A traveller receives a meningococcal ACWY conjugate vaccine. Two years later, serum bactericidal antibody (SBA) titres are checked and found to be ≥1:128. What does this SBA titre indicate?

• A The individual has active meningococcal infection
• B Vaccine failure requiring immediate booster
• C Cross-reactive antibodies from recent N. gonorrhoeae infection
• D Seroprotection — functional antibodies capable of complement-mediated killing of N. meningitidis ✓

Explanation

SBA titre ≥1:4 (with human complement) or ≥1:128 (with rabbit complement) is the accepted correlate of protection for meningococcal vaccines. The SBA assay measures the ability of patient serum antibodies to activate complement-mediated killing of Neisseria meningitidis in vitro, directly reflecting functional immunity. A titre of 1:128 indicates robust seroprotection. This assay is the gold standard surrogate for efficacy in vaccine trials. Cross-reactivity from gonococcal infection does not produce this level of specific bactericidal activity.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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