Recurrent Mumps in a Fully Vaccinated Young Adult With a Prior Breakthrough Infection: A Case Report.

Mumps is a vaccine-preventable viral disease that typically presents with parotid gland swelling and constitutional symptoms. Although widespread immunization has substantially reduced disease incidence, outbreaks continue to occur among vaccinated populations, raising concerns regarding waning immunity and vaccine failure. Recurrent mumps following both prior natural infection and complete vaccination is uncommon and remains infrequently reported. We describe a 21-year-old woman who presented with a four-day history of fever, malaise, myalgia, and progressive painful swelling of the left parotid gland associated with difficulty chewing and speaking. She had received two doses of measles-mumps-rubella (MMR) vaccine according to the national immunization schedule and had a documented history of mumps infection at nine years of age, occurring after she had already completed her two-dose MMR series (vaccine-breakthrough infection). Physical examination revealed tender unilateral parotid enlargement, cervical lymphadenopathy, and evidence of undernutrition with a body mass index of 16.7 kg/m². Laboratory evaluation demonstrated iron deficiency anemia. Further evaluation, including positive mumps RT-PCR and positive anti-mumps IgM serology, confirmed acute mumps infection, while investigations excluded alternative causes of parotid enlargement. The patient was managed conservatively with isolation, supportive care, nutritional supplementation, and symptomatic treatment, resulting in complete clinical recovery without complications. This case highlights that recurrent mumps can occur despite prior natural infection and complete MMR vaccination. Although waning immunity and host factors such as undernutrition may have contributed to susceptibility, a causal relationship cannot be established from a single case. Continued surveillance and awareness of breakthrough infections remain important, particularly in resource-limited settings.