This large mumps outbreak was unexpected at a campus with high two-dose MMR coverage among its students. The overall reported clinical mumps attack rate among the nearly 19,000 undergraduate students was 0.9% through May 8, 2006. In five reported college outbreaks (student populations ranging from 1,500 to 9,000 students) that occurred in 1986–1987, before the two-dose MMR recommendation was made, attack rates ranged from 0.4% to 2.7%, with a mean attack rate of 1.6%.11
While differences in attack rates can be expected because of differences in students' mumps vaccination coverage and immunity from childhood exposure to wild-type virus, other factors might also influence this estimate (e.g., whether mumps exposure was indeed widespread, the timing of the outbreak relative to the academic year, case definition, and case ascertainment).
Before notification of this outbreak, most physicians who evaluated the Kansas university students with parotitis did not suspect mumps as the etiology. After many years of high vaccine coverage among children and adolescents, mumps had become an illness rarely reported (and likely rarely seen) by clinicians in the U.S. Even after the outbreak was identified, diagnosing mumps was not always straightforward. Many students sought initial care early in their illness, presenting with rather nonspecific signs and symptoms. Furthermore, from clinical descriptions during the pre-vaccine era, the submandibular gland may be the only gland involved in 10% to 15% of patients12
and submandibular gland swelling can be virtually indistinguishable from anterior cervical lymphadenopathy.13
This can lead to over- or underdiagnosis of mumps. In the pre-vaccine era, epididymo-orchitis occurred in 20% to 38% of postpubertal males.14,15
In our investigation, testicular involvement was described in the medical records of 11% of men, but 33% of men reported testicular pain or swelling during our interview. The lower proportion of cases with orchitis by medical record review might be because medical care was sought before orchitis had developed, while interviews were usually conducted after recovery and assessed the full course of mumps.
Several risk factors were found to be associated with mumps in the case-control study; some of these factors were also identified in the case-cohort study performed during the outbreak.16
In our study, younger students had increased odds of developing mumps, and students aged 18 to 19 years had the highest odds. At large universities such as this one, first- and second-year students are often in classes with larger numbers of students than those who are in their third and fourth years, potentially increasing exposure to infectious people during outbreaks. Although details of social interactions were not assessed in our investigation, differences in behavior may have increased mumps exposure for younger students. During a mumps outbreak among young adults in Canada, attendance at a specific type of party (rave) was a risk factor for mumps, while attending bars, clubs, concerts, or other social events was not.17
At three Illinois universities with mumps outbreaks in 1986–1987, students in lower classes had an increased risk of mumps compared with students in upper classes by univariate analyses, and this held true for dormitory residents at the one university in which stratification of data was possible.18
At the Illinois universities, the difference in risk was believed to be caused by underimmunization (with one MMR) of younger students who did not have previous exposure to wild-type virus. Although underimmunization was not a factor in our outbreak, it is possible that older students at our university were better protected because of prematriculation exposure to wild-type virus, while younger students were less well protected in 2006 because they had no prior exposure to wild-type virus and were protected only by vaccine. Given the low reported incidence of mumps in the U.S. when the Kansas university students were children, however, a substantial difference in exposure between birth cohorts separated by only a few years is not very likely.
Other factors, such as working or volunteering on campus, may have increased disease odds by allowing exposure to a greater number of students. While dormitory residents may also be expected to have increased odds of disease because of possible increased exposure, this was not a consistent finding in our analysis. In our study, women in dormitories were at increased odds of developing mumps compared with men in dormitories. Only five mumps cases occurred among men in dormitories; why this number is so low is not clear. Women in dormitories may have been more likely than men to associate in groups with close physical contact, facilitating transmission. One could also postulate that men were less likely to seek medical care for an illness that has no specific treatment. This assessment, however, is not supported by the finding that among non-dormitory residents, there was no difference in disease odds between men and women. In the Illinois college outbreaks, only results of univariate analyses were available; dormitory residents had an increased risk of mumps and there was no significant difference in mumps risk by gender among all students.18
Close, prolonged exposure to someone with mumps was also associated with developing disease in our study, but only 33% of cases reported such exposure. In the remaining majority of cases, transmission may have occurred from an infectious person who never developed classic mumps, or from an infectious person before developing classic mumps (and the exposed student was never aware of the index case's illness). Only 38% of case subjects in the Illinois college outbreaks18
and 48% in a Tennessee high school outbreak (with high one-dose coverage)19
reported prior contact with someone who had mumps. Despite differences in vaccination status among these outbreak populations, the similarly low proportion of case students who reported exposure suggests a universal explanation, and transmission from minimally symptomatic people seems most likely. Viral shedding from unvaccinated, minimally symptomatic people has been documented;20,21
however, the frequency of such infections among people with two MMR doses is not known.
If transmission from minimally symptomatic people played an important role in this outbreak, one may question whether isolation of clinical cases was a valuable control measure. The effectiveness of isolating mumps cases had previously been questioned in unvaccinated children because of transmission from cases before the onset of parotitis and from minimally symptomatic people.21
Implementing isolation recommendations required substantial effort among SHS, university housing, and LHD staff.22
The change in the recommended number of isolation days created confusion among clinicians, public health practitioners, and students. Increasing the recommended days of isolation near the end of the semester might have exaggerated the decrease in reported cases; some physicians reported this change led them to be more stringent in diagnosing mumps because of the perceived educational and social impact the longer isolation period had on students. Additionally, anecdotal reports indicated that some students with mumps did not seek medical care at the end of the semester, fearing isolation orders would prevent them from completing final examinations. Ideally, studies would be performed to assess the overall benefits and risks of isolation measures for mumps among highly vaccinated university populations; in practice, such studies are exceedingly difficult to perform.
This investigation had several limitations. First, the enrollment rate was low among potential control students, raising the concern that enrolled control students may not have been highly representative of all students without mumps. The enrolled control students, however, were similar to all potential control students in factors available for comparison. Second, we investigated case students who sought medical care; how many cases, if any, did not seek medical care and how these differed from cases that did is not known. Third, our case definition relied largely on clinical criteria; however, our definition was similar to the CDC/Council of State and Territorial Epidemiologists definition, and by reviewing medical records we were able to document mumps-specific features for most cases.