DURHAM, N.H. (CBS) — There have been six cases of confirmed or probable mumps at the University of New Hampshire in the last two weeks, the New Hampshire Division of Public Health Services announced Friday. Four have been confirmed.

All of the cases are among students who have previously been fully vaccinated and have been in close contact with one another, the DHHS said. “There is not broader campus transmission currently identified.”

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The university is reaching out to select groups of students and recommending a third dose of the MMR vaccine. According to UNH, those people include members of a sorority.

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“The MMR vaccine is approximately 88% effective at preventing mumps after a 2-dose series, but it is still possible for vaccinated individuals to develop mumps. There is evidence of decreasing vaccine effectiveness over time, which is contributing to an increasing number of mumps outbreaks nationally in previously vaccinated populations living in close communal settings, like colleges and universities,” said the DHHS. “A third dose of the MMR vaccine has been studied in outbreak settings in these populations, and has been shown to increase immunity and help preventing transmission.”

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Mumps is a viral infection that causes swelling in the salivary glands and cheeks. While it is usually mild, complications from mumps include meningitis and encephalitis.