MMR vaccine
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Despite a successful US childhood vaccination program, mumps outbreaks still occur. A recent study looked at whether a third dose of MMR vaccine could control mumps outbreaks.

The success of the measles, mumps, and rubella (MMR) vaccine in reducing mumps cases in the US is evident, yet outbreaks continue to be reported, particularly in college settings. Although the vast majority of children do receive the recommended vaccinations, it is likely that immunity diminishes by the time they reach young adulthood. Therefore, it is not surprising that college students are at an increased risk of exposure to mumps. Health authorities have considered using MMR vaccination regardless of vaccination status to control outbreaks, yet data regarding a third dose of MMR vaccine is both limited and inconclusive.

A recent observational study in The New England Journal of Medicine investigated whether a third dose of MMR vaccine could control an outbreak of mumps at the University of Iowa. Most students (98.1%) had received at least two doses of the MMR vaccine before the outbreak, as per university enrolment requirements. Researchers conducted a 10-day vaccination campaign during the early stages of the outbreak period, offering free MMR vaccines to students under 25 years of age (20,496 students altogether). By the end of the outbreak, a quarter of the students (25.3%) had received three or more MMR doses.

During the outbreak period, 259 students received a diagnosis of mumps. These students were approximately two times less likely to have received a third MMR dose, and 14 times more likely to have received their second MMR dose more than 13 years ago. These results suggest that a third dose of MMR vaccine decreases the risk of mumps.

Statistical modeling showed that the risk of mumps was 61% lower seven days after receiving the third MMR dose compared to the second dose, possibly due to a booster response. The risk was even lower 28 days after receiving the third MMR dose (78%), at which time the immune response to mumps has fully developed. However, the effectiveness of two MMR doses compared to no dose at reducing the risk of mumps was dependent on the time that had elapsed since receiving the second dose. Two MMR doses were approximately three times more effective (89.4%) at reducing the risk of mumps in students who had received their second dose less than 13 years before the outbreak.

The authors noted the possibility that unmeasured factors may have affected the effectiveness of the vaccine. Despite these limitations, the results suggest that vaccination with a third dose of MMR vaccine has the potential to control mumps outbreaks in highly vaccinated populations and that it is particularly important to target individuals who received their last MMR vaccination more than 13 years ago. These results are especially relevant for other institutes with high-density populations, such as schools and correctional facilities, which are hotspots for communicable diseases. Further research into the long-term effects of a third dose of MMR vaccine is required to help implement public health strategies designed to control future outbreaks.

Written by Natasha Tetlow, PhD

Reference:

Cardemil CV, Dahl RM, James L, et al. Effectiveness of a third dose of MMR vaccine for mumps outbreak control. N Engl J Med. 2017. Available from: doi.10.1056/NEJMoa1703309.

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