Every year, a new influenza vaccine is developed. what is the most likely reason that doctors do not use the same vaccine each year? a. the vaccine turns into a new virus that causes a recurrence of the same influenza each year. b. the virus which causes influenza adapts each year to the last vaccine in order to survive and reproduce. c. any vaccine can only last for a year before it dies. d. a new, different virus that causes influenza enters the food chain through plants about once every year, infecting humans.