Bacteriology: Vi polysaccharide vaccine: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Bacteriology: Vi polysaccharide vaccine

Sample item

A paediatrician considers typhoid vaccination for a 12-month-old child. Why is the unconjugated Vi capsular polysaccharide vaccine unsuitable at this age?

AIt is a T-independent antigen that gives a poor response under two yearsBIt is a live vaccine that is contraindicated in infantsCIt protects only against Salmonella Paratyphi ADIt contains whole killed bacilli that cause severe reactions

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. It is a T-independent antigen that gives a poor response under two years

The Vi vaccine is a pure polysaccharide, a T-independent antigen, so infants under about two years respond poorly and without memory. Typhoid conjugate vaccines link Vi to a carrier protein and can be used in younger children.

Option by option

A. It is a T-independent antigen that gives a poor response under two years (correct)
Correct. A T-independent polysaccharide gives a poor infant response without memory.
B. It is a live vaccine that is contraindicated in infants
The Vi vaccine is not a live vaccine; the live typhoid vaccine is the oral Ty21a.
C. It protects only against Salmonella Paratyphi A
Vi antigen is found on S. Typhi and not on Paratyphi A, but this does not explain the age limit.
D. It contains whole killed bacilli that cause severe reactions
The Vi vaccine is a purified antigen, whereas whole-cell killed vaccine was the older reactogenic form.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, It is a T-independent antigen that gives a poor response under two years. The Vi vaccine is a pure polysaccharide, a T-independent antigen, so infants under about two years respond poorly and without memory.

Why is "It is a live vaccine that is contraindicated in infants" not the answer?

The Vi vaccine is not a live vaccine; the live typhoid vaccine is the oral Ty21a.

Why is "It protects only against Salmonella Paratyphi A" not the answer?

Vi antigen is found on S. Typhi and not on Paratyphi A, but this does not explain the age limit.

Why is "It contains whole killed bacilli that cause severe reactions" not the answer?

The Vi vaccine is a purified antigen, whereas whole-cell killed vaccine was the older reactogenic form.

What concept does this question test?

It tests Bacteriology: Vi polysaccharide vaccine in Microbiology. Typhoid conjugate vaccines link Vi to a carrier protein and can be used in younger children.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Bacteriology: Vi polysaccharide vaccine?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.