Massachusetts ? Most of the college students who got the mumps in a big outbreak in 2006 had received the recommended two vaccine shots, according to a study that raises questions about whether a new vaccine or another booster shot is needed.

The outbreak was the biggest in the U.S. since shortly before states began requiring a second shot for youngsters in 1990.

Nearly 6,600 people became sick with the mumps, mostly in eight Midwest states, including Kansas and Missouri, and the hardest-hit group was college students ages 18 to 24. Of those in that group who knew whether they had been vaccinated, 84 percent had had two mumps shots, according to the study by the Centers for Disease Control and Prevention and state health departments.

That “two-dose vaccine failure” startled public health experts, who hadn’t expected immunity to wane so soon – if at all.

The research is reported in today’s New England Journal of Medicine.