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A face-to-face educational method used among Orthodox Jews apparently led to a US outbreak of mumps in 2009 and 2010 even though most of those infected had been properly vaccinated, according to a US study.
The outbreak, detailed in the New England Journal of Medicine, indicates how close, repeated contact with an infected person can overwhelm the mumps vaccine, the researchers said.
"The risk of infection with mumps may be higher when the exposure dose of virus is large or intensely transmitted," wrote lead author Albert Barskey, of the US Centers for Disease Control and Prevention's (CDC) National Center for Immunization and Respirators Diseases, and colleagues.
This may also explain why the mumps vaccine tends to be less effective among household contacts than among school or community contacts, they added.
In the mumps outbreak, 3,502 cases were reported over a one-year period beginning in June 2009 in New Jersey, New York City and New York State's Orange and Rockland counties. A camp in the Catskill Mountains was the source.
The researchers, from the involved state health departments and the CDC, studied 1,648 of those cases, nearly all of them Orthodox Jews. The researchers found that 89 per cent had received the recommended two doses of mumps vaccine.
Many attended a religious school known as a yeshiva, where boys receive intense training with a study partner known as a chavrusa, who sits across a narrow table. The teaching method often involves animated discussions and the partners are switched several times a day.
The researchers wrote that "chavrusa study, with its prolonged, face-to-face contact," probably resulted in high exposures to the virus, and these "overcame vaccine-induced protection in individual students."
The large families in Orthodox Jewish communities also contributed to the spread, Barskey told Reuters Health in a telephone interview.
"As the outbreak went on, we started to see younger and older cases, and females as well. What that suggests is there was spread in the households. From family it would jump to a new school," he said.
"The chavrusas played the biggest role. The households played a lesser role."
The source of the outbreak was eventually traced to an 11-year-old boy, who had himself received two vaccine doses but nonetheless picked up the disease in the United Kingdom, where fears about vaccination had led to a large mumps outbreak.
That child attended the camp, which had about 400 Orthodox Jewish boys.