The report is published in the Feb. 12 edition of the CDC's Morbidity and Mortality Weekly Report.
Although the outbreak started among boys and remains mostly among them, the disease has spread to some adults and girls, Gallagher said. The concentration of the outbreak among boys is largely due to the separation of these boys and girls in school and other social and religious activities.
Most of the people who have become sick had received the mumps, measles and rubella vaccine (MMR), according to the report. In fact, 88 percent had received at least one dose of the vaccine and 75 percent had received two doses.
However, the vaccine is not 100 percent effective, the CDC team noted. Studies have found one dose 73 percent to 91 percent effective, while the effectiveness of two doses ranged from 79 percent to 95 percent, according to the report.
"That means if you were to expose 100 people to mumps, 10 of them might still come down with the disease," Gallagher said. In this outbreak, these percentages are holding, she added.
Although the vaccine doesn't protect everyone, it does work for most people and effectively prevents outbreaks from becoming worse. "We think that if we didn't have such high levels of MMR coverage, we would have even more spread," Gallagher said.
Outbreaks of mumps are not all that unusual, Gallagher said.
"We have had outbreaks of mumps in communities that have had two doses before," she said. In 2006, there was a large outbreak of mumps among college students in the Midwest, she noted.
And mumps remains prevalent in other areas of the world.
"While we are well-vaccinated against mumps in the United States, many countries throughout the world don't vaccinate at all against mumps," she said. "There is a lot of mumps circulating globally. This means that anytime there is a potential for importation like happened with this summe
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