Epidemiology: vaccine efficacy: 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: vaccine efficacy

Sample item

In a vaccine trial the attack rate is 10 percent among unvaccinated and 2 percent among vaccinated participants. What is the vaccine efficacy?

A80 percentB98 percentC20 percentD8 percent

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

The answer and the reasoning

A. 80 percent

Vaccine efficacy equals (attack rate in unvaccinated minus attack rate in vaccinated) divided by attack rate in unvaccinated. This is (10 minus 2) divided by 10, which is 80 percent.

Option by option

A. 80 percent (correct)
Correct. 8 divided by 10 is 80 percent.
B. 98 percent
98 percent is the proportion of vaccinated who did not get disease.
C. 20 percent
20 percent is the relative risk, 2 divided by 10, expressed as a percentage.
D. 8 percent
8 percent is the absolute difference, not the proportional reduction.

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 PSM question

What is the answer to this PSM question?

The best answer is A, 80 percent. Vaccine efficacy equals (attack rate in unvaccinated minus attack rate in vaccinated) divided by attack rate in unvaccinated.

Why is "98 percent" not the answer?

98 percent is the proportion of vaccinated who did not get disease.

Why is "20 percent" not the answer?

20 percent is the relative risk, 2 divided by 10, expressed as a percentage.

Why is "8 percent" not the answer?

8 percent is the absolute difference, not the proportional reduction.

What concept does this question test?

It tests Epidemiology: vaccine efficacy in PSM. This is (10 minus 2) divided by 10, which is 80 percent.

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: vaccine efficacy?

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.