One recent study found that people were able to burn up an extra 450 calories a day with one hour of moderate exercise. That can include walking briskly, biking or swimming.

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Arielle Schacter, 17, holds her hearing aid in a New York subway station. Hearing loop technology funnels a transit worker's voice into her ear, blocking out the subway noise behind her. Ashley Milne-Tyte for NPR hide caption

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What's in this vial changes from year to year, but the process of creating the flu vaccine remains pretty constant.

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