To further develop your story, consider these questions:
Does the character tell anyone?
Do they rebel or go along with it, feeling paranoid now that the authorities always know where they are?
What is the vaccine protecting against?
How does your main character find out about the tracking device? Are there just rumors or does the character have proof?
How does your character balance their desire for immunization against illness that the vaccine provides with the discomfort relating to the tracking device?