A large outbreak of mumps occurred at the University of Illinois at Urbana-Champaign during April 2015–May 2016; 89% of patients with mumps had received at least 2 doses of measles-mumps-rubella (MMR) vaccine, and a third dose was recommended as a control measure. The rationale for the recommendation of a third MMR dose included a consideration of seasonal trends and characteristics of the at-risk population. These were weighed against potential drawbacks, which included the potential for vaccine-related side effects, associated costs, and the lack of evidence of the effectiveness of a third MMR dose.

Mumps is an acute viral disease characterized by fever and swelling of the parotid or other salivary glands. On May 1, 2015, the Illinois Department of Public Health (IDPH) confirmed a mumps outbreak at the University of Illinois at Urbana-Champaign. IDPH and the Champaign-Urbana Public Health District (C-UPHD) conducted an investigation and identified 317 cases of mumps during April 2015–May 2016. Because of sustained transmission in a population with high 2-dose coverage with measles-mumps-rubella (MMR) vaccine, a third MMR dose was recommended by IDPH, C-UPHD, and the university’s McKinley Health Center. No formal recommendation for or against the use of a third MMR dose has been issued by the Advisory Committee on Immunization Practices (ACIP) (1). However, CDC has provided guidelines for use of a third dose as a control measure during mumps outbreaks in settings in which persons are in close contact with one another, where transmission is sustained despite high 2-dose MMR coverage, and when traditional control measures fail to slow transmission (2).

On April 15, 2015, the university health center reported to C-UPHD a male aged 21 years with fever and parotitis beginning April 9. Mumps was suspected; however, confirmatory testing was not performed. During the following 2 weeks, five additional suspected cases of mumps were identified. Each patient received a diagnosis of parotitis without laboratory confirmation of mumps. All patients with suspected mumps had documentation of receipt of 2 doses of MMR vaccine. On May 1, 2015, a seventh suspected mumps case was confirmed by a positive real-time reverse transcription–polymerase chain reaction (rRT-PCR) test of a buccal swab conducted at the IDPH state laboratory. The six previous suspected cases were epidemiologically linked to the same academic program as the confirmed case, which enabled IDPH to establish the existence of a mumps outbreak at the university.

Confirmed, probable, and suspected cases were identified using the standard case definition for mumps (3). Patients were considered to be infectious from 2 days before until 5 days after the onset of parotitis. The exposure period was defined as 12–25 days before the onset of parotitis. Outbreak control measures recommended to the university health center by C-UPHD included standard and droplet precautions for patients in health care facilities and isolation of ill patients. Ill students were directed to return home or were provided alternative housing during their infectious period. Investigators identified contacts of mumps patients to verify receipt of 2 doses of MMR vaccine and recommended vaccination of susceptible close contacts if they were not fully vaccinated.* Susceptible close contacts who had a contraindication to vaccination or who refused vaccination were excluded from public settings for 14 days (from days 12–25 following exposure to a person with probable or confirmed mumps). University vaccination records were reviewed, and 2-dose MMR vaccination coverage was estimated at >97% among all students. On May 26, 2015, IDPH posted a notification on CDC’s Epidemic Information Exchange (Epi-X) and issued a memorandum to health departments to request information on cases among persons who returned home from the university during the summer semester.

Despite high 2-dose MMR coverage and a reduced student population on campus, cases continued to occur during the summer semester (Figure). By July 31, a total of 70 cases had been reported. On August 4, IDPH, C-UPHD, and the university health center issued a recommendation for all students and staff members born during or after 1957† to receive an additional dose of MMR vaccine (2). Notifications were sent to students and their families, and an Epi-X notification was posted to inform state health agencies of the recommendation. An estimated 50,000 students and staff members were targeted for this intervention.

A total of 8,200 doses of MMR vaccine were administered at five vaccination clinics held on the university campus during August 6–27. An unknown number of additional vaccine doses were administered to students and staff members living off-campus during the summer, who were encouraged to received vaccine from a health care provider or pharmacy before returning to school. C-UPHD and the university health center administered an additional 3,300 doses throughout the fall and spring semesters. Persons vaccinated were monitored for 15 minutes after receiving the vaccine and were given a vaccine information statement. No serious vaccine-related adverse events were reported.

Investigators identified 317 probable and confirmed mumps cases with onset during April 9, 2015–May 27, 2016. One hundred (32%) cases were laboratory confirmed by rRT-PCR, and 217 (68%) were classified as probable. Cases occurred in persons who ranged in age from 16–55 years, with a median age of 20 years. Twenty-two (7%) patients were evaluated at the emergency department, and three (1%) were hospitalized (one to treat meningitis, one to rule out meningitis, and one for parotitis pain management). Two (1%) patients experienced orchitis, a recognized complication of mumps (2). No deaths were reported. Specimens from four cases were genotyped at the CDC Measles, Mumps, Rubella, and Herpesvirus Laboratory Branch; all were mumps genotype G. All cases were epidemiologically linked to the university; 278 (88%) cases occurred in students, three (1%) in staff members, and 36 (11%) in persons not affiliated with the university, but who had contact with university students or the campus. Several sub-clusters occurred within the larger outbreak in certain academic programs, athletic facilities, and community workplaces.

Among the 317 cases identified, at the time of parotitis onset, 50 (16%) mumps patients had received 3 doses of MMR vaccine, 232 (73%) had received 2 doses, 12 (4%) had received 1 dose, seven (2%) were unvaccinated, and 16 (5%) had unknown vaccination status. Forty-five (90%) of the 50 patients with a third dose received it during this outbreak, and five (10%) received it in prior years for reasons unrelated to this outbreak. Some of the 45 persons who received a third dose during this outbreak might have been exposed before vaccine-induced immunity was boosted. Eleven (24%) of the 45 patients had parotitis onset on the same day or within 2 weeks after receiving the third dose, six (13%) within 2–4 weeks, and 27 (60%) >4 weeks after. One (2%) patient received a third dose 3 days after parotitis onset.