A mumps outbreak that began in the country last fall is continuing across the United States. New research suggests that even small populations of unvaccinated individuals may be fueling the outbreak.
The United States is in the midst of its largest mumps outbreak in a decade, sparking new research into how vaccination rates impact “herd immunity” for this contagious disease.
A mumps outbreak in 2016 saw 5311 cases reported nationwide. In 2017, the current national case count is 1077. The last time the country experienced an outbreak this large was in 2006, when more than 6500 cases of the mumps were reported.
Mumps is a highly contagious virus that has become less common in the United States since the introduction of the measles, mumps, and rubella (MMR) vaccine more than 50 years ago. Over the past decade, the disease—marked by swelling in the cheeks and jaws along with fever, muscle aches, and tiredness—has affected as few as 229 people in a year, and so the current outbreak has health officials across the country on alert.
The mumps virus is most likely to take hold in communities with low MMR vaccination rates, and in settings where people have prolonged close contact, such as college dormitories. With the recommended two doses of the MMR vaccine, the Centers for Disease Control and Prevention (CDC) states that the mumps component is about 88% effective at preventing infection. A new study by Boston Children’s Hospital researchers published in the journal The Lancet recently found that substandard vaccination coverage may be the cause of the current outbreak in Arkansas, the hardest-hit state in the 2016-2017 mumps outbreak.
For the highly-contagious virus, the researchers note that a 96% vaccination rate with both doses of the MMR vaccine may be needed to maintain herd immunity. In Arkansas, which has seen 2898 cases of the mumps in the current outbreak, 90% to 95% of school-aged children and 30% to 40% of adults affected by the virus have been fully immunized. The authors of the new Lancet study found that self-reported vaccination rates may be higher than actual vaccination rates, and that as few as 70% of people in Arkansas’ affected communities may have received both doses of the vaccine.
Through a project called HealthMap, the research team collects data on contagious disease outbreaks around the country. One of the outbreaks the team has tracked is in the Chicago suburb of Barrington, where area health officials have confirmed at least 11 mumps cases and additional suspected cases since reporting the first case nearly three weeks ago. The outbreaks have occurred among students at area middle and high schools, and county health officials are offering a free vaccination clinic for any student who has not received both doses of the MMR vaccine.
In Washington, the majority of the 664 mumps cases that have occurred in the state since the current outbreak began in October of 2016 have occurred in Spokane and King counties. The outbreak seems to be affecting younger children, and area health officials and parents worry that too many unvaccinated children may lead to more cases.
“We’re asking people, especially in these two counties, to make sure everyone in their family has been fully vaccinated,” said the state’s health officer Kathy Lofy, MD, in an earlier statement. “While most people who get mumps will have a mild illness which goes away within a week or so, some people may experience serious health complications.”