Third MMR vaccination may help reduce the risk of mumps during an outbreak in a university setting

EBM Focus - Volume 12, Issue 38

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Reference: N Engl J Med 2017 Sep 7;377(10):947 (level 2 [mid-level] evidence)

  • Mumps outbreaks may occur in high-density settings, even among populations who have had 2 MMR vaccinations. There is some evidence that a third MMR vaccination may help control such outbreaks.
  • A retrospective cohort study of over 20,000 students evaluated the efficacy of a third MMR vaccination in controlling a mumps outbreak at the University of Iowa in 2015-2016.
  • Three MMR vaccinations was associated with a reduced rate of mumps attacks during the outbreak compared to 2 MMR vaccinations (0.67% vs. 1.45%, p < 0.001).

The Centers for Disease Control and Prevention (CDC) recommends Measles, Mumps, and Rubella (MMR) vaccination at 1-1.5 years old with a second MMR vaccination at 4-6 years old (CDC 2017). Mumps outbreaks occasionally occur in high-density settings even among populations with high vaccination rates (Public Health Rep 2009, MMWR 2016 Jul, MMWR 2012 Dec). Some evidence exists that a vaccination campaign offering a third MMR vaccination may help control outbreaks, but additional studies are needed (MMWR 2016 Jul, MMWR 2012 Dec, Pediatrics 2012, Pediatr Infect Dis J 2013). A recent retrospective cohort study evaluated the efficacy of a third MMR vaccine to control a mumps outbreak at the University of Iowa in the United States from August 2015 to May 2016. In the study, 20,496 students aged 18-24 years old were assessed via university records and data from the outbreak investigation. Almost all students had 2 MMR vaccinations before the outbreak, and a third MMR vaccination was offered during the outbreak, mostly through a vaccination campaign in November. In total, 5,110 students (24.9%) had 3 MMR vaccinations before symptom onset (in students who contracted mumps) or by the end of the outbreak, and 15,206 students (74.2%) had only 2 MMR vaccinations.

Students with 3 MMR vaccinations had a lower rate of mumps attacks during the outbreak than students with 2 MMR vaccinations (0.67% vs. 1.45%, p < 0.001). Because time between a third MMR vaccination and immune response is unclear, multivariable analyses were conducted to assess the risk of a mumps attack starting at 7, 14, 21, and 28 days after the third vaccination. The benefit of a third MMR vaccination was apparent for all times, with an adjusted hazard ratio of 0.4 (95% CI 0.26-0.62) for the risk ≥ 7 days after the third vaccination and consistent results for the other days. In an analysis of students with ≥ 2 MMR vaccinations, a longer interval since the second MMR vaccination was associated with an increased mumps attack rate: 1.76% among students with a second MMR vaccination 16-23 years previously, 1.13% for 13-15 years, 0.39% for 3-12 years, and 0.16% for within the past 2 years (p < 0.001 for trend). The increased risk with a longer interval since the second MMR vaccination was also apparent in multivariable analyses of students who had 3 MMR vaccinations. Adverse events were not reported.

This retrospective cohort study provides additional evidence that a third MMR vaccination may help reduce the risk of mumps during an outbreak in a high-density setting of highly vaccinated persons. It also provides evidence that increased time since a second MMR vaccination may be associated with an increased risk of mumps attack during an outbreak. The authors note that factors other than a third MMR vaccination likely contributed to risk reduction as well, including implementation of protocols for case detection, rapid testing, and case isolation; heightened student awareness; and recent second MMR vaccinations due to a university requirement of 2 MMR vaccinations before enrollment. These results are most relevant for outbreak control, but they also support some consideration of a third MMR vaccination for persons entering a high-density setting who had a second MMR vaccination many years previously.

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