A child immunized against measles at 9 months develops measles at age 3 years. Which immunological mechanism BEST explains vaccine failure in this scenario?
- A Primary vaccine failure due to maternal antibody interference
- B Secondary vaccine failure due to waning immunity ✓
- C Cold chain failure causing vaccine inactivation
- D Wrong route of administration
Explanation
Secondary vaccine failure (waning immunity) occurs when an initial immune response is mounted but protection diminishes over time, resulting in susceptibility to infection years later. Primary vaccine failure at 9 months — especially in resource-limited settings — can also occur due to residual maternal antibodies interfering with seroconversion. However, the scenario specifies a 3-year gap with disease at age 3, suggesting seroconversion occurred initially (primary vaccine success) followed by waning (secondary failure). The two-dose MR schedule (9 months + 15 months) was designed partly to overcome primary failure and provide a booster.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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