Mumps, which many parents had assumed was a disease of the past, is back.
Last year, a mumps epidemic swept across the United States—5,642 people were infected, most between 18 and 25 years of age. Nineteen separate outbreaks occurred on college campuses. Ten years earlier, in 2006, another mumps epidemic swept across the Midwest, affecting more than 6,000 people; again mostly older teenagers and young adults sickened. More recently, a surge in cases has been reported in the past two months in Virginia, Oklahoma, Illinois, and Washington state. These outbreaks haven’t centered on people who never received the mumps vaccine. Rather, they’ve been caused by something else.
Let’s start at the beginning.
In 1963, Jeryl Lynn Hilleman woke up her father in the middle of the night. “My neck hurts,” she said. Her father, Maurice Hilleman, a researcher at Merck, Sharpe & Dohme, noticed that the salivary glands located at the angle of her jaws (called parotid glands) were swollen, causing her to look like a chipmunk. “You’ve got the mumps,” he told her. Then Hilleman did something that no father does. He put his daughter back to bed, drove to his research laboratory, picked up some cotton swabs and culture broth, drove back home, gently woke up his daughter, and swabbed the inside of her mouth.
During the next four years, Hilleman weakened Jeryl Lynn’s mumps virus in the laboratory by adapting it to grow in chick embryo cells. As the virus got better and better at growing in the chick cells, it got worse and worse at growing in human cells. This highly modified (attenuated) mumps virus could now induce an immune response in people without causing disease. Called the “Jeryl Lynn” strain, Hilleman’s mumps vaccine was licensed by the Food and Drug Administration in 1967 and later recommended for all children at 12 months of age. In 1971, Maurice Hilleman’s mumps vaccine was combined with two others he had invented—which prevented measles and rubella—into a single shot.
In 1989, following a massive measles epidemic that swept across the United States, the Centers for Disease Control and Prevention recommended a second dose of measles-mumps-rubella (MMR) vaccine for all children to be given between 4 and 6 years of age. Now, children in the United States received two doses of MMR. As a consequence, the number of cases of mumps declined from 200,000 per year to about 200—a 99.9 percent decrease. As one journalist noted, “Jeryl recovered from mumps virus, but mumps virus never recovered from infecting Jeryl.”
Despite the documented effectiveness of this vaccine, most people who came down with mumps last year had received two doses of MMR. Some have speculated that the strain of mumps currently circulating in the United States (called the G strain) has drifted far enough away from the Jeryl Lynn strain that immunization no longer protects against disease. But that’s not it. Serum taken from people immunized with MMR still neutralizes the circulating mumps strain. The problem with the mumps vaccine is that immunity fades. As a consequence, 10 years after the second dose of MMR, a significant number of older teenagers and young adults are susceptible to mumps.
The fading immunity found following the mumps vaccine hasn’t been a problem with the measles or rubella vaccines. As a consequence, we eliminated measles from the United States in 2000 and rubella in 2005. On the other hand, we’ve never completely eliminated mumps. (Unlike the mumps outbreak last year, measles outbreaks in 2014 and 2015 occurred in children whose parents had chosen not to vaccinate them. The mumps epidemic occurred primarily in people who had been vaccinated.)
One could reasonably ask, so what? Mumps virus causes swelling of the parotid glands that, while painful, is usually transient and harmless. Indeed, my first understanding of mumps (and frankly of all infectious diseases) came from a song written by The Coasters in 1959. It was titled “Poison Ivy”:
Measles make you bumpyAnd mumps'll make you lumpyAnd chicken pox'll make you jump and twitchA common cold'll fool yaAnd whooping cough'll cool yaBut poison ivy, Lord'll, make you itch
The problem with mumps, however, is that it doesn’t just make you “lumpy.” Mumps virus can also travel to the lining of the brain and spinal cord and cause meningitis, or to the brain itself to cause encephalitis. Indeed, in its heyday, mumps was the most common cause of acquired deafness in the United States. The virus can also infect the testes (orchitis) or ovaries (oophoritis), causing sterility, or the pancreas, causing pancreatitis. For these reasons, mumps was a feared disease. One interesting observation about the current outbreak is that these serious consequences of mumps infections haven’t been observed. And they should have been. As many as 30 percent of males infected with mumps will develop orchitis. And as much as 15 percent of boys and girls infected with mumps will develop meningitis. So while previous vaccination with two doses of mumps-containing vaccine doesn’t protect everyone from the disease, it does appear to protect against the severe consequences of disease.
During the next year, it is likely that the CDC will determine whether to give a third dose of MMR vaccine during adolescence. This may be the only way to stop these sporadic outbreaks.
Paul A. Offit is a professor of pediatrics and director of the Vaccine Education Center at the Children’s Hospital of Philadelphia and the author of Pandora’s Lab: Seven Stories of Science Gone Wrong (National Geographic Press, April 2017)