Just this week, an outbreak of mumps among more than 1,000 people in New Jersey and New York has raised alarm among infectious disease authorities. The outbreak itself is not unusual, though. What's unusual is that the health authorities slipped up and admitted that most of the people infected with mumps had already been vaccinated against mumps. In Ocean County health department, New Jersey, county spokeswoman Leslie Terjesen told CNN that 77% of those who caught mumps had already been vaccinated against mumps.