Mumps cases are still being reported in the United States, primarily among vaccinated people, including children.

Mumps cases fell by more than 99 percent in the United States after a vaccine against the highly contagious respiratory infection was developed in 1967. Cases fell to 231 in 2003, from more than 152,000 in 1968. However, cases began to rise again in 2006, with 6,584 reported, the majority of which were in vaccinated people.

According to the Centers for Disease Control and Prevention, one-third of mumps cases in the United States between 2007 and 2019 were reported in children and adolescents. As many as 94 percent of those who became ill had been immunized.

“Previously, large outbreaks of mumps among fully vaccinated people, including vaccinated children, were not common,” said Mariel Marlow, an epidemiologist at the CDC who led the new study. “However, in vaccinated people, disease symptoms are usually milder, and complications are less common.”

Experts aren’t sure why vaccinated people get mumps, but a number of factors appear to be affecting immunity in vaccinated people, including a lack of prior exposure to the virus, waning immunity, and the spread of genotypes that the vaccine doesn’t contain.

The mumps virus is spread through direct contact with saliva or respiratory droplets from an infected person’s mouth, nose, or throat. A person who is infected can spread the virus by coughing, sneezing, talking, or sharing drinks, or by engaging in close-contact activities such as sports. Almost 91 percent of the U.S. population has received at least one dose of the two-dose measles, mumps, and rubella, or MMR, vaccine, which is given between the ages of 12 months and 6 years and is 88 percent effective against the disease.

Cases in recent years have been primarily driven by large localized outbreaks, though a peak in 2016 and 2017 included more than 150 outbreaks reported in 37 states and Washington, D.C., totaling approximately 9,000 cases. Mumps cases fell last year compared to the previous six years, but the illness continued to spread in the United States despite precautions such as isolation, lockdowns, and masking. 32 health departments reported 142 mumps cases between April 1, 2020, and the end of the year.

The numbers are still low, but they aren’t reason to believe vaccines are no longer effective, according to Joseph Lewnard, an assistant professor of epidemiology at the University of California, Berkeley’s School of Public Health.

Mumps vaccination antibodies in some people decline over time, reducing protection. According to Lewnard, older adolescents are more vulnerable during outbreaks among young people because they are more likely than younger children to have weakened immunity as a result of waning vaccine protection.

According to Marlow, because most people aren’t routinely exposed to mumps, there is also less immunologic boosting — when people are exposed to mumps, their immunity is boosted but they don’t get sick. Because mumps has continued to circulate globally during the pandemic, she expects cases and outbreaks of mumps to continue across the country, which could be exacerbated by a larger unvaccinated population.

According to Dr. Amesh Adalja, a senior scholar at the Johns Hopkins Bloomberg School of Public Health’s Center for Health Security, U.S. mumps vaccines contain genotype A strain, which no longer circulates in the country. However, this does not appear to reduce the effectiveness of the vaccines.

“One of the mysteries of understanding this is that when the genotype A vaccine is given during an outbreak, it still works,” he said. “We’ve seen that a third dose of MMR is enough to stop outbreaks on college campuses.”

Adalja believes that combating new outbreaks could be as simple as increasing the MMR vaccine dose schedule from two to three doses. It’s nothing new to change the schedule: In 1977, the CDC’s Advisory Committee on Immunization Practices recommended a single dose of the mumps vaccine for routine use, which was later increased to two doses in 1989.

During large outbreaks, the panel suggested in 2017 that people at high risk of contracting mumps receive a third dose of the MMR vaccine.