Public Health South has issued a formal notification about the mumps outbreak.
There have been 12 confirmed cases in Dunedin, most of them at the University of Otago, the authority said in a statement yesterday.
Last week, the Otago Daily Times reported the university was providing free booster vaccinations in response to about 10 reported cases.
Public Health South had adopted a ''manage it'' approach to the outbreak.
''This is because there is spread within the community, and so case-by-case investigation is no longer an effective means of controlling the outbreak.
''Efforts are shifting towards vaccinating at-risk populations,'' Public Health South said.
Medical officer of health Naomi Gough said vaccination was the best defence.
''We are encouraging vaccination of at-risk groups to prevent them from getting infected prior to being exposed.
''You won't get it walking past someone on the street with mumps, but if you are not immune and you spend time in close proximity with someone who is sick and infectious, you are at risk,'' Dr Gough said.
Symptoms include mild fever, aches and pains, and swelling around the face.
