NEW YORK (Reuters Health) - A face-to-face educational technique used among Orthodox Jews apparently led to an outbreak of mumps in 2009 and 2010 even though most of those infected were properly vaccinated, according to a new study.
The outbreak, detailed in the November 1 New England Journal of Medicine, illustrates how close repeated contact with an infected person can overwhelm the mumps vaccine.
"The risk of infection with mumps may be higher when the exposure dose of virus is large or intensely transmitted," the researchers concluded in their report. 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 all, 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 research team, from the involved state health departments and the U.S. Centers for Disease Control and Prevention, studied 1,648 of those cases, nearly all of them Orthodox Jews. The researchers found that 89 percent had received the recommended two doses of the mumps vaccine.
Many attended a religious school, known as a yeshiva, where boys receive intense training with a study partner, 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, lead author Albert Barskey, of the CDC's National Center for Immunization and Respiratory Diseases, 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 the family, it would jump to a new school," he said.
"The chavrusas played the biggest role. The households played a lesser role."
The most common complication among sick males over 11 was orchitis, an inflammation of the testicles, which can lead to sterility. It affected 120 people, or seven percent of those infected, but was less common among boys and men who had been vaccinated twice.
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.
"It typically takes 16 to 18 days for an exposed child to develop signs and symptoms, and to be infectious," said Barskey.
The researchers said the fact that mumps did not spread to other communities shows that, in most cases, the system of giving two doses of the vaccine is effective.
And proper vaccination did generally reduce both the severity of disease and transmission to others when exposure was "less intense," they noted.
The previous large mumps outbreak in the United States involved nearly 6,600 cases in six Midwestern states in 2006.
SOURCE: http://bit.ly/Y1Q482 New England Journal of Medicine, online October 31, 2012.