Mumps

Definition

Mumps is an acute viral infection characterized by inflammation of the parotid and other salivary glands. Prior to the introduction of vaccination in the United States, mumps was a common childhood illness. The incidence of mumps declined dramatically after the introduction of the vaccine, but mumps remains endemic. Cases and even outbreaks can occur in previously vaccinated children and adults.

Epidemiology

Humans are the only natural host of mumps virus. Mumps is highly contagious and spreads by close person-to-person contact via respiratory droplets. Transplacental transmission has rarely been reported. The incubation period between exposure to the onset of illness ranges from 12 to 25 days. Infected individuals are most likely to be contagious up to 2 days before through 5 days after the onset of symptoms, but individuals who have asymptomatic infections can also transmit the disease.

The introduction of mumps vaccine into the routine childhood immunization schedule for children ages 12 months and older in 1977 led to a dramatic decline in mumps disease in the United States, from over 150,000 cases of mumps in 1968 to about 3000 cases in 1985. In 1989, a second dose of measles-mumps-rubella (MMR) vaccine for children ages 4 to 6 years was added to the routine childhood immunization schedule and led to a further reduction, with an average of fewer than 300 reported cases annually through the early 2000s. ^,

However, mumps remains endemic in the United States, and Canada and multiple large outbreaks of mumps continue to occur, primarily involving fully vaccinated adolescents and young adults in densely populated settings. For example, a multistate mumps outbreak in 2006 involved more than 6500 people across multiple midwestern college campuses. During 2009 to 2010, large outbreaks occurred in Guam and in New York City, where more than 3500 individuals were infected in a close-knit religious community. From 2015 to 2019, multiple large outbreaks involved schools, universities, athletic teams, and religious groups. In a large outbreak in a close-knit community in Arkansas from 2016 to 2017, approximately 3000 individuals became infected. Approximately 900 mumps cases were identified across 57 detention facilities in 19 states between 2018 and 2019.

Mumps cases and outbreaks among fully vaccinated persons are likely due to a combination of factors. One set of factors relates to the limits of vaccination. First is the potential for a mismatch between the MMR vaccine and the mumps strains in circulation. For example, sera from vaccinated persons against genotype A strain used in vaccines in the United States fully neutralizes the genotype G strain, which is the main genotype that has circulated in the United States since 2006, but geometric mean antibody titers are lower relative to titers against the vaccine strain. In addition, the mumps component of the MMR vaccine is less effective than the measles and rubella components, with an average initial seroconversion rate of 91% and a median two-dose effectiveness of 88%. Furthermore, immunity following mumps-containing vaccine wanes over time. ^, A second set of issues relates to infectivity. Persons may be contagious prior to the onset of symptoms or may be asymptomatic throughout their infections. Challenges with diagnostic testing, especially in vaccinated persons, may result in delayed or missed diagnoses, thereby allowing infected individuals to spread the disease. Finally, settings with intense exposures (e.g., colleges and universities, close-knit communities with large congregate gatherings) are conducive to outbreaks.