Image: CC/CDC. Mumps virions.
Key study points
1. Mumps outbreaks continue to occur in populations with predominantly two-dose coverage with the MMR vaccine.
2. There is a potential role for a third MMR vaccine dose for better mumps outbreak control in these populations.
Primer: Prior to the development of the mumps vaccine, mumps was the leading cause of viral meningitis and of unilateral sensorineural hearing loss in school-aged children. The mumps vaccine has been available since 1967, and the combined measles, mumps and rubella (MMR) vaccine has been recommended for all children in a two-dose vaccination regimen since 1989. Coincident with the development of the vaccine, mumps cases in the US have declined 99% since the late 1960s. Despite high MMR vaccination coverage since that time, however, there have been several large mumps outbreaks in the US recently, most notably in 2006. Factors contributing to these outbreaks include proximity and waning immunity, with insufficient vaccine coverage to provide herd immunity. Strategies for mumps outbreak control in the past have included vaccination of populations that were either not vaccinated or under-vaccinated, but in populations with high pre-existing two-dose vaccine coverage, outbreak control has relied on standard measures such as isolation of infected patients. This paper addresses the efficacy of a third MMR vaccine dose as an additional outbreak control method in populations with pre-existing two dose MMR coverage.
This [prospective, cohort] study included sixth to 12th grade students in Orange County, New York, who had had two previous doses of the MMR vaccine. As part of an outbreak control measure, these students were offered a third dose of MMR vaccine during a mumps outbreak that occurred in that county in late 2009. Of the 2265 students eligible for the third dose of MMR, 1755 students elected to receive it. The calculated risk of contracting mumps among students who received three MMR doses was eightfold lower than among students who had only received two doses, although this difference was not statistically significant (0.06% vs 0.48%, p=0.097). The paper additionally notes that after the third dose, there was an overall reduction in risk of mumps infection in the three weeks after, as compared with the three weeks prior to the vaccine (a 75.6% decline), with a statistically significant decline in the target 11- to 17-year-old age group (96% relative decline, p<0.05). This decline was also seen among unvaccinated students in the same classes as the vaccinated students, suggesting that the intervention may have provided “heard-immunity” effects as well.
In sum: This study demonstrates that a third MMR vaccination may confer protective benefits in a population with high pre-existing two-dose vaccination coverage during a mumps outbreak. After the intervention, there was a reduction in mumps cases in all age groups, but the decline was statistically significant only among vaccinated children aged 11-17 years. These results are suggestive that a third MMR vaccine may be used effectively as a method of outbreak control among previously vaccinated populations. The paper is limited, however, in that the intervention occurred immediately after the peak of the outbreak, which makes it impossible to exclude the possibility that the decline in mumps incidence post-vaccination could be unrelated to the intervention.
By [EH] and [DB]
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