Public health officials say millions of Americans who are at risk for the flu this winter still need to be vaccinated -- and some people who have already gotten shots might need a second dose. The CDC reports that a particularly dangerous strain of the flu is about this year...and even some people who have been vaccinated may not be fully protected. About 5 percent of people who got flu shots this year received a batch of vaccine that isn't as potent as it should have been. NPR's Richard Harris reports.
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