Mumps, once a ubiquitous childhood disease, was nearly eliminated in the United States after a vaccine came out in 1967. (It was folded into the three-part measles, mumps, rubella [MMR] vaccine in 1971.) But the disease is making a comeback, this time in young adults; there have been hundreds of outbreaks across U.S. college campuses since the early 2000s. Now, a new study reveals why: Protection from the mumps vaccine fades over time. A booster shot around age 18 could solve the problem, the researchers say.
Spread through coughing, sneezing, or kissing, mumps causes an uncomfortable swelling of the salivary glands, leading to a swollen neck, fever, and headaches in young children; in adults, the virus can cause painfully swollen testicles, meningitis, and even permanent hearing loss.
When a mumps outbreak hit Harvard University in 2016, epidemiologist Joseph Lewnard and immunologist Yonatan Grad, both at the Harvard T. H. Chan School of Public Health in Boston, wanted to know why. They saw two possibilities: Either today’s mumps strains have evolved to elude the immune response triggered by the vaccine, or protection from the vaccine simply wanes over time.
The pair compiled data from six previous studies of the vaccine’s effectiveness carried out in the United States and Europe between 1967 and 2008. (None of the studies is part of a current fraudulent claims lawsuit against U.S. vaccinemaker Merck.) Based on these data, they estimated that immunity to mumps lasts about 16 to 50 years, or about 27 years on average. That means as much as 25% of a vaccinated population can lose immunity within 8 years, and half can lose it within 19 years, researchers report today in Science Translational Medicine.
The team then built mathematical models using the same data to assess how declining immunity might affect the susceptibility of the U.S. population. When they ran the models, their findings lined up with reality. For instance, the model predicted that 10- to 19-year-olds who had received a single dose of the mumps vaccine at 12 months were more susceptible to infection; indeed, outbreaks in those age groups happened in the late 1980s and early 1990s. In 1989, the Centers for Disease Control and Prevention added a second dose of the vaccine at age 4 to 6 years. Outbreaks then shifted to the college age group.
Lewnard and Grad did not find evidence that the vaccine is any less effective today than it was a half a century ago. If that were the case, they would have expected to see outbreaks in younger people, which aren’t happening.
The researchers say future mumps outbreaks could be prevented by giving all 18-year-olds a third dose; they recommend clinical trials to test whether that approach works. Already, the U.S. Advisory Committee on Immunization Practices has recommended that people exposed to outbreaks get a booster shot. “You can see that when we give these vaccines during outbreaks, the outbreaks stop,” says Laura Pomeroy, a disease ecologist at The Ohio State University in Columbus who was not involved in the study. The strategy has also worked well for the military, which has not seen mumps outbreaks since it began giving all new recruits an MMR dose in 1991.
Stanley Plotkin, a veteran vaccine expert with VaxConsult in West Chester, Pennsylvania, is not totally convinced that virus evolution doesn’t also play a role. Some studies suggest that the vaccine triggers a less potent reaction against today’s mumps viruses than those of 50 years ago, he says; that may play a role in the resurgence, in addition to waning immunity. “From my point of view, both factors are important,” Plotkin says.
But he supports the idea of a third dose for college students. “Something needs to be done,” he says. “The simplest thing, the thing we can do today, is to propose a third dose of MMR on entry to university colleges, which would help to prevent outbreaks from occurring.”