ATLANTA — The mumps outbreak that began in December 2005 at an Iowa university totaled 5,824 cases in 45 states by mid-October—and it isn't over yet, Dr. Gustavo H. Dayan said at a meeting of the Centers for Disease Control and Prevention's Advisory Committee on Immunization Practices.
The outbreak—which has primarily affected young adults aged 18–24—appeared to have peaked in mid-April of this year, when 25% of the known cases were diagnosed. The number dropped between May and September, while most college students were on break. However, since the resumption of classes in August, mumps clusters have been reported at three campuses, one in Illinois (85 cases), another in Kansas (22), and a third in Virginia (12).
Since January, the number of reported cases per state has ranged from 1 to 1,971. Seven states reported 100 or more cases (Iowa, Kansas, Illinois, Wisconsin, Nebraska, South Dakota, and Missouri). Three states reported 50–99 cases, 18 had 10–49, while 17 states reported 1–5 cases. Only five states have not reported any cases (Connecticut, Delaware, Maine, Montana, and Vermont).
These numbers, reported by Dr. Dayan at the ACIP meeting, are updated from those published in the October 27th issue of the CDC's Morbidity and Mortality Weekly Report (www.cdc.gov/mmwr
Approximately two-thirds of cases have been female. The reason for this is not known, but it may relate to the fact that college women tend to congregate closely together more often than men, and perhaps are more likely to seek health care, said Dr. Dayan of the CDC's Division of Viral Diseases.
In the seven states with the most mumps cases (4,538), parotitis was reported in 68% and orchitis in about 6%. Other manifestations, such as meningitis, encephalitis, deafness, oophoritis, and mastitis have been reported in less than 1%. Approximately 2% of patients have been hospitalized. Overall, “the complications are much lower than in the prevaccine era,” Dr. Dayan noted.
Among those 4,538 cases, 46% had received two doses of mumps vaccine, 20% received one dose, and 1% received three doses. Vaccination status was unknown in 30%. Four percent were unvaccinated. However, following the CDC's updated recommendation for receipt of a second dose of measles-mumps-rubella vaccine in June (MMWR 2006;55:629–30), the proportion who had received two doses was higher in the three recent college clusters: 93% in Illinois, 95% in Kansas, and 100% in Virginia.
Preliminary data do not suggest that waning immunity plays a major role. Even with two doses, a vaccine efficacy of 90%–95% still might allow for accumulation of enough susceptible individuals to sustain periodic outbreaks, he said.