My daughter Kate has a deadly combination of ZERO tolerance for pain AND acute overly dramatic syndrome which combines to mean any scratch, any bump, bruise or bonk means it's showtime for drama and despair!  She claims the debilitating effects of a flu shot render her arm useless for at least a day!  And the Oscar goes to...

Truth is for a lot of people the arm ache thing after certain shots is real and the real reason your arm hurts the next day is legit.

A University of Pittsburgh professor of family medicine says the pain you feel is due to an immunological response, and actually means the vaccination is working. It means your immune system recognizes the injected inactivated virus as an invader and begins to attack it and that causes inflammation, which is why you feel pain the next day.

The sore arm is not universal however, with only about one in five people have this local reaction, and that number varies by vaccine.  Why is that? Researchers don't know - time for more research!  (PopSci)