Why so many mumps outbreaks around the country?
A recent outbreak in North Texas has involved more than 100 confirmed and suspected cases in Johnson County and surrounding counties, largely in school-aged kids. Students who have not been fully vaccinated have been told to stay home until they’re either up on their vaccinations or the illness is no longer present in the schools.
But this outbreak is tiny compared with Arkansas, where more than 1,800 people have fallen ill so far.
According to the U.S. Centers for Disease Control and Prevention, there were 5,311 mumps cases reported nationwide in 2016 (as of December 31), with eight states reporting more than 100 cases. That’s well over twice the number of mumps cases for all of 2015—but not quite the size of the big mumps outbreak of 2006 that affected mostly colleges in the Midwest.
Colleges are natural environments for outbreaks because students live in close quarters, pretty much 24/7.
Still, that doesn't explain why mumps outbreaks affect far more people than, say, measles. It was big news when the country saw 667 cases of measles in 2014, with about half of those coming from an outbreak in unvaccinated Amish communities in Ohio. That’s by far the highest number in the past seven years, and yet it’s only about one-tenth of the 2006 mumps outbreak.
And why are large numbers of vaccinated people getting sick with the mumps in these outbreaks?
After all, just about as many people get vaccinated for mumps as for measles. Both are included in the MMR vaccine—or measles, mumps, rubella—generally given as one shot. Both measles and mumps are highly contagious.
But the mumps vaccine, while highly effective, doesn’t have as high a success rate as the measles vaccine. For those who have received the recommended two shots for both diseases, the mumps vaccine is 88 percent effective, compared with 97 percent for measles.
It makes sense that many of the people who have caught the mumps had received at least partial vaccination against it, simply because most people in school and college have been vaccinated--they make up the vast majority of most populations. Close to 95 percent of U.S. children have received both recommended MMR vaccine doses by kindergarten.
Let's say 60 percent of those who get sick were previously vaccinated. It doesn't mean that the vaccine made people more likely to catch the mumps or that the vaccine wasn't at all effective. Almost everyone is vaccinated against mumps in the U.S. If the vaccine didn't prevent illness at all, we would expect a similarly high percentage — 95 percent, rather than 60 percent — of those falling ill to have gotten the vaccine.
It’s not known how many students in the affected Texas schools were vaccinated, because Texas law does not require the reporting of immunization rates for individual schools. That's something we at The Immunization Partnership hope to see changed so that parents can know more about the safety and health of their children's schools.
School districts do report the numbers, however, and the vaccination rates for the school districts in this case are high: more than 95 percent of kindergartners and roughly 99 percent of 7th graders have received two doses of the MMR. This suggests that while the number of cases is high in North Texas, many more people could have fallen ill if vaccination coverge weren't so high.
Outbreak investigators are still researching the causes of the outbreaks and why there were a number of "breakthrough" cases in vaccinated individuals. But health officials overhwelmingly agree that the MMR vaccine is still currently the best defense against mumps. Thanks to the vaccine, the number of cases has fallen by more than 99 percent over the past half century, and even when vaccinated people do catch mumps, they appear much better off for having been inoculated.
“We have only seen a few cases with complications, like swelling of the brain or testicles,” the Arkansas Department of Health reported. “Normally, we would expect to see many more persons with complications. This tells us that even though some vaccinated individuals are still getting the mumps, they are experiencing mild disease.”
Mumps is a milder disease in children than in adults, so for those who never got their protective shots, this is a great time to talk to their doctor about doing so.
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