Sonia Despiar, right, a nurse with Gouverneur Healthcare Services, injects Imelda Silva with flu vaccine on Friday, Jan. 11, 2013, in New York. At least 10 elderly people and two children in New York have died from the flu and hospitalizations are climbing as the illness hits every county in the state. Bebeto Matthews/AP hide caption

itoggle caption Bebeto Matthews/AP

Four-year-old Gabriella Diaz gets a flu shot at the Whittier Street Health Center in Boston, Mass., on Wednesday, the same day the city declared a public health emergency. Charles Krupa/ASSOCIATED PRESS hide caption

itoggle caption Charles Krupa/ASSOCIATED PRESS

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

Justin Sullivan/Getty Images hide caption

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