A 3-year-old child presents with fever, rash, and Koplik spots on the buccal mucosa. The child developed measles despite receiving one dose of measles vaccine at 9 months. The most likely explanation is:
- A Waning immunity after a single dose
- B Vaccine failure due to maternal antibody interference at 9 months ✓
- C Wrong route of administration
- D Viral mutation causing vaccine escape
Correct answer: B. Vaccine failure due to maternal antibody interference at 9 months
Explanation
Primary vaccine failure at 9 months is the leading explanation; maternally derived measles antibodies persist in some infants up to 9 months and can neutralise the live attenuated vaccine virus, preventing adequate seroconversion. This is why IAP recommends a second dose at 15 months (or as MR at 9–12 months with a second at 16–24 months), and a catch-up dose for primary failures. Waning immunity requires years and would not manifest by age 3 after vaccination at 9 months.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.