During a school outbreak investigation in Delhi, a 12-year-old girl presents with bilateral parotid swelling, fever (38.2°C), and mild trismus. She reports that a classmate had similar symptoms 2 weeks ago. The girl's immunization card shows she received two doses of MMR vaccine at ages 12 months and 18 months. Laboratory testing confirms mumps virus RNA by RT-PCR. What is the most likely explanation for vaccine failure in this case?