Meningitis vaccine rollout begins but why is the jab important?

Students receive a vaccine in the sports hall at the University of Kent campus in Canterbury, where the rollout of a meningitis B vaccine to about 5,000 students has begun (Gareth Fuller/PA) - (PA Wire)

Students from the Canterbury campus of the University of Kent have begun to receive the meningitis B vaccine amid an outbreak of the bug which has left two young people dead.

But how is the rollout working and how long does it take for the jab to take effect?