The large mumps outbreak in the United States in 2006 was the first ever caused by the failure of the vaccine, federal health experts said Thursday.

The outbreak was the worst in 20 years (6584 cases), the disease spreading fast in eight states including Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota and Wisconsin. They all had 85 percent of the cases. Most of the people infected with mumps had received the second recommended dose of a mumps vaccine, the researchers reported and still they became ill.

“Despite a high coverage rate with two doses of mumps-containing vaccine, a large mumps outbreak occurred, characterized by two-dose vaccine failure, particularly among Midwestern college-age adults who probably received the second dose as schoolchildren. A more effective mumps vaccine or changes in vaccine policy may be needed to avert future outbreaks and achieve the elimination of mumps,” Gustavo Dayan of the Centers for Disease Control and Prevention in Atlanta and colleagues wrote in a study appearing in the April 11 issue of the New England Journal of Medicine.

Eighty-five people needed hospitalization. Fortunately, there were no deaths. However, 11 people lost their hearing and 22 developed meningitis, according to the report.

“It would have been tens of thousands of cases if we didn’t have the coverage,” said Jane Seward, deputy director of the CDC’s division of viral disease, according to Reuters.

Mumps is a disease caused by a virus that usually spreads through saliva and can infect many parts of the body, but mostly the parotid salivary glands. These glands are located towards the back of each cheek, in the area between the ear and jaw. In cases of mumps, these glands typically swell and become painful.

After the 2006 outbreak, there hasn’t been another of the same proportion in the U.S. However, the CDC has already set a goal of eliminating mumps by 2010 in the U.S. But, according to the report, the current vaccine is only 90 percent effective after two doses.

“So, even if you had a 100 percent vaccination rate, you would still have 10 out of every 100 people susceptible to mumps,” study co-author Amy A. Parker, from the CDC explained.

Therefore, in order to achieve the 2010 goal, Parker said the vaccine needs to be changed in order to be effective or the number of shots should be considered. Now two shots are recommended, one at 12 to 15 months and the other at age 4 to 6.

There is another reason that might stay in the way of reaching the 2010 goal with so many parents not vaccinating their children because fears about the safety of the combined measles, mumps and rubella (MMR) vaccine. But, health experts say these fears are grounded and parents who do not vaccinate their children put them at risk.