In 2016, the US CDC identified the highest number of mumps cases in the US since 2006 - approximately 5,310 cases in 46 states. The state most affected by disease activity during 2016 was Arkansas, accounting for nearly 40 percent of the total number of cases reported countrywide.
Despite the recent mumps outbreak in the US, it is important to note that the number of cases in the US has declined by more than 99 percent since the start of the mumps vaccination program in 1967. Prior to the start of this program, health officials in the US reported roughly 186,000 mumps cases per year.
Case counts in the US now range from a couple hundred to a few thousand per year. However, due to the increasing number of mumps outbreaks in highly vaccinated communities, federal health officials announced in February that they would be looking into whether a third dose of the measles, mumps, and rubella (MMR) vaccine should be recommended instead of two doses.
What is Mumps?
Mumps is a highly contagious viral disease spread through airborne droplets created when an infected individual coughs, sneezes, or speaks. Infected individuals can also expose people around them by contaminating surfaces with secretions from the nose, mouth, and throat.
Symptoms of mumps typically start with fever, headache, muscle ache, tiredness, and loss of appetite. Symptoms then progress to characteristic swelling and tenderness of the salivary glands. These symptoms generally develop 16-18 days after exposure but can occur as late as 25 days after exposure.
Most people recover from mumps in 7-10 days. However, mumps can lead to serious medical complications, including permanent deafness and, in rare cases, infertility. Fortunately, people who are vaccinated against mumps are less likely to develop serious complications than unvaccinated people.
2016-2017 Mumps Outbreak in Arkansas
As of March 15, nearly 2,900 suspected and confirmed cases of mumps have been reported in Arkansas since the start of the outbreak in August 2016. The outbreak began in a community of Marshall Islanders in the northwestern part of the state. Members of this community typically live in multi-family, multi-generational households, often with 15-20 individuals living in a single home. According to media reports, Marshall Islanders have accounted for at least 2,200 of the cases.
According to the Arkansas Department of Health, 90-95 percent of school-aged children and 30-40 percent of adults involved in the outbreak have been fully immunized against mumps. However, it is important to note that studies have shown that self-reported vaccination rates are usually greater than actual vaccination rates. Therefore, it is highly likely that a lower percent of individuals in Arkansas involved in the outbreak had been fully immunized against the disease. However, due to the surge in activity among Marshall Islanders, health officials are also investigating whether the MMR vaccine is less effective in certain populations.
How Effective is the Mumps Component of the MMR Vaccine?
The US CDC currently recommends that children receive only two doses the MMR vaccine – the first dose at 12-15 months of age and the second dose 4-6 years of age. Two doses of the MMR vaccine are about 88 percent (range: 66-95 percent) effective in preventing mumps, and one dose is about 78 percent (range: 49-92 percent) effective. Mumps outbreaks in highly vaccinated communities mostly occur in close-contact settings; such as schools and colleges. According to media reports, at least 19 outbreaks were reported on college campuses in 2016.
Due to the increasing number of mumps outbreaks in highly vaccinated communities, the Advisory Committee on Immunization Practices (ACIP) – a group of experts that provides advice regarding vaccine recommendations for children and adults in the US – announced in February that they will be looking into whether a third dose of the MMR vaccine should be recommended. The outcome of this deliberation is not expected until February 2018.
Currently, only two doses of the MMR vaccine are recommended to prevent mumps. According to the US CDC, individuals who receive two doses of the vaccine are about nine times less likely to get mumps than someone who is not vaccinated against the disease. However, individuals who are vaccinated can still become infected, especially if they have prolonged, close contact with an infected person. In recent years, the number of mumps outbreaks in the US in highly vaccinated communities has increased, and health officials are considering whether a third dose of the vaccine should be recommended.