Epidemiology: herd immunity with an imperfect vaccine: practice question

A PSM 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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PSM · Epidemiology: herd immunity with an imperfect vaccine

Sample item

An infection has a basic reproduction number (R0) of 5. A vaccine is 90% effective in protecting those vaccinated. What approximate vaccination coverage is needed to reach the herd immunity threshold?

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

The answer and the reasoning

C. About 89%

The herd immunity threshold is 1 - 1/R0 = 1 - 0.2 = 80% of the population immune. If the vaccine protects only 90% of recipients, coverage must be higher: 80 / 90 = about 89%. Imperfect efficacy therefore raises the coverage needed.

Option by option

  • A. 80%

    80% is the threshold of immune people and would be sufficient only with a vaccine that is 100% effective.

  • B. About 95%

    95% would be required for a much more transmissive infection such as measles, not for an R0 of 5.

  • C. About 89% (correct)

    Dividing the immune proportion required (80%) by vaccine efficacy (0.9) gives about 89% coverage.

  • D. About 72%

    Multiplying 0.8 by 0.9 wrongly lowers the target, whereas imperfect efficacy should raise it.

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.

More PSM questions

Questions about this PSM question

What is the answer to this PSM question?

The best answer is C, About 89%. The herd immunity threshold is 1 - 1/R0 = 1 - 0.

Why is "80%" not the answer?

80% is the threshold of immune people and would be sufficient only with a vaccine that is 100% effective.

Why is "About 95%" not the answer?

95% would be required for a much more transmissive infection such as measles, not for an R0 of 5.

Why is "About 72%" not the answer?

Multiplying 0.8 by 0.9 wrongly lowers the target, whereas imperfect efficacy should raise it.

What concept does this question test?

It tests Epidemiology: herd immunity with an imperfect vaccine in PSM. 2 = 80% of the population immune. If the vaccine protects only 90% of recipients, coverage must be higher: 80 / 90 = about 89%. Imperfect efficacy therefore raises the coverage needed.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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 Epidemiology: herd immunity with an imperfect vaccine?

Practise biostatistics with small calculations until sensitivity, predictive values and risk measures are automatic. Then use the flashcards and the other PSM 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.