Apparently the CDC literally changed its definition. From the Miami Herald (but you can check against other sources):

> Before the change, the definition for “vaccination” read, “the act of introducing a vaccine into the body to produce immunity to a specific disease.” Now, the word “immunity” has been switched to “protection.” The term “vaccine” also got a makeover. The CDC’s definition changed from “a product that stimulates a person’s immune system to produce immunity to a specific disease” to the current “a preparation that is used to stimulate the body’s immune response against diseases.”

Webster dictionary broadened its definition too. From USA Today:

> Merriam-Webster revised its "vaccine" definition to replace "immunity" with "immune response."