MUMPS MANIFESTATIONS The continuum of mumps illness ranges from asymptomatic infection to parotitis (the most well-known manifestation) to rare but severe complications.8 Table 2 lists potential complications of mumps in order of frequency. Complications vary by age and sex but tend to occur more often in adults.4,11
Immunization modifies the clinical presentation of mumps11 and likely decreases complications. Although one analysis of the 2006 outbreak identified no difference in complication rates between vaccinated and unvaccinated patients, the authors attributed this to misclassification of patients’ vaccination status; they did find lower reported rates of mumps complications compared with complication rates before widespread vaccination.13 A study of the 2009-2010 outbreak found that complication rates were lower among vaccinated patients.18
RISK FACTORS FOR OUTBREAKS Of the three components of the MMR vaccine, the least effective is the mumps portion. One dose confers 78% immunity and two doses, 88%, which the CDC characterizes as incomplete protection.1 Compare this to the measles vaccine, which is 97% effective with two doses,19 and the rubella vaccine, which is 97% effective after a single dose in conferring immunity.11
In 2000, as a result of high rates of vaccination, the US determined that endemic measles had been eliminated. A similar conclusion was reached about rubella in 2004, and both determinations were reaffirmed in 2011.20 In contrast, mumps has never been eliminated.1
Waning immunity While antibodies to mumps as a result of vaccination persist into adulthood, they decline over time. A 2009 CDC study found that, 12 years after a second MMR dose, mumps antibody levels in adolescents and young adults had declined to levels similar to those measured before the second dose.21 Other analyses of major outbreaks suggested the need for further studies to determine optimal timing for the second MMR dose (eg, at a later age) or if a third dose would provide longer-lasting immunity.13,22 Waning immunity among young adults, coupled with high-density living environments that intensify exposure to the virus, increase risk for the disease.23
Further complicating the situation is the lack of evidence about the required level of antibodies needed to confer protection against mumps infection.17 To date, the antibody titer threshold of mumps-specific IgG at which an individual is protected from the disease is unknown.11,17
Imported risks Mumps remains endemic in many parts of the world, with vaccination employed in only 61% of countries belonging to the World Health Organization.4 Several recent outbreaks were traced to index cases originating outside the US.13 It is likely that importation of the virus from abroad will continue.
