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SAN DIEGO -- The effectiveness of the mumps vaccine wanes at an average of 27 years after the last dose, potentially explaining the resurgence of cases over the past 11 years, researchers said here.

According to an analysis of epidemiological studies, the introduction of the measles-mumps-rubella (MMR) vaccine in the 1960s lowered case rates by more than 99%, according to the CDC, and annual cases in recent years have typically numbered in the hundreds, reported Joseph Lewnard, PhD, and Yonathan H. Grad, MD, PhD, both of the Harvard TH Chan School of Public Health in Boston.

But mumps cases spiked in 2006 and again in 2016 and 2017, they said in a presentation at the annual IDWeek meeting, sponsored jointly by the (IDSA), the (PIDS), the (SHEA), and the (HIVMA).

The findings point to the need for booster doses in adults, according to Lewnard. "If you want to put mumps on the track to elimination, it's not an unreasonable expectation," he stated.

"We've largely eliminated it, but the [MMR] vaccine is not a perfect dose," said Paul Offit, MD, of the Children's Hospital of Philadelphia, to 木瓜直播.

The CDC received reports of more than 6,300 cases in 2016 compared with just 229 in 2012.

For 2017, the CDC has received reports of 4,439 mumps cases as of Sept. 7, 2017, with large outbreaks of 300 or more cases in Texas, Arkansas, Missouri, and New York.

The number is likely higher because mumps is not reportable. Only Wyoming, South Dakota and West Virginia reported no cases.

According to the CDC, "include the known effectiveness of the vaccine, lack of previous exposure to wild-type virus, and the intensity of the exposure setting (such as a college campus) coupled with behaviors that increase the risk of transmission."

Lewnard's group pooled epidemiological data from multiple studies, and concluded that vaccine effectiveness persists for an average of 27 years after the last dose is given. "Changes in age-specific susceptibility due to vaccine waning and declining transmission track with the current resurgence in cases among young adults in the US, and explain outbreaks reported among vaccinated adolescents during the late 1980s," the authors reported.

In another study, Huong Q. Mclean, PhD, MPH, of the Marshfield Clinic Research Institute at Emory University and Emma Seagle, MPH, of the CDC, both in Atlanta, and colleagues analyzed 302 Wisconsin children who received the second dose of the MMR vaccine at ages 4-6 years, and were followed up to 12 years after vaccination. They said that of the 79% of mumps cases with reported vaccination status, 88% had received one or more doses and 60% had received two or more doses of the MMR vaccine. The median age of patients was 20 years (range <1-88 years), reflecting the high number of outbreaks among colleges.

Of 1,379 cases with symptom and complications data, complications included: orchitis in 7% of males, oophoritis in 2% of females, and hearing loss, mastitis, encephalitis, or pancreatitis, each in 1% or fewer patients.

Geometric mean titer to mumps declined 9.2% per year, the researchers reported, but 88% still maintained seropositivity to mumps. While "a fast rate of decline and high degree of variation was observed for mumps," no predictors of the decline were identified," they wrote.

Offit, who was not involved in the studies, said it's possible that a third dose of the MMR vaccine will be recommended to cover young adults through later high school, college, and perhaps a bit beyond. But for now, he said he doesn't think booster shots are advisable outside of a specific risk like an outbreak situation.