Researchers say changes in the polio virus were to blame for hundreds of deaths in the Republic of Congo in 2010.
Originally published at - http://learningenglish.voanews.com/media/video/study-identifies-problem-in-polio-vaccine/2455999.html
Published 2014-10-03 · Watch on YouTube ↗ Availability and playback restrictions are controlled by the original channel.
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