A 15-month-old child from Delhi is brought to the primary health centre for routine immunisation review. The immunisation card is missing, and the mother cannot recall which vaccines the child has received. The health worker needs to determine the child's immunity status to measles before deciding on catch-up vaccination strategy. Which investigation would be most appropriate to confirm measles immunity status?

Explanation

## Serological Assessment of Measles Immunity in UIP ### Clinical Scenario A child with unknown immunisation history requires confirmation of measles immunity status to guide catch-up vaccination decisions in India's Universal Immunisation Programme. ### Investigation of Choice: Serum Measles IgG Antibody Detection **Key Point:** Measles IgG antibodies indicate past infection or successful vaccination-induced immunity. Detection of IgG confirms protective immunity and guides the decision to vaccinate or defer. **High-Yield:** In UIP: - Measles IgG positivity (≥1:16 or ≥120 mIU/mL by ELISA) indicates protective immunity - IgG is the marker of choice for assessing immunity status in children with unknown vaccination history - This test prevents unnecessary revaccination in already-immune children and identifies susceptible children requiring catch-up doses ### Serological Markers in Measles | Antibody | Indicates | Clinical Use | Timing | |----------|-----------|--------------|--------| | **IgM** | Acute/recent infection | Diagnosis of active measles | Early infection (first week) | | **IgG** | Past infection or vaccination | Immunity assessment & catch-up decisions | Lifelong after infection/vaccination | | **Neutralising Ab** | Functional immunity | Research, not routine practice | Persists after vaccination | **Clinical Pearl:** Measles IgG testing is the cornerstone of immunisation surveillance in UIP. A positive IgG result in a child with unknown history confirms immunity and eliminates the need for catch-up measles vaccination, reducing vaccine wastage and unnecessary injections. ### Mnemonic: **IgG = Immunity Gone Good** - **Ig**G antibodies indicate past exposure (infection or vaccine) - **G**uides catch-up vaccination decisions - **G**ood for assessing population immunity levels **Warning:** Do not confuse IgM (acute infection marker) with IgG (immunity marker). IgM is used to diagnose active measles disease, not to assess immunity status. [cite:Park 26e Ch 7]