The outbreak was traced to a child who went to Britain - where the illness is more common because of lower levels of vaccination - and then attended a summer camp upstate, apparently infecting dozens of kids.It can also cause death in rare cases.
City health officials say there are 57 confirmed or probable cases and they are investigating more. Those infected are mostly between 10 and 15 years old.
Puzzlingly, 75% of the victims had the normal two doses of mumps vaccine, which is supposed to protect against the illness 90% of the time.
"We know that approximately one in every 20 people who are vaccinated may not develop antibodies," said Dr. Jane Zucker, assistant commissioner of immunization. "If the vaccine was not effective we would have many, many more cases. "
The infection caused by the mumps virus was a common scourge of childhood - and a leading cause of deafness - until vaccination largely ended it in the developed world.
Typically, it causes a painful swelling of the salivary glands. In rarer cases, patients can be rendered deaf.
One of the reasons that it is so important to get vaccinations is that there is a herd immunity imparted on the entire community so that if there are any instances of the disease, they are limited in size and scope; anti-vaccination campaigns by the uninformed and misguided have brought down the number of people who get the immunizations as recommended.
The mumps vaccine is given as part of the MMR series (measles, mumps, rubella).