Between August 2015 and May 2016, an outbreak of mumps occurred on the campus of the University of Iowa. In response, the Centers for Disease Control and Prevention (CDC) and local health officials responded by administering MMR vaccine (Cardemil CV, Dahl RM, James L, et al. Effectiveness of a third dose of MMR vaccine for mumps outbreak control. New England Journal of Medicine 2017 Sep 7;377(10):947-56).
Of the 20,496 students who were enrolled during the 2015-2016 academic year, 259 were diagnosed with mumps. Most of those infected had been previously immunized. Before the outbreak, 98.1 percent of the students had received at least two doses of MMR. During the outbreak, 4,783 received a third dose. The infection rate was lower among students who had received three doses than among those who had received two doses (6.7 cases vs. 14.5 cases per 1,000 in the population; p<0.001). Further, students who had received MMR vaccine more than 13 years before the outbreak were nine times more likely to develop mumps than those who had received the vaccine more recently.
The authors concluded that a third dose of mumps lessened the risk of the disease in those who had previously received two doses of MMR, writing, “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.”