Simply put, as the cell line ages it loses its capability to differentiate itself into various tissues and thus is less useful.

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"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues), " it said.

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"Less than 1% develop a serious neurologic illness such as encephalitis or meningitis (inflammation of the brain or surrounding tissues), " the CDC said.

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