Thus, by all measures, the 2006 mumps outbreak in the United States was an aberration. While the CDC reports that the average number of mumps cases nationwide from 2001 to 2005 was 265 per year, there were 3,200 confirmed cases in 2006 across 12 states, primarily in the Midwest.
Some infectious disease experts noted that the 2006 measles outbreak might have been due to the relatively low number of college students who complied with CDC's recommendation in 1989 that everyone receive a second "booster shot" for measles, mumps and rubella.
"I think the degree of two-shot uptake is very important in preventing large outbreaks," said Dr. James Turner, executive director of the Department of Student Health and professor of Internal Medicine at the University of Virginia. "In [this] study… two shot uptake was only 84 percent. I think there is a higher threshold that must be reached before the vaccine prevents widespread disease."
Dr. William Schaffner, professor and chair of the Department of Preventive Medicine at Vanderbilt University School of Medicine, said the only real preventive measure in preventing future outbreaks is to require all U.S. citizens to get a second dose of the vaccine.
"All developed countries should provide all their children with two doses of mumps vaccine," Schaffner said. "Had this been routine practice in England, there would not have been a mumps outbreak there, there would not have been an importation [of mumps into] the U.S. and there would not have been an outbreak here."
But while some experts said the CDC should be more vigilant in its recommendations that everyone receive a second booster shot, others maintain that the most important measure to ensure fewer outbreaks of the disease is public health education.
"No vaccine can protect 100 percent of people 100 percent of the time," said Dr. Mark Slifka, associate scientist at the Vaccine and Gene Therapy Institute at Oregon Health and Science University. "Increased surveillance at college campuses and increased awareness of the hazards associated with [the disease] through increased education efforts … will also reduce the incidence of outbreaks [and] improve response time in controlling outbreaks."
The CDC's Seward added that although the one-dose mumps vaccine is about 80 percent effective at preventing contraction of the disease, and the two-dose vaccine is about 90 percent effective, there are additional factors that put young adults between the ages of 18 and 24 at higher risk.
"We think close-contact living conditions like on college campuses and also decreased vaccine immunity over time contributed to the  outbreak," Seward said.
Although the study, published in the New England Journal of Medicine, points out that the cause of the outbreak didn't have to do with the vaccine failure, many experts cautioned against citing the study as an example of the vaccine's failure without also highlighting its successes.
"While technically the study reports 'vaccine failures,' it, in fact, reports vaccine successes," said Dr. Martin Myers, executive director of the National Network for Immunization Information. "Most of those who had received two doses of vaccine did not develop mumps after exposure, mumps did not spread to the primary or secondary schools, and the outbreak did not expand into an epidemic of the proportion that occurred in Great Britain, [which experienced] 60,000 cases."
Additionally, experts pointed out that those who contracted mumps despite having received their childhood immunizations did not exhibit the more severe symptoms of the disease, such as inflammation of tissues in the central nervous system or inflammation of the ovaries or the testicles.
"So I would assert that the vaccine, though not totally preventing disease," Virginia's Turner said, "seems to prevent complications."