I had my Flu shot about a week ago. Maybe it will protect me from the Flu and maybe it won't since it's specific to the type of Flu the drug company thinks will be most widespread, but doesn't cover all of them.
A consensus of Infectious Disease specialists from the CDC, WHO, and other I.D. centers selects the strains of flu for immunization, based on the prevalence of specific types of Flu A and B seen last year and strains seen in Asia in the past 6 months.
They do a pretty good job of predicting, most years.
They also rely on the probability that folks immunized against specific strains last year and the year before that are still immune to those strains, so the overall "herd" immunity is going to be pretty high against the "old" virus strains.
