Four African countries have reported new cases of polio linked to the oral vaccine meant to protect against the disease.

Health numbers show more children are now being paralyzed by viruses from the vaccinations than wild polio viruses.

Last week, the World Health Organization, or WHO, and partners noted nine new polio cases caused by the oral vaccine in Nigeria, Congo, Central African Republic and Angola. Seven other African countries have similar cases, they said.

WHO and its partners also noted the problem in Asia. Of the two countries where polio remains widespread – Afghanistan and Pakistan – cases linked to the vaccine have been found in Pakistan.

In rare situations, the live virus in the oral vaccine can change into a form capable of causing polio infections. All the current cases that came from oral vaccines have been linked to a Type 2 virus contained in the vaccine. The Type 2 wild virus was eradicated years ago.

Polio, short for poliomyelitis, is a highly infectious disease. It spreads in water or food and usually affects children under the age of 5. About one in 200 infections leads to paralysis. Among those, a small percentage die when their breathing muscles no longer work.

Last week, donors promised $2.6 billion to fight polio as part of a plan aimed at eradicating the disease by the year 2000. The plan was launched in 1988. Since then, numerous targets for defeating the disease have been missed.

To eradicate polio, more than 95 percent of a population needs to be vaccinated. For a long time, WHO and partners have depended on oral polio vaccines partly because production costs are low. In addition, the oral vaccines are easy to give, requiring only two drops per dose. Western countries use a more costly injectable polio vaccine that contains an inactivated virus, one that cannot cause polio.

WHO officials work with the Independent Monitoring Board, or IMB, to measure polio eradication. In a report this month, the IMB warned that polio virus from vaccines is "spreading uncontrolled in West Africa." The report said its spread raises important questions and problems for the eradication process.

I'm Alice Bryant.

The Associated Press reported this story. Alice Bryant adapted it for Learning English. George Grow was the editor.

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oral – adj. of or relating to the mouth

paralyze – v. to make a person or animal unable to move or feel all of part of the body

eradicate – v. to eliminate or destroy something harmful

dose – n. the amount of a medicine, drug, or vitamin that is taken at one time