A 5-year-old child presents to a travel clinic before visiting a typhoid-endemic region. The physician chooses the Vi polysaccharide typhoid vaccine (Typhim Vi) over the Ty21a oral vaccine. The Vi polysaccharide vaccine is classified as which type of immune stimulus, and what is its major immunological limitation?

• A T-cell-dependent antigen; limitation is short duration of protection
• B Recombinant subunit antigen; limitation is requirement for cold chain storage at -20°C
• C Live attenuated organism; limitation is risk of vaccine-derived disease in immunocompromised hosts
• D T-cell-independent antigen; limitation is poor immunogenicity in children under 2 years and lack of memory response ✓

Explanation

The Vi capsular polysaccharide vaccine is a purified polysaccharide, making it a T-cell-independent type 2 (TI-2) antigen. TI-2 antigens cannot be presented via MHC class II and fail to engage T follicular helper cells, so they produce poor responses in infants and young children (under 2 years), lack affinity maturation, and do not generate immunological memory. The newer Vi-CRM197 conjugate vaccine overcomes this limitation.

Reference: Plotkin's Vaccines, 7th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.