Why is the first dose of measles-containing vaccine scheduled at 9 completed months of age rather than earlier in India's routine programme?

• A Infants below 9 months lack the thymic capacity to respond to live viral antigens
• B The vaccine strain causes measles-like illness in younger infants because of immature cellular immunity
• C Measles incidence peaks only after the ninth month, so earlier vaccination offers no benefit
• D Maternally derived IgG antibodies neutralize the vaccine virus and blunt seroconversion before this age ✓

Explanation

Transplacentally acquired maternal IgG against measles persists for roughly the first 6 to 9 months and neutralizes the attenuated Edmonston-derived vaccine virus, producing primary vaccine failure if given too early. The 9-month timing balances early protection against the risk of waning maternal antibody leaving a susceptible window. Young infants can mount responses to live viral vaccines, as shown by successful outbreak-response measles vaccination at 6 months followed by a repeat dose at 9 months.

Reference: Nelson Textbook of Pediatrics, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.