An Afghan health worker administers the polio vaccine to a child during a vaccination campaign in Kabul, Afghanistan, in 2010. Majid Saeedi/Getty Images hide caption

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Microsoft founder Bill Gates, his wife Melinda French Gates and British Prime Minister David Cameron at the World Economic Forum on January 28, 2011, in Davos, Switzerland. Johannes Eisele/AFP/Getty Images hide caption

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A student at Carlin Springs Elementary School receives a flu shot last January in Arlington, Va. Mandel Ngan/AFP/Getty Images hide caption

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A study that measured antibodies and the disease history of more than  200 kids found that two doses of vaccine were 98 percent effective in  preventing chickenpox compared with 86 percent for a single dose. AP hide caption

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