Effectiveness of a Third Dose of MMR Vaccine for Mumps Outbreak Control

Abstract

Background: The effect of a third dose of the measles-mumps-rubella (MMR) vaccine in stemming a mumps outbreak is unknown. During an outbreak among vaccinated students at the University of Iowa, health officials implemented a widespread MMR vaccine campaign. We evaluated the effectiveness of a third dose for outbreak control and assessed for waning immunity.

Methods: Of 20,496 university students who were enrolled during the 2015-2016 academic year, mumps was diagnosed in 259 students. We used Fisher's exact test to compare unadjusted attack rates according to dose status and years since receipt of the second MMR vaccine dose. We used multivariable time-dependent Cox regression models to evaluate vaccine effectiveness, according to dose status (three vs. two doses and two vs. no doses) after adjustment for the number of years since the second dose.

Results: Before the outbreak, 98.1% of the students had received at least two doses of MMR vaccine. During the outbreak, 4783 received a third dose. The attack rate was lower among the students who had received three doses than among those who had received two doses (6.7 vs. 14.5 cases per 1000 population, P<0.001). Students had more than nine times the risk of mumps if they had received the second MMR dose 13 years or more before the outbreak. At 28 days after vaccination, receipt of the third vaccine dose was associated with a 78.1% lower risk of mumps than receipt of a second dose (adjusted hazard ratio, 0.22; 95% confidence interval, 0.12 to 0.39). The vaccine effectiveness of two doses versus no doses was lower among students with more distant receipt of the second vaccine dose.

Conclusions: Students who had received a third dose of MMR vaccine had a lower risk of mumps than did those who had received two doses, after adjustment for the number of years since the second dose. Students who had received a second dose of MMR vaccine 13 years or more before the outbreak had an increased risk of mumps. These findings suggest that the campaign to administer a third dose of MMR vaccine improved mumps outbreak control and that waning immunity probably contributed to propagation of the outbreak. (Funded by the Centers for Disease Control and Prevention.).

Figure 1.. Age at the Time of Receipt of the Measles-Mumps-Rubella (MMR) Vaccine among Students Attending the University of Iowa (2015–2016).

Among the students who received a first dose of MMR vaccine (MMR1), 82.7% were vaccinated between the ages of 12 months and 23 months. Among those who received a second dose of MMR vaccine (MMR2), 81.6% were vaccinated between the ages of 4 years and 6 years. Among those who received a third dose of MMR vaccine (MMR3), 94.7% were vaccinated between the ages of 18 years and 24 years.

Figure 2.. Probability of Freedom from Mumps among Vaccine Recipients of MMR3 versus MMR2.

The probability of remaining mumps-free was higher with receipt of the third MMR vaccine dose (MMR3) at 28 days after vaccination than with the second dose (MMR2). The inset shows the same data on an expanded y axis, with shaded areas indicating 95% confidence intervals. The data have been adjusted for the years since the receipt of MMR2. Graphs for models of data at 7 days, 14 days, and 21 days after vaccination are provided in Figure S3 in the Supplementary Appendix.