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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itoggle caption Justin Sullivan/Getty Images

Three years ago, drugstores like Walgreens began training pharmacists to give customers vaccines. Since then, tens of thousands of pharmacists have been certified to give shots.

Francis Ying for Kaiser Health News/NPR hide caption

itoggle caption Francis Ying for Kaiser Health News/NPR