Recently, 41 students at Harvard University came down with mumps and, according to the Public Health department in Cambridge, every single one of those students had been vaccinated.4
Four other campuses in Boston are also starting to see cases, as have four universities in Indiana. About 13 cases of mumps have also cropped up in California.
One ridiculous explanation offered by Dr. Amesh Adalja, an infectious-disease specialist at the University of Pittsburgh Medical Center's Center for Health Security, is that the vaccine only works if the exposure to the virus is low; it can't be expected to work if there are high amounts of exposure, such as in dorms:5
"The exposure that they have to mumps is so high in these situations that it overcomes the ability of the vaccine to protect them," Adalja told Live Science. "It may be that, in these special situations, a much higher level of antibodies [against mumps] is needed to keep the virus at bay."