Federal health officials are evaluating the benefit of an additional dose of the mumps vaccine because of the increasing number of mumps outbreaks since 2006. More than 5,000 cases of the contagious viral illness were reported last year in the United States, the most in a decade.

Among the outbreaks in recent years, 19 occurred last year on college campuses. Arkansas has been battling an outbreak that began in one community last summer and has since infected 2,815 people, the largest recorded in that state.

Unlike outbreaks of measles and whooping cough, which have taken place in populations with significant numbers of unvaccinated people, the mumps outbreaks have been occurring in communities with high rates of immunisation and residents who often have received both recommended doses of the vaccine.

Federal officials said Thursday that they are looking into whether mumps immunity decreases over time and whether there would be benefits to a third dose. State and local health authorities are particularly interested in that additional shot as a preventive measure, Mona Marin told the Advisory Committee on Immunisation Practices.