Natural history of infection: serial interval: 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 · Natural history of infection: serial interval

Sample item

In a village outbreak of a viral infection, case A developed symptoms on 1 January. Case B, infected by A, developed symptoms on 12 January, and case C, infected by B, developed symptoms on 22 January. What is the mean serial interval in this chain?

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

The answer and the reasoning

B. 10.5 days

The serial interval is the time between symptom onset in a primary case and symptom onset in the secondary case it infects. Here it is 11 days from A to B and 10 days from B to C, so the mean is (11 + 10) / 2 = 10.5 days. It differs from the incubation period, which is measured from infection to onset in a single person.

Option by option

  • A. 11 days

    This is the first interval only (A to B) and ignores the second pair.

  • B. 10.5 days (correct)

    Correct. The mean of the two successive onset-to-onset intervals, 11 and 10 days, is 10.5 days.

  • C. 7 days

    This divides the 21-day span by the three cases instead of the two intervals.

  • D. 21 days

    This is the total span from A to C across two generations, not the interval per generation.

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 B, 10.5 days. The serial interval is the time between symptom onset in a primary case and symptom onset in the secondary case it infects.

Why is "11 days" not the answer?

This is the first interval only (A to B) and ignores the second pair.

Why is "7 days" not the answer?

This divides the 21-day span by the three cases instead of the two intervals.

Why is "21 days" not the answer?

This is the total span from A to C across two generations, not the interval per generation.

What concept does this question test?

It tests Natural history of infection: serial interval in PSM. Here it is 11 days from A to B and 10 days from B to C, so the mean is (11 + 10) / 2 = 10.5 days. It differs from the incubation period, which is measured from infection to onset in a single person.

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 Natural history of infection: serial interval?

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.