Third MMR dose shows promise in curbing mumps outbreaks

A vaccine campaign to administer a third dose of measles, mumps, and rubella (MMR) vaccine as part of the response to a mumps outbreak on the University of Iowa campus helped control the event and showed that waning immunity probably contributes to mumps outbreaks in highly vaccinated groups.

Researchers from the US Centers for Disease Control and Prevention and state and local health officials reported their findings from an outbreak during the 2015-16 school year yesterday in the New England Journal of Medicine.

College settings ripe for outbreaks

Mumps outbreaks are often reported on college campuses, though many schools require students to have two doses of MMR vaccine before starting classes. The disease can spread in settings where people are in close proximity, such as in college dorms and on sports teams.

Waning immunity after the second dose has been suspected as one of the factors fueling the outbreaks, and a few colleges—including the University of Iowa—have offered third doses of MMR as a booster shot to prevent the spread of the disease. Until now, however, little was known about the impact of the practice, which kept the question open about whether the strategy was a useful tool and practical because of limited resources.

A quarter of target population reached

Of 20,496 students enrolled at the University of Iowa that school year, mumps was confirmed in 259. Before the outbreak, 98.1% had received at least two doses of MMR vaccine.

As one of the response steps, the university held eight mass vaccination clinics over 10 days targeting students younger than age 25 that reached 4,494 students.

The attack rate was lower in students who received three doses than in their peers who had gotten only two—6.7 versus 14.5 cases per 1,000 population. The researchers calculated that the vaccine was associated with a 60% to 78% reduction in mumps risk.

Signs of waning immunity

Also, students who had received their second MMR dose 13 or more years before the outbreak began had 9 to 14 times the risk of mumps as students who got the second dose more recently.

The improvement noted in vaccine effectiveness between the two- and three-dose groups at 7 days after vaccination suggests that the benefit occurs shortly after vaccination, which is likely due to a booster response, the authors wrote.

They noted that the campaign's timing, just before the highest outbreak peak, helped the researchers assess differences between the groups.

They said other factors, such as a coordinated public health response, implementation of a standard case definition, rapid testing, isolation, and enhanced awareness, also probably played a role in the decline of the outbreak, which occurred rapidly.

The authors concluded that longer-term benefits of routine three-dose MMR administration requires further study, but for now, a third-dose may be useful for outbreak control. And the drop-off seen in immunity may be useful in helping health officials decide if a third dose would be helpful in stemming outbreaks in highly vaccinated populations.

See also:

Sep 6 N Engl J Med report

Apr 8, 2016, CIDRAP News story "Early reports show mumps on the rise in 2016, including on campuses"

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