The indication in the article was that those who were vaccinated relied on the body's immune system to provide antibodies; for some unknown reason, no antibodies produced.
No indication of booster shots.
Those immediately impacted were all members of the swimming team, with various others (friends, girlfriends, etc.) possibly exposed.
The information contained in the article was somewhat superficial and didn't provide much additional information.
Yes it was quite lacking in details. If correct that no antibodies formed, that makes me wonder if the vaccine or protocol was somehow defective. Mumps is so contagious, It’s hard to understand why there would not be antibodies.