Can a Child Still Get Mumps Despite Being Immunized?
Yes, a child can still develop mumps even after receiving the routine MMR vaccine, though the risk is substantially reduced—approximately 88% of vaccinated children are protected, meaning about 12% remain susceptible to infection. 1
Primary Vaccine Failure Explains Breakthrough Infections
- Approximately 5% of children fail to develop immunity to one or more components after a single dose of MMR vaccine, a phenomenon called primary vaccine failure. 2
- The two-dose MMR schedule (first dose at 12-15 months, second at 4-6 years) was specifically designed to address this gap—almost all children who don't respond to the first dose will develop immunity after the second dose. 2
- Even with two doses, vaccine effectiveness against mumps is approximately 88%, which is lower than the protection conferred against measles or rubella. 1
Waning Immunity Contributes to Mumps Cases
- Anti-mumps antibody levels remain relatively stable over 10 years following vaccination, but breakthrough infections still occur in highly vaccinated populations. 2, 3
- Mumps outbreaks have been documented in settings where substantial numbers of cases occurred among persons who previously received a single dose of mumps-containing vaccine. 2
- Research shows that children who received only one dose of MMR have significantly lower seroprevalence (78.4%) compared to those with two doses (96.5%), and protection is negatively correlated with time since vaccination. 4
Clinical Presentation May Differ from Classic Mumps
- Classic parotitis (salivary gland swelling) appears in only 30-40% of mumps infections; 15-20% are completely asymptomatic, and up to 50% present with nonspecific or respiratory symptoms. 5
- In MMR-vaccinated children with mumps-like symptoms, the actual cause is often not mumps virus—one study found that only 14% of such cases were due to other viruses (Epstein-Barr virus 7%, parainfluenza 4%, adenovirus 3%). 6
- Laboratory confirmation through CSF PCR, parotid-duct swabs, or serum antibody testing is essential to distinguish true mumps from mimicking illnesses. 5
Post-Exposure Vaccination Does Not Prevent Mumps
- Unlike measles, administering MMR vaccine after mumps exposure does not prevent infection or modify clinical severity. 5, 7
- This is a critical distinction: post-exposure MMR prophylaxis is ineffective for mumps, though it may provide some protection against measles if given within 72 hours. 7
Key Clinical Caveats
- Children born before 1957 are presumed immune due to natural infection, but this does not guarantee protection—during outbreaks, MMR should be considered even for this cohort. 2
- Persons with equivocal serologic test results should be considered susceptible unless they have physician-diagnosed mumps or laboratory evidence of immunity. 2
- The benefit of a second MMR dose is primarily to reduce the proportion of persons who remain susceptible due to primary vaccine failure, not to boost waning immunity. 2
- A second dose given later in childhood has only a minor and transient effect on anti-measles and anti-rubella titers, but does have a boosting effect on anti-mumps antibody levels. 3