Top story: Nearly 6,000 confirmed cases of mumps reported in 2006

December 2006

ATLANTA – A total of 5,824 confirmed cases of mumps were reported between Jan. 1 and Oct. 14, 2006.

Seven states accounted for the majority (87%) of confirmed cases; Iowa had the highest number, followed by Kansas, Wisconsin, Illinois, Nebraska, South Dakota and Missouri. The outbreak occurred mainly in people aged 18 to 24; many were college students.

“Health care workers should remain alert to suspected mumps, conduct appropriate diagnostic testing and report these cases to local or state health departments,” researchers wrote in the CDC’s Morbidity and Mortality Weekly Report. At the Advisory Committee on Immunization Practices meeting, Gustavo Dayan, MD, and colleagues from the CDC stressed the need to use every available opportunity to ensure adequate immunity, especially for high-risk populations, such as health care workers and college students.

The effectiveness of two doses of mumps vaccine is in the range of 90% to 95%, therefore mumps may still occur in people who received two doses of measles, mumps and rubella (MMR) vaccine, according to Dayan. He also said physicians should suspect the disease in people with compatible symptoms, even if they are adequately vaccinated.

Characteristics of the outbreak

The average age of mumps patients was 22 years. The second highest age range of incidence was the 5 to 17 age group, followed by 25 to 39.

Each of the seven states with the highest number of reported cases, including Iowa, all had 100 or more cases of mumps. A peak was discovered in the last two weeks of April.

According to preliminary data provided by the seven most affected states, 4% of the patients had not been vaccinated, 20% received one dose, 46% two doses, 1% three doses. In 30% of the patients, the vaccination status could not be determined. The disease appeared to be mild with low rates of complications and hospitalizations.

“The number of reported cases decreased during May to September, when most students were not attending college,” the researchers wrote. “However, since students began returning to school in August, mumps clusters have been reported from three college or university campuses in Illinois, 84 cases; Kansas, 22 cases; and Virginia, 32 cases. “According to the researchers, most of the cases were reported in people who had previously received two doses of measles-mumps-rubella vaccine. “Because two doses of mumps-containing vaccine are not 100% effective, in a setting with high vaccination coverage such as the United States, most mumps cases likely will occur in people who have received the two doses,” the researchers wrote.

Dayan also said the clusters on college campuses occurred because of the high transmission potential in colleges, especially among students living in dorms.

What was learned?

There were few infants and day care outbreaks, as well as no spread to unvaccinated populations. The researchers noted that cases with mild clinical presentation among vaccinated people might have delayed the recognition of cases.

Researchers noted more cases in women, but could not say why this occurred. Dayan said this could be because women may be more likely to seek medical care or because women gather and socialize more than men, facilitating easier transmission.

Diagnosis of mumps is challenging. Immunoglobulin M (IgM) tests may have a varied performance and a delayed or absent IgM response has not been observed, especially in vaccinated persons. Immunoglobulin G (IgM) testing requires paired serum specimens to detect a significant rise in antibody titers, which difficults feasibility. Viral isolation or viral detection using reverse transcriptase-polymerase chain reaction (RT-PCR) testing have shown a low yield, especially when the specimens are not collected early in the course of disease.

In outbreaks such as this, intervention strategies are limited. Although isolation guidance is suggested, it may not be feasible to truly isolate in dormitory settings.

The outbreak occurred because of delayed recognition and the high potential for transmission in a college setting. “High MMR vaccine coverage levels and vaccine effectiveness likely prevented thousands of additional mumps cases; nine out of 10 exposures that may have resulted in infection in two-dose vaccine recipients were prevented,” Dayan said.

Future considerations

Dayan noted this outbreak demonstrated a need for improved surveillance, better case-definitions for mumps, better laboratory diagnosis and the development of new tests. He also suggested that adequate guidelines for isolation in colleges should be developed, and the effectiveness of the current vaccine and the need of more immunogenic and effective mumps vaccines should be reviewed. Health care workers should remain alert. At the initial visit, the researchers recommend that the physicians obtain a serum specimen to be tested for mumps IgM antibodies and a swab from the parotid duct or other affected salivary gland ductus for viral isolation, RT-PCR testing or both. The first serum specimen should be collected within five days of the onset of illness. If IgM antibody titers are negative, then a second specimen should be collected two to three weeks after the onset of symptoms to detect a delayed IgM response. The paired serum specimens also can be used to detect a significant rise in IgM. Negative laboratory results, especially in vaccinated people, should not be used to rule out a mumps diagnosis, because these tests are not sensitive enough to detect infection in all people with clinical illness.

Reinforcing vaccine importance

According to the researchers, the MMWR report reinforces the importance of the combination MMR vaccine for high-risk populations. The vaccine is required for school entry and is usually administered when children are aged 12 to 15 months and again at 4-years-old to 6-years-old.

In response to the large number of mumps cases this year, the Advisory Committee on Immunization Practices (ACIP) recommendations for prevention and control of mumps were updated. Evidence of immunity through documentation of vaccination is now defined as one dose of live mumps vaccine for preschool-aged children and adults not at high risk for exposure and infection and two doses of live mumps vaccine for school-aged children (ie, grades kindergarten through 12) and adults at high risk for exposure and infection (ie, health-care workers, international travelers and students at post-high-school education institutions). Additional recommendations for outbreak control include administering a second dose of MMR for preschool children and adults not at high risk for exposure and infection if these persons are part of a group that is experiencing an outbreak. “To ensure high levels of immunity, especially among groups at high risk for exposure and infection, every opportunity should be used to provide the first or second dose of MMR vaccine to those without adequate evidence of immunity.”

Dayan said there is currently not a lot of mumps activity in the United States. The cases in the affected colleges seem to be slowing down.

For more information:

• Dayan G. Mumps outbreak United States, 2006. Presented at: Advisory Committee on Immunization Practices meeting; Oct. 25-26, 2006; Atlanta.
• Reef S, Dayan G, Bellini W, et al. Brief report: update: mumps activity – United States, Jan. 1-Oct. 7, 2006. MMWR. Oct 2006;55:1152-1153.