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January 12, 2023

Mumps outbreaks have become more common in recent decades. Scientists are trying to determine why

The trend has persisted despite high vaccination rates in the U.S. Researchers have two main theories – waning immunity and new viral strains

A mumps outbreak spread through Temple University four years ago, infecting 186 people and prompting students, faculty and staffers to line up for booster doses. 

The outbreak underscored the increased number of mumps cases seen in the United States in recent decades. Though mumps cases decreased by more than 99% in the decades after the mumps vaccine became available, they have increased since 2006 – despite high vaccination rates. Large outbreaks particularly have popped up in settings where people have frequent close contact, including colleges.

The same trend has played out in other countries with high vaccination rates. What's behind it? Scientists have two theories – waning immunity in the years after vaccination and the emergence of new virus strains that the immune system doesn't recognize.   

In the U.S., children are immunized against mumps using the measles-mumps-rubella vaccine, which is administered in two doses during early childhood. But the strength of protection, and the length of immunity, differs among the three diseases targeted by the MMR vaccine. It cuts the risk of infection for measles and rubella by about 97%, but only by 88% for mumps, according to the U.S. Centers for Disease Control and Prevention. That more robust protection against measles and rubella also does not tend to wane. 

A new study, published Monday, found that waning immunity offers the best explanation for the recurring mumps outbreaks. The researchers used a mathematical model to test the two theories, using data from the CDC and other sources. 

Still, research remains conflicting. In 2018, Harvard University researchers also determined that waning immunity was likely the cause of the recent outbreaks, noting that vaccine protection persists for an average of 27 years after a person's last dose. But a 2019 study by Emory University, found both theories may be plausible. 

In that study, the majority of participants who had been vaccinated as children had antibodies against mumps, but about 10% of them didn't have any detectable mumps-specific memory B cells. These cells help trigger the immune system's response to the virus. In addition, the antibodies produced were not as effective at neutralizing wild strains of mumps.

Are mumps boosters necessary?

In 2017, the CDC's Advisory Committee on Immunization Practices approved a third dose of the MMR vaccine to be used during mumps outbreaks, but boosters are not recommended in general.

Some research has shown that giving people a third MMR vaccine dose at age 18 – and then booster shots every 10 years afterward – could increase protection against mumps in people with low antibody levels. According to one study however, the added boost only lasts for about a year.

And though the immunity provided by the vaccine wanes, it still provides protection against severe mumps, vaccine experts told U.S. News & World Report. Waning immunity means a partial loss of protection, they stressed. Plus, mumps cases still remain rare, despite their increasing numbers. 

Mumps cases dropped from 152,209 in 1968 – the year after the mumps vaccination program began in the U.S. – to 231 cases in 2003, according to the CDC. Since then, they have ranged from a few hundred to about 6,000 each year. In 2022, there were 322 cases in the U.S., including 12 in Pennsylvania. New Jersey didn't report any. 

"These outbreaks are still distinctive and rather unusual events," Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases, told U.S. News & World Report. 

Immunologists say it is still unclear why some vaccines offer protection for life, yet others only provide protection for just a few weeks. Vaccine-derived immunity against pertussis, meningococcal disease and yellow fever also tends to fade. Understanding vaccine durability is vital if scientists want to make all vaccines better, they emphasize.

Mumps, which is known for causing puffy cheeks, is caused by paramyxovirus. Symptoms generally appear 2 to 3 weeks after exposure and initially resemble flu-like symptoms. The most common symptoms are fever, headache, muscle aches or pain, lack of appetite, fatigue and swelling of the salivary glands. Though the infection is usually mild, serious complications like brain inflammation and hearing loss can occur.

The mumps virus is transmitted through direct contact with saliva or respiratory droplets. People can spread it by coughing, sneezing and talking, as well as sharing water bottles and cups, or through close-contact activities, like sports, dancing and kissing. 

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