Mumps cases continue to circulate in the U.S., largely among vaccinated people, including children.
Cases of mumps, once a common childhood illness, declined by more than 99 percent in the U.S. after a vaccine against the highly contagious respiratory infection was developed in 1967. Cases dropped to just 231 in 2003, down from more than 152,000 in 1968. But cases began climbing again in 2006, when 6,584 were reported, most of them in vaccinated people.
According to the report from the Centers for Disease Control and Prevention, one-third of mumps cases in the U.S. from 2007 to 2019 were reported in children and adolescents. As many as 94 percent of those who contracted the illness had been vaccinated.
“Before that, large outbreaks of mumps among people who were fully vaccinated were not common, including among vaccinated children,” said Mariel Marlow, an epidemiologist at the CDC who led the new study. “But the disease symptoms are usually milder and complications are less frequent in vaccinated people.”
Experts aren’t sure why vaccinated people get mumps, but multiple factors appear to be affecting immunity in vaccinated people, including a lack of prior exposure to the virus, waning immunity and the circulation of genotypes the vaccine doesn’t contain.
The mumps virus is spread through direct contact with saliva or respiratory droplets from the mouth, the nose or the throat of an infected person. An infected person can spread the virus by coughing, sneezing, talking or sharing drinks or during close-contact activities, such as sports. Nearly 91 percent of the U.S. population has had at least one dose of the two-dose measles, mumps and rubella, or MMR, vaccine, which is administered between 12 months and 6 years and is 88 percent effective against the disease.
Cases in recent years have largely been driven by big localized outbreaks, although a peak in 2016 and 2017 included more than 150 outbreaks reported in 37 states and Washington, D.C., amounting to about 9,000 cases. Mumps cases decreased last year compared to the previous six years, but the illness continued to circulate in the U.S. despite distancing, lockdowns and masking. From April 1, 2020, to the end of the year, 32 health departments reported 142 mumps cases.