Epidemiology: 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

Sample item

If the basic reproduction number (R0) of an infection is 4, roughly what proportion of the population must be immune to achieve herd immunity?

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

The answer and the reasoning

A. 75%

The herd immunity threshold is 1 − 1/R0. With R0 = 4 it is 1 − 0.25 = 0.75, or 75%. Higher R0 requires a higher proportion immune, which is why measles needs about 95% coverage.

Option by option

  • A. 75% (correct)

    Correct. 1 − 1/4 = 0.75.

  • B. 25%

    25% is 1/R0, which is the fraction that can remain susceptible, not immune.

  • C. 90%

    90% would correspond to R0 = 10.

  • D. 50%

    50% would correspond to R0 = 2.

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 A, 75%. The herd immunity threshold is 1 − 1/R0.

Why is "25%" not the answer?

25% is 1/R0, which is the fraction that can remain susceptible, not immune.

Why is "90%" not the answer?

90% would correspond to R0 = 10.

Why is "50%" not the answer?

50% would correspond to R0 = 2.

What concept does this question test?

It tests Epidemiology in PSM. With R0 = 4 it is 1 − 0.25 = 0.75, or 75%. Higher R0 requires a higher proportion immune, which is why measles needs about 95% coverage.

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 Epidemiology?

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.