Lessons Were Learned in the Mumps Outbreak

Kansas also vacillated between one and two doses of mumps vaccine as its definition of adequate protection before ultimately deciding that patients who receive one dose of measles-mumps-rubella (MMR) vaccine are adequately vaccinated. Separate guidelines and algorithms were established for health care workers and day care workers that rely on self-reported vaccination history data.

Immunization history available on 85% of cases revealed that 73% had received one dose of MMR vaccine and 7% were unvaccinated, and 64% of all vaccinated patients had a history of two doses.

Laboratories were able to communicate those results to clinicians, but at times, there weren't enough qualified workers or materials to perform the necessary testing. After the testing, it wasn't clear how to interpret negative results and how to convince local authorities that it was still mumps. “Negative results do not rule out disease,” Ms. Hill said.

Delayed recognition of the outbreak, enhanced transmission in colleges, and unrecognized importations all contributed to the outbreak, according to Dr. Dayan. “In addition, two doses of mumps vaccine may confer about 90%–95% vaccine effectiveness, which may result in accumulation of susceptible persons sufficient to sustain transmission and a sizeable outbreak on a periodic basis,” he said. There was no evidence of genetic drift, although the role of waning immunity is unclear.

“However, high MMR vaccine coverage levels and vaccine effectiveness likely prevented thousands of additional mumps cases, the incidence was relatively low, and the disease appeared to be modified with low rates of complications,” Dr. Dayan said.