Despite routine use of a childhood vaccine, the United States still sees outbreaks of mumps. Now, a new study reinforces the belief that it's due to waning immunity post-vaccination.
Mumps is a viral infection best known for causing puffy cheeks, a swollen jaw, fever and general misery. While it's usually relatively mild, mumps occasionally causes serious complications like brain inflammation and hearing loss.
Because of that, children in the United States are routinely immunized against mumps, using the measles-mumps-rubella (MMR) vaccine.
Yet mumps has seen a resurgence in the past two decades. Since 2006, there has been an uptick in annual cases -- often among college-age adults who were vaccinated as children, according to the U.S. Centers for Disease Control and Prevention.
Similar patterns have been seen in other countries where vaccination rates are high, leading to two theories on why: waning immunity to mumps in the years after vaccination; or the emergence of new mumps strains that evade the vaccine-induced immune response.
The new study — published Jan. 9 in the Proceedings of the National Academy of Sciences — supports the waning-immunity view.
Using mathematical modeling, researchers found that waning immunity could largely explain the resurgence of mumps in the United States in recent years. They also estimated that about one-third of vaccinated kids start to lose their mumps immunity by age 18.
But experts who were not involved in the study stressed there is no reason to doubt the strong protection provided by the MMR vaccine.
For one, while the United States does record mumps outbreaks in certain settings, like universities, the numbers pale in comparison to the pre-vaccination era.
Yearly case numbers have varied since 2006, according to the CDC -- ranging from a few hundred to around 6,000. That compares with more than 100,000 mumps cases per year in the pre-vaccine days.
"These outbreaks are still distinctive and rather unusual events," said Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases.
Beyond that, he said, "waning immunity" means there's a partial -- not total -- loss in the immune response.
People who received the recommended two doses of the MMR would still have good protection from becoming severely ill, according to Schaffner, who is also a professor at Vanderbilt University Medical Center in Nashville, Tenn.
Dr. Aaron Glatt, a spokesman for the Infectious Diseases Society of America, agreed.
"The vaccine isn't perfect, but it's great," he said.
The MMR is so highly effective, Glatt noted, that measles was declared eliminated in the United States in 2000 -- meaning the disease is no longer constantly present. (Outbreaks do continue to happen when the virus is "imported" from a traveler who interacts with unvaccinated people.)
So, the bottom line for parents remains the same as always, Glatt said: Make sure your child receives both MMR vaccine doses.
It has long been known, both doctors noted, that the mumps component of the MMR is the relative weak link. The two-dose regimen given in childhood slashes the risk of contracting measles or rubella by about 97%, versus being unvaccinated, according to the CDC.
By comparison, it cuts mumps risk by a still substantial 88%.
It's also widely accepted, Schaffner said, that mumps immunity gradually fades, while immunity against measles and rubella persists.
He said the debate over whether mumps outbreaks are due to waning immunity or new viral strains is largely one between U.S. and European scientists -- with those in the United States favoring the waning-immunity side.
The new study, led by Deven Gokhale at the University of Georgia in Athens, adds more evidence for that.
The researchers used CDC figures on decades' worth of mumps cases in the United States, along with other data sources, to construct mathematical models testing the two mumps theories. Overall, they found, waning immunity appeared to explain the resurgence of the disease in more recent years.
Does that mean adults should get MMR boosters? No, both Glatt and Schaffner said.
In fact, Schaffner said, the CDC has already examined that question and concluded that boosters should be used only strategically. If there's an outbreak on a college campus, for example, the local health department can offer boosters there.
In general, Schaffner noted, mumps does take close contact to spread. It's transmitted through direct contact with saliva or respiratory droplets, and is often passed by sharing items like water bottles or cups, or during activities like sports.
The U.S. Centers for Disease Control and Prevention has a primer on mumps.
SOURCES: Aaron Glatt, MD, chief, infectious diseases and hospital epidemiologist, Mount Sinai South Nassau, Oceanside, N.Y., and professor, medicine, Icahn School of Medicine at Mount Sinai, New York City; William Schaffner, MD, medical director, National Foundation for Infectious Diseases, Bethesda, Md., and professor, medicine and preventive medicine, Vanderbilt University Medical Center, Nashville, Tenn.; Proceedings of the National Academy of Sciences, Jan. 9, 2023, online