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On June 17, 2009, an 11-year-old boy returned home from the U.K., which was experiencing a large number of mumps cases at the time. He then went to a summer camp for Orthodox Jews in upstate New York.
This turned out to be the spark that led to an outbreak of mumps among Orthodox Jewish communities in and around New York City. Ultimately, more than 3,500 people got sick.
Many of them were children who had already received the standard immunization — two shots of the measles-mumps-rubella, or MMR, vaccine.
The outbreak, which lasted for about a year, was largely confined to three neighborhoods in Brooklyn. There were also cases in a few towns in Orange, Rockland and Sullivan counties in New York and Ocean County, New Jersey.
But mumps spread so widely within the Orthodox Jewish communities in these areas that public health officials concluded that a quarantine, the usual response to a mumps outbreak, wouldn't work.
So they tried something new.
After receiving approval from the New York State Department of Health and the Institutional Review Boards at the Centers for Disease Control and Prevention, the officials administered a third dose of MMR vaccine to children who hadn't yet contracted the disease.
Why didn't the standard two-shot dose of the vaccine protect the children? Public health officials, who published their findings in this week's New England Journal of Medicine, say it was the unique style of study in yeshivas, religious schools for Orthodox Jews.
In a yeshiva, students are paired up in partnerships called chavrusas. The two students in a chavrusa share the same desk and the same book and engage in a vigorous dialogue about the day's lesson. Throughout the school day, which lasts up to 15 hours in a yeshiva, the students rotate among different chavrusas, changing their study partner each time.
Albert Barskey, an epidemiologist with the CDC who worked to contain the outbreak, says the chavrusa style of studying probably exacerbated the spread of mumps. The mumps virus gets around on respiratory droplets, saliva and other bodily fluids.
"Because of the close prolonged contact, uninfected students were probably bombarded with large amounts of the virus from the infected students, and the virus overwhelmed the vaccine," Barskey tells Shots.
Fortunately, there were no deaths in this outbreak. Less than 5 percent of the people infected with mumps reported serious complications, though Barskey says there was one case of permanent deafness.
There were also very few side effects to the third dose of the MMR vaccine. Of the 1,755 students who received an extra dose, only 115 reported any side effects. Half of those consisted of pain or swelling at the injection site. Public health officials released their findings on the side effects of the third dose in a separate paper published in the journal Vaccine.
Barskey is quick to note that Orthodox Jews aren't any more or less susceptible to mumps than any other group. Recent outbreaks have also been reported at summer camps and in college dorms.
"It's not necessarily any particular behavioral or religious custom," he says. "It's just being in close quarters."