio, though there are pockets of unvaccinated people among the poor and among those whose beliefs lead them to refuse vaccinations.

The last naturally occurring polio case in the United States was diagnosed in 1979.

Although most of the U.S. population has been vaccinated, an imported strain of the virus might still pose a threat, according to an editorial accompanying the report.

"It is prudent to acquire a U.S. polio vaccine stockpile, despite the existing formidable barriers," wrote Dr. John Modlin of the Dartmouth-Hitchcock Medical Center in Lebanon, New Hampshire.

SOURCE: Journal of the American Medical Association, October 13, 2004.

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