Mumps Virus Infections Are on the Rise Across the U.S.

A vaccine record "yellow card" tracks immunizations. Frankie Leon / Flickr Creative Commons

Late last summer, a family in northwest Arkansas was stricken with mumps, possibly brought by a relative visiting from Iowa. The virus spread quickly among public school students in Springdale, the fourth-largest city in Arkansas. Immigrants from the Marshall Islands, a prominent community in the region, were particularly hard-hit initially, most likely because they tend to live in overcrowded homes due to poverty. But the virus, which is extremely contagious, was not confined to any particular population. At the height of the outbreak, which was near the end of 2016, about 50 new cases were reported to the Arkansas Department of Health each day.

Across the U.S., mumps has been making a surprising comeback. Outbreaks in Washington state, upstate New York, Oklahoma, Indiana and elsewhere in 2016 pushed the virus to its severest spread since 2006. The infections are not confined to unvaccinated children, so parents who chose not to immunize their children cannot be blamed entirely. Rather, the door was likely opened by the frustrating imperfection of the vaccine. But rising vaccine abstention may allow the outbreaks to persist.

The mumps virus, a member of a family called Rubulavirus, is made of a single strand of RNA inside a protein bag. The virus targets the central nervous system, which is why it can sometimes lead to encephalitis or meningitis. More commonly, a case of mumps leads to painfully swollen salivary glands, fever, muscle pain, headaches and tiredness. In some men, mumps swells testicles, occasionally resulting in infertility. Some children emerge from the illness permanently deaf. No medical treatment for mumps exists, so although most cases resolve on their own, the risk of damaging side effects warrants preventive measures.

A vaccine for mumps has been available since 1967. Created by Maurice Hilleman, a microbiologist who developed more than 40 vaccines, the inoculation protects against two strains of the virus originally cultured from the throat of Hilleman's daughter, Jeryl Lynn (the vaccine strains are known as Jeryl Lynn 1 and 2).

The Centers for Disease Control and Prevention recommends two doses—one at age 12-15 months and one at 4-6 years old—of the "MMR" vaccine, a shot that protects against measles, mumps and rubella. Completing the full vaccine schedule prevents an average of 88 percent of cases. In other words, of every 100 people vaccinated against mumps, an average of 12 people are still susceptible to the virus. By contrast, the measles vaccine is 97 percent effective.

Largely because of the vaccine's incomplete protection, mumps continues to infect. "Outbreaks can still occur in highly vaccinated U.S. communities," says Ian Branam, spokesperson for the CDC, "particularly in close-contact settings." College campuses are popular breeding grounds for the virus because of the high number of students thrust together through dormitory living. That it spreads through saliva—kissing, sharing cups and cigarettes—makes campuses particularly conducive habitats for the virus.

Still, the number of cases in the U.S. in 2016, reported by almost all states, was unusual, says Paul Throne, a public health physician with Washington State Department of Health. Since the total number of mumps cases spiked above 6,000 in 2006, the incidence had remained below 3,000 per year. In 2016, states reported more than 5,000 cases total. Eight states reported more than 100 infections. (The CDC has a map of U.S. cases available online here.)

According to the CDC, 2017 cases have already exceeded the expected amount, with infections reported in every U.S. region. The agency could not comment to Newsweek before press time about why the number of mumps cases rose in 2016. Throne says the outbreaks are likely due to the incomplete protection conferred by the vaccine combined with fading immunity over time, a known phenomenon of many inoculations. The virus could also have mutated away from the two Jeryl Lynn strains included in the shot given to American children.

Throne and Dirk Haselow, Arkansas state epidemiologist, both expressed concern about unvaccinated children. Arkansas allows children whose families object to immunizations on philosophical grounds to attend public schools (some states forbid this practice). The state's health department barred unvaccinated children from school during the 2016 outbreak as a preventive measure. Although hundreds of previously non-vaccinating children were immunized after the Arkansas outbreak hit, about 50 were not, and Haselow is concerned about the ramifications of the prolonged school absence on the children and their families.

Throne worries that the increase in cases indicates rising resistance to the CDC's recommended immunization schedule. He explains that halting the spread of mumps requires that 90 percent of the population receive a complete MMR vaccine. "The outbreaks are a sign that we are not at that level of protection," says Throne. Indeed, MMR vaccine rates in the U.S. appear to be falling.

The most worrisome side effects were rare among the 2016 mumps outbreaks.

Arkansas has reported no cases of meningitis and only five hospitalizations among the more than 2,700 cases. "There is no doubt at all that we are seeing a milder course of illness," says Haselow. The illnesses in the Washington state, where the virus spread mainly through public schools in several counties, were also mild.

Throne wonders if the phenomenon may be akin to that seen with the flu vaccine, which may diminish the sickness even if it doesn't prevent an infection. "It may be a sign that the vaccine is protecting people from the worst of the disease," says Throne.

Although the Arkansas outbreak appears to be waning, the state is still seeing about five cases per day and authorities remain vigilant. "We can't afford to relax until it's truly over," says Haselow. In Washington, the outbreak continues, with nearly 400 cases reported on February 7. "We do not see any sign of it slowing down," says Throne.