Third MMR Shot Lowered Mumps Risk in Iowa Students

Marcia Frellick

September 09, 2017

Students who received a third dose of the measles-mumps-rubella (MMR) vaccine had a lower risk for mumps than students who received only two, according to researchers who studied a recent college outbreak in Iowa.

Cristina V. Cardemil, MD, MPH, an epidemiologist from the Centers for Disease Control and Prevention, Atlanta, Georgia, and colleagues, obtained student vaccination and demographic records from the University of Iowa, Iowa City. Of 20,496 students who were enrolled during the 2015 to 2016 academic year, 259 were diagnosed with mumps.

The researchers compared the likelihood of getting mumps among students who received three, two, or no doses of the vaccine and also looked at the number of years since the second dose to see how that affected outcomes.

They published their findings in the September 7 issue of the New England Journal of Medicine.

Before the outbreak, 98.1% of the students had received at least two doses of the vaccine, which is required by the university. During the outbreak, 4783 students received a third dose.

The mumps attack rate was twice as low among students who received a third dose compared with those who received only two doses (6.7 vs 14.5 cases per 1000 population, P < 0.001).

At 28 days after vaccination, those who received a third vaccine dose had a 78.1% lower risk for mumps than those who received a second dose (adjusted hazard ratio, 0.22; 95% confidence interval, 0.12 - 0.39).

Students had an increased risk for mumps if they received their second dose at least 13 years before the outbreak. Waning immunity probably contributed to the outbreak, the researchers conclude.

"Most of the students had received their first and second doses of vaccine during childhood, according to the recommendations of the Advisory Committee on Immunization Practices, which resulted in a highly vaccinated population before the outbreak. However, students who had received the second dose of MMR vaccine 13 years or more before the outbreak had 9 to 14 times the risk for mumps as did those who had received the second dose more recently," the researchers explain.

The university held mass-vaccination clinics targeting students younger than 25 years for a third dose.

Evidence on the effectiveness of these campaigns in other outbreaks has been inconclusive, the authors note.

However, this study shows that the University of Iowa campaign resulted in one in four of the targeted students getting a third dose and a 60% to 78% reduction in mumps risk.

"We did not investigate whether the routine administration of a third dose of MMR vaccine might be a potential solution to achieving further mumps control, so our findings are best interpreted in the short-term context of an outbreak," the authors write.

The study was supported by the Centers for Disease Control and Prevention. The authors have disclosed no relevant financial relationships.

N Engl J Med. 2017;377:947-956. Abstract

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