Multiple factors led to mumps outbreak in Midwest

May 2008

The United States experienced the largest mumps outbreak in almost 20 years during 2006 — one that quickly spread across states in the Midwest.

In January 2006, health officials identified the first cases of the mumps in colleges in eastern Iowa. By April 2006, the outbreak had reached 40 states and infected 2,786 people. The highest incidence occurred among college students aged 18 to 24 years.

“There were a combination of factors that probably caused this outbreak,” Patricia Quinlisk, MD, MPH, medical director and state epidemiologist at the Iowa Department of Public Health and member of the Infectious Disease News editorial advisory board, said in an interview. “It appears that the two-dose mumps vaccine that was administered to these students when they entered school about 12 years prior may only be about 80% effective, and because respiratory and saliva-spread disease like this are easier to spread in places like college dormitories. We also think that some of the early cases may have not been diagnosed as mumps since the disease had been rare in Iowa for about 20 years and because the disease was not expected in vaccinated people.”

Reported mumps cases

Quinlisk and colleagues in other states and CDC officials collected and reviewed mumps cases reported to the National Notifiable Diseases Surveillance System from state health departments and reported their findings in The New England Journal of Medicine. A patient with a confirmed case of mumps met the clinical case definition and also had a positive laboratory test or was epidemiologically linked. Probable cases were defined as those that fit the clinical case definition but were not laboratory confirmed.

Data were then reviewed from the U.S. Immunization Survey, the National Immunization Survey, the National Health Interview Survey and school vaccination surveys.

Case information included vaccination status and self-reported race or ethnic groups for the eight states most affected: Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, South Dakota and Wisconsin. Four of the states also included information on college attendance.

Results of the study indicated that 6,584 mumps cases were reported in 2006; 85 people were hospitalized, and no deaths directly attributed to mumps were reported.

Eighty-five percent of these college students with mumps lived in the eight neighboring Midwestern states. Eighty-four percent of the 1,020 individuals aged 18 to 24 years with known vaccine status received both doses of the mumps vaccine. “This is not surprising since almost all college students have been vaccinated, with only 4% of those younger than 30 never vaccinated,” said Quinlisk. “Thus it would be expected that higher number of cases occur in those vaccinated students in whom immunity was not sustained than in those who were never vaccinated.”

Waning immunity may be secondary to lack of natural exposure. People aged 18 to 24 years were born in the 1980s when mumps activity was sufficiently low and many of them were never exposed to the disease. A second dose of the vaccine was probably administered during the 1990s, a time when boosting from exposure to the natural virus became increasingly rare. These Midwestern students entered college at a time of increased risk for the importation of the mumps virus from other countries, such as the 2004 to 2006 outbreak in the United Kingdom that affected more than 70,000 people.

According to the researchers, incidence was higher among women than among men (13.5 vs. 7.7 per 100,000 people). This was not explained by differences in vaccination status or age. In response to this outbreak, the CDC and the American College Health Association now recommend that colleges require students to receive the two-dose measles-mumps-rubella vaccine (M-M-R II, Merck).

The study concluded that a more effective mumps vaccine or changes in the vaccine policy may be needed to avert future outbreaks and achieve the elimination of mumps.

Already, the Advisory Committee on Immunization Practices updated their recommendations in (June 2006), which now advise eligible children and health care workers to receive two doses of the mumps vaccine, separated by at least 28 days.

For more information:

• Dayan G, Quinlisk P, Parker A, et al. Recent resurgence of mumps in the United States. N Engl J Med. 2008;358:1580-1589.