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This chavrusa style of study may have allowed for particularly efficient transmission of mumps virus. Although mumps is a respiratory infection, it is spread through droplets and requires closer exposure than do more contagious respiratory infections transmitted by the airborne route, such as measles.
21 We postulate that chavrusa study, with its prolonged, face-to-face contact, resulted in high-inoculum exposures and that such exposures overcame vaccine-induced protection in individual students.22 The role that the transmitted dose of a respiratory virus plays in the determination of vaccine effectiveness is difficult to study in humans, but challenge studies in animals that have been vaccinated with an avian influenza vaccine,
23 epidemiologic investigations involving patients with measles who had previously been vaccinated,
24,25 and biologic plausibility, as suggested from a comparison of challenge methods among volunteers who had been administered an inactivated mumps-virus vaccine,
26 all suggest that the risk of infection with mumps may be higher when the exposure dose of virus is large or intensely transmitted. This phenomenon can also explain why the efficacy of the mumps vaccine tends to be lower among household contacts than among school or community contacts.27 If, in fact, the intensity of exposure reduced the effective protection provided by the mumps vaccine, the frequent daily changing of partners helps to explain why the disease spread in the yeshiva setting.
The finding that transmission of mumps to non-Orthodox persons in the affected communities occurred rarely and was not sustained in that population supports the conclusion that intense exposure is necessary to overcome an individual person's vaccine-induced immunity. Although Orthodox Jewish persons generally cluster geographically and socially, they often interact with their non-Orthodox neighbors and others who work within their communities or visit their communities. However, exposures in these settings would typically not be as intense as those in yeshivas. Notably, a large mumps outbreak in Jerusalem, Israel, during 2009 was linked to an importation from the U.S. outbreak. Orthodox Jewish adolescent boys, typically yeshiva students, were also disproportionately affected in that outbreak, even though their vaccination rates were high and they mixed regularly with their non-Orthodox neighbors.
28
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The fact that the outbreak did not spread to surrounding communities highlights the effectiveness of the two-dose MMR vaccine schedule in most settings. Previous studies have shown that two doses of mumps vaccine have an effectiveness of approximately 88% (range, 79 to 95) in preventing clinical mumps,
8,31,37 and this schedule has been successful in controlling mumps in the general U.S. population
6; a similar schedule has resulted in the elimination of mumps in Finland.
38 Since this outbreak, rates of mumps in the United States have been at near-record low levels, with only 370 provisional cases reported by the end of 2011.
39 (...)
