Most pediatric mumps cases occur in vaccinated kids. Why experts aren't worried.

As vaccines come under renewed scrutiny amid the COVID-19 pandemic, a recent report found the majority of mumps cases in kids occurred in those who were fully vaccinated against the virus. 

Since 2007, about one-third of mumps cases reported in the United States were in children and teenagers, according to the report published last week by the Centers for Disease Control and Prevention. 

Of those approximately 9,000 cases, up to 94% of patients were vaccinated against the mumps, which can cause fever, headache, painful swollen glands and sometimes hearing loss in children. Although this may alarm some parents, health experts say they aren’t surprised.

“People take from that headline that the vaccine doesn’t work,” said Dr. Paul Offit, an infectious diseases expert at The Children’s Hospital of Philadelphia. But the vaccine has "virtually eliminated what was at one time the most common cause of deafness” in children.

Mumps is one of three illnesses targeted by the MMR vaccine, which also protects against measles and rubella. The vaccine contains a weakened live virus that causes a harmless infection and helps the body arm an immune response. 

Health experts say the study's findings are not a cause for concern. Mumps is endemic in the U.S., like the seasonal flu, and most children are vaccinated. Still, no vaccine is 100% effective, and there's evidence of waning immunity.  

The study's findings raises awareness among pediatricians so they can identify mumps symptoms among vaccinated children and not mistake them for another virus, said senior author Dr. Mariel Marlow, epidemiologist and mumps program lead at the division of viral diseases at the Centers for Disease Control and Prevention. 

She emphasizes the MMR vaccine has been key in reducing mumps cases in the U.S. by 99% since 1967. 

"Before the mumps vaccine was introduced, mumps was a common childhood illness," Marlow said. "We had over 100,000 cases reported each year and that’s just cases reported to the CDC, we don’t know how may weren’t reported to us." 

Like COVID-19, mumps is a disease caused by a virus that spreads through respiratory droplets. Symptoms may include fever, headache, muscle aches, tiredness, loss of appetite, and swollen or tender glands under the ears or jaw, according to the CDC. 

An infection can occasionally cause complications such as swelling in the brain, testicles, ovaries or breasts, and can cause temporary or permanent deafness. But experts say the vaccine has rendered complications rare and most people fully recover.

“This vaccine has closed homes for the deaf because it’s that effective,” Offit said.

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Unlike measles and rubella, mumps is endemic in the United States, said Dr. Camille Sabella, a pediatric infectious diseases specialist at Cleveland Clinic Children’s. While measles and rubella infections usually come from overseas, the mumps virus is constantly circulating. 

The CDC recommends children should get two doses of the MMR vaccine, starting with the first dose at 12 to 15 months of age and the second dose at 4 through 6 years of age. Children can get their second dose earlier as long as it is at least 28 days after the first dose.

As of August, the agency reported 90.8% of people received their MMR vaccine by 24 months. Health experts aren't surprised children infected with mumps are more likely to be vaccinated because vaccination rates are so high.

“As more and more people are vaccinated, given that the vaccine isn’t 100% effective, more and more of those who are hospitalized will be vaccinated,” Offit said.

Two doses of the MMR vaccine is 97% effective against measles and rubella, but it’s 88% effective against mumps, the CDC reports. Health experts say immunity wanes over time and some people may not be protected against mumps later in life.

“So if 90% of the population is vaccinated and the vaccine is about 88% effective but every year that goes by you become less protected and the virus is already here, then it makes sense that we have pretty significant outbreaks and that there are a large number of cases of mumps every year,” Sabella said.

More than half of pediatric mumps cases occurred among children and adolescents age 11 to 17, according to the CDC report, which health experts say demonstrate how waning immunity plays a role in breakthrough infections.

Mumps outbreaks are rare, but most occur on college campuses, Sabella said. When this happens, public health authorities recommend an additional dose of the MMR vaccine to people at risk of exposure.

“It’s really easy to think that vaccines are going to be very effective and all vaccines are created equally but in reality that’s not the case,” he said. “Every one of these viruses is a little bit different and every vaccine we have against these viruses is going to be different.”

Experts say Americans shouldn't be concerned by the CDC study as the mumps vaccine is effective at preventing severe complications from the disease. However, pediatricians shouldn't dismiss the possibility of a mumps diagnosis when patients exhibit symptoms, even if they're fully vaccinated.

"Unvaccinated people are still at higher risk for mumps and still at higher risk for mumps complications," Marlow said. "High vaccination coverage maintains control of mumps in the U.S. so we don’t return to those days of 100,000 cases reported every year." 

Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT. 

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